configuration of the server There are many configuration parameters that affect the behavior of the database system. In the first section of this chapter we describe how to interact with configuration parameters. The subsequent sections discuss each parameter in detail. All parameter names are case-insensitive. Every parameter takes a value of one of five types: boolean, string, integer, floating point, or enumerated (enum). The type determines the syntax for setting the parameter: Boolean: Values can be written as on, off, true, false, yes, no, 1, 0 (all case-insensitive) or any unambiguous prefix of one of these. String: In general, enclose the value in single quotes, doubling any single quotes within the value. Quotes can usually be omitted if the value is a simple number or identifier, however. Numeric (integer and floating point): A decimal point is permitted only for floating-point parameters. Do not use thousands separators. Quotes are not required. Numeric with Unit: Some numeric parameters have an implicit unit, because they describe quantities of memory or time. The unit might be kilobytes, blocks (typically eight kilobytes), milliseconds, seconds, or minutes. An unadorned numeric value for one of these settings will use the setting's default unit, which can be learned from pg_settings.unit. For convenience, settings can be given with a unit specified explicitly, for example '120 ms' for a time value, and they will be converted to whatever the parameter's actual unit is. Note that the value must be written as a string (with quotes) to use this feature. The unit name is case-sensitive, and there can be whitespace between the numeric value and the unit. Valid memory units are kB (kilobytes), MB (megabytes), GB (gigabytes), and TB (terabytes). The multiplier for memory units is 1024, not 1000. Valid time units are ms (milliseconds), s (seconds), min (minutes), h (hours), and d (days). Enumerated: Enumerated-type parameters are written in the same way as string parameters, but are restricted to have one of a limited set of values. The values allowable for such a parameter can be found from pg_settings.enumvals. Enum parameter values are case-insensitive. The most fundamental way to set these parameters is to edit the file postgresql.confpostgresql.conf, which is normally kept in the data directory. A default copy is installed when the database cluster directory is initialized. An example of what this file might look like is: # This is a comment log_connections = yes log_destination = 'syslog' search_path = '"$user", public' shared_buffers = 128MB One parameter is specified per line. The equal sign between name and value is optional. Whitespace is insignificant (except within a quoted parameter value) and blank lines are ignored. Hash marks (#) designate the remainder of the line as a comment. Parameter values that are not simple identifiers or numbers must be single-quoted. To embed a single quote in a parameter value, write either two quotes (preferred) or backslash-quote. Parameters set in this way provide default values for the cluster. The settings seen by active sessions will be these values unless they are overridden. The following sections describe ways in which the administrator or user can override these defaults. SIGHUP The configuration file is reread whenever the main server process receives a SIGHUP signal; this signal is most easily sent by running pg_ctl reload from the command line or by calling the SQL function pg_reload_conf(). The main server process also propagates this signal to all currently running server processes, so that existing sessions also adopt the new values (this will happen after they complete any currently-executing client command). Alternatively, you can send the signal to a single server process directly. Some parameters can only be set at server start; any changes to their entries in the configuration file will be ignored until the server is restarted. Invalid parameter settings in the configuration file are likewise ignored (but logged) during SIGHUP processing. In addition to postgresql.conf, a PostgreSQL data directory contains a file postgresql.auto.confpostgresql.auto.conf, which has the same format as postgresql.conf but should never be edited manually. This file holds settings provided through the command. This file is automatically read whenever postgresql.conf is, and its settings take effect in the same way. Settings in postgresql.auto.conf override those in postgresql.conf. The system view pg_file_settings can be helpful for pre-testing changes to the configuration file, or for diagnosing problems if a SIGHUP signal did not have the desired effects. PostgreSQL provides three SQL commands to establish configuration defaults. The already-mentioned command provides a SQL-accessible means of changing global defaults; it is functionally equivalent to editing postgresql.conf. In addition, there are two commands that allow setting of defaults on a per-database or per-role basis: The command allows global settings to be overridden on a per-database basis. The command allows both global and per-database settings to be overridden with user-specific values. Values set with ALTER DATABASE and ALTER ROLE are applied only when starting a fresh database session. They override values obtained from the configuration files or server command line, and constitute defaults for the rest of the session. Note that some settings cannot be changed after server start, and so cannot be set with these commands (or the ones listed below). Once a client is connected to the database, PostgreSQL provides two additional SQL commands (and equivalent functions) to interact with session-local configuration settings: The command allows inspection of the current value of all parameters. The corresponding function is current_setting(setting_name text). The command allows modification of the current value of those parameters that can be set locally to a session; it has no effect on other sessions. The corresponding function is set_config(setting_name, new_value, is_local). In addition, the system view pg_settings can be used to view and change session-local values: Querying this view is similar to using SHOW ALL but provides more detail. It is also more flexible, since it's possible to specify filter conditions or join against other relations. Using on this view, specifically updating the setting column, is the equivalent of issuing SET commands. For example, the equivalent of SET configuration_parameter TO DEFAULT; is: UPDATE pg_settings SET setting = reset_val WHERE name = 'configuration_parameter'; In addition to setting global defaults or attaching overrides at the database or role level, you can pass settings to PostgreSQL via shell facilities. Both the server and libpq client library accept parameter values via the shell. During server startup, parameter settings can be passed to the postgres command via the -c command-line parameter. For example, postgres -c log_connections=yes -c log_destination='syslog' Settings provided in this way override those set via postgresql.conf or ALTER SYSTEM, so they cannot be changed globally without restarting the server. When starting a client session via libpq, parameter settings can be specified using the PGOPTIONS environment variable. Settings established in this way constitute defaults for the life of the session, but do not affect other sessions. For historical reasons, the format of PGOPTIONS is similar to that used when launching the postgres command; specifically, the -c flag must be specified. For example, env PGOPTIONS="-c geqo=off -c statement_timeout=5min" psql Other clients and libraries might provide their own mechanisms, via the shell or otherwise, that allow the user to alter session settings without direct use of SQL commands. PostgreSQL provides several features for breaking down complex postgresql.conf files into sub-files. These features are especially useful when managing multiple servers with related, but not identical, configurations. include in configuration file In addition to individual parameter settings, the postgresql.conf file can contain include directives, which specify another file to read and process as if it were inserted into the configuration file at this point. This feature allows a configuration file to be divided into physically separate parts. Include directives simply look like: include 'filename' If the file name is not an absolute path, it is taken as relative to the directory containing the referencing configuration file. Inclusions can be nested. include_if_exists in configuration file There is also an include_if_exists directive, which acts the same as the include directive, except when the referenced file does not exist or cannot be read. A regular include will consider this an error condition, but include_if_exists merely logs a message and continues processing the referencing configuration file. include_dir in configuration file The postgresql.conf file can also contain include_dir directives, which specify an entire directory of configuration files to include. These look like include_dir 'directory' Non-absolute directory names are taken as relative to the directory containing the referencing configuration file. Within the specified directory, only non-directory files whose names end with the suffix .conf will be included. File names that start with the . character are also ignored, to prevent mistakes since such files are hidden on some platforms. Multiple files within an include directory are processed in file name order (according to C locale rules, i.e. numbers before letters, and uppercase letters before lowercase ones). Include files or directories can be used to logically separate portions of the database configuration, rather than having a single large postgresql.conf file. Consider a company that has two database servers, each with a different amount of memory. There are likely elements of the configuration both will share, for things such as logging. But memory-related parameters on the server will vary between the two. And there might be server specific customizations, too. One way to manage this situation is to break the custom configuration changes for your site into three files. You could add this to the end of your postgresql.conf file to include them: include 'shared.conf' include 'memory.conf' include 'server.conf' All systems would have the same shared.conf. Each server with a particular amount of memory could share the same memory.conf; you might have one for all servers with 8GB of RAM, another for those having 16GB. And finally server.conf could have truly server-specific configuration information in it. Another possibility is to create a configuration file directory and put this information into files there. For example, a conf.d directory could be referenced at the end of postgresql.conf: include_dir 'conf.d' Then you could name the files in the conf.d directory like this: 00shared.conf 01memory.conf 02server.conf This naming convention establishes a clear order in which these files will be loaded. This is important because only the last setting encountered for a particular parameter while the server is reading configuration files will be used. In this example, something set in conf.d/02server.conf would override a value set in conf.d/01memory.conf. You might instead use this approach to naming the files descriptively: 00shared.conf 01memory-8GB.conf 02server-foo.conf This sort of arrangement gives a unique name for each configuration file variation. This can help eliminate ambiguity when several servers have their configurations all stored in one place, such as in a version control repository. (Storing database configuration files under version control is another good practice to consider.) In addition to the postgresql.conf file already mentioned, PostgreSQL uses two other manually-edited configuration files, which control client authentication (their use is discussed in ). By default, all three configuration files are stored in the database cluster's data directory. The parameters described in this section allow the configuration files to be placed elsewhere. (Doing so can ease administration. In particular it is often easier to ensure that the configuration files are properly backed-up when they are kept separate.) data_directory (string) data_directory configuration parameter Specifies the directory to use for data storage. This parameter can only be set at server start. config_file (string) config_file configuration parameter Specifies the main server configuration file (customarily called postgresql.conf). This parameter can only be set on the postgres command line. hba_file (string) hba_file configuration parameter Specifies the configuration file for host-based authentication (customarily called pg_hba.conf). This parameter can only be set at server start. ident_file (string) ident_file configuration parameter Specifies the configuration file for user name mapping (customarily called pg_ident.conf). This parameter can only be set at server start. external_pid_file (string) external_pid_file configuration parameter Specifies the name of an additional process-ID (PID) file that the server should create for use by server administration programs. This parameter can only be set at server start. In a default installation, none of the above parameters are set explicitly. Instead, the data directory is specified by the -D command-line option or the PGDATA environment variable, and the configuration files are all found within the data directory. If you wish to keep the configuration files elsewhere than the data directory, the postgres -D command-line option or PGDATA environment variable must point to the directory containing the configuration files, and the data_directory parameter must be set in postgresql.conf (or on the command line) to show where the data directory is actually located. Notice that data_directory overrides -D and PGDATA for the location of the data directory, but not for the location of the configuration files. If you wish, you can specify the configuration file names and locations individually using the parameters config_file, hba_file and/or ident_file. config_file can only be specified on the postgres command line, but the others can be set within the main configuration file. If all three parameters plus data_directory are explicitly set, then it is not necessary to specify -D or PGDATA. When setting any of these parameters, a relative path will be interpreted with respect to the directory in which postgres is started. listen_addresses (string) listen_addresses configuration parameter Specifies the TCP/IP address(es) on which the server is to listen for connections from client applications. The value takes the form of a comma-separated list of host names and/or numeric IP addresses. The special entry * corresponds to all available IP interfaces. The entry 0.0.0.0 allows listening for all IPv4 addresses and :: allows listening for all IPv6 addresses. If the list is empty, the server does not listen on any IP interface at all, in which case only Unix-domain sockets can be used to connect to it. The default value is localhost, which allows only local TCP/IP loopback connections to be made. While client authentication () allows fine-grained control over who can access the server, listen_addresses controls which interfaces accept connection attempts, which can help prevent repeated malicious connection requests on insecure network interfaces. This parameter can only be set at server start. port (integer) port configuration parameter The TCP port the server listens on; 5432 by default. Note that the same port number is used for all IP addresses the server listens on. This parameter can only be set at server start. max_connections (integer) max_connections configuration parameter Determines the maximum number of concurrent connections to the database server. The default is typically 100 connections, but might be less if your kernel settings will not support it (as determined during initdb). This parameter can only be set at server start. When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server. superuser_reserved_connections (integer) superuser_reserved_connections configuration parameter Determines the number of connection slots that are reserved for connections by PostgreSQL superusers. At most connections can ever be active simultaneously. Whenever the number of active concurrent connections is at least max_connections minus superuser_reserved_connections, new connections will be accepted only for superusers, and no new replication connections will be accepted. The default value is three connections. The value must be less than the value of max_connections. This parameter can only be set at server start. unix_socket_directories (string) unix_socket_directories configuration parameter Specifies the directory of the Unix-domain socket(s) on which the server is to listen for connections from client applications. Multiple sockets can be created by listing multiple directories separated by commas. Whitespace between entries is ignored; surround a directory name with double quotes if you need to include whitespace or commas in the name. An empty value specifies not listening on any Unix-domain sockets, in which case only TCP/IP sockets can be used to connect to the server. The default value is normally /tmp, but that can be changed at build time. This parameter can only be set at server start. In addition to the socket file itself, which is named .s.PGSQL.nnnn where nnnn is the server's port number, an ordinary file named .s.PGSQL.nnnn.lock will be created in each of the unix_socket_directories directories. Neither file should ever be removed manually. This parameter is irrelevant on Windows, which does not have Unix-domain sockets. unix_socket_group (string) unix_socket_group configuration parameter Sets the owning group of the Unix-domain socket(s). (The owning user of the sockets is always the user that starts the server.) In combination with the parameter unix_socket_permissions this can be used as an additional access control mechanism for Unix-domain connections. By default this is the empty string, which uses the default group of the server user. This parameter can only be set at server start. This parameter is irrelevant on Windows, which does not have Unix-domain sockets. unix_socket_permissions (integer) unix_socket_permissions configuration parameter Sets the access permissions of the Unix-domain socket(s). Unix-domain sockets use the usual Unix file system permission set. The parameter value is expected to be a numeric mode specified in the format accepted by the chmod and umask system calls. (To use the customary octal format the number must start with a 0 (zero).) The default permissions are 0777, meaning anyone can connect. Reasonable alternatives are 0770 (only user and group, see also unix_socket_group) and 0700 (only user). (Note that for a Unix-domain socket, only write permission matters, so there is no point in setting or revoking read or execute permissions.) This access control mechanism is independent of the one described in . This parameter can only be set at server start. This parameter is irrelevant on systems, notably Solaris as of Solaris 10, that ignore socket permissions entirely. There, one can achieve a similar effect by pointing unix_socket_directories to a directory having search permission limited to the desired audience. This parameter is also irrelevant on Windows, which does not have Unix-domain sockets. bonjour (boolean) bonjour configuration parameter Enables advertising the server's existence via Bonjour. The default is off. This parameter can only be set at server start. bonjour_name (string) bonjour_name configuration parameter Specifies the Bonjour service name. The computer name is used if this parameter is set to the empty string '' (which is the default). This parameter is ignored if the server was not compiled with Bonjour support. This parameter can only be set at server start. tcp_keepalives_idle (integer) tcp_keepalives_idle configuration parameter Specifies the number of seconds of inactivity after which TCP should send a keepalive message to the client. A value of 0 uses the system default. This parameter is supported only on systems that support the TCP_KEEPIDLE or TCP_KEEPALIVE symbols, and on Windows; on other systems, it must be zero. In sessions connected via a Unix-domain socket, this parameter is ignored and always reads as zero. On Windows, a value of 0 will set this parameter to 2 hours, since Windows does not provide a way to read the system default value. tcp_keepalives_interval (integer) tcp_keepalives_interval configuration parameter Specifies the number of seconds after which a TCP keepalive message that is not acknowledged by the client should be retransmitted. A value of 0 uses the system default. This parameter is supported only on systems that support the TCP_KEEPINTVL symbol, and on Windows; on other systems, it must be zero. In sessions connected via a Unix-domain socket, this parameter is ignored and always reads as zero. On Windows, a value of 0 will set this parameter to 1 second, since Windows does not provide a way to read the system default value. tcp_keepalives_count (integer) tcp_keepalives_count configuration parameter Specifies the number of TCP keepalives that can be lost before the server's connection to the client is considered dead. A value of 0 uses the system default. This parameter is supported only on systems that support the TCP_KEEPCNT symbol; on other systems, it must be zero. In sessions connected via a Unix-domain socket, this parameter is ignored and always reads as zero. This parameter is not supported on Windows, and must be zero. authentication_timeout (integer) timeoutclient authentication client authenticationtimeout during authentication_timeout configuration parameter Maximum time to complete client authentication, in seconds. If a would-be client has not completed the authentication protocol in this much time, the server closes the connection. This prevents hung clients from occupying a connection indefinitely. The default is one minute (1m). This parameter can only be set in the postgresql.conf file or on the server command line. ssl (boolean) ssl configuration parameter Enables SSL connections. Please read before using this. The default is off. This parameter can only be set at server start. SSL communication is only possible with TCP/IP connections. ssl_ca_file (string) ssl_ca_file configuration parameter Specifies the name of the file containing the SSL server certificate authority (CA). The default is empty, meaning no CA file is loaded, and client certificate verification is not performed. (In previous releases of PostgreSQL, the name of this file was hard-coded as root.crt.) Relative paths are relative to the data directory. This parameter can only be set at server start. ssl_cert_file (string) ssl_cert_file configuration parameter Specifies the name of the file containing the SSL server certificate. The default is server.crt. Relative paths are relative to the data directory. This parameter can only be set at server start. ssl_crl_file (string) ssl_crl_file configuration parameter Specifies the name of the file containing the SSL server certificate revocation list (CRL). The default is empty, meaning no CRL file is loaded. (In previous releases of PostgreSQL, the name of this file was hard-coded as root.crl.) Relative paths are relative to the data directory. This parameter can only be set at server start. ssl_key_file (string) ssl_key_file configuration parameter Specifies the name of the file containing the SSL server private key. The default is server.key. Relative paths are relative to the data directory. This parameter can only be set at server start. ssl_ciphers (string) ssl_ciphers configuration parameter Specifies a list of SSL cipher suites that are allowed to be used on secure connections. See the ciphers manual page in the OpenSSL package for the syntax of this setting and a list of supported values. The default value is HIGH:MEDIUM:+3DES:!aNULL. It is usually reasonable, unless you have specific security requirements. Explanation of the default value: HIGH Cipher suites that use ciphers from HIGH group (e.g., AES, Camellia, 3DES) MEDIUM Cipher suites that use ciphers from MEDIUM group (e.g., RC4, SEED) +3DES The OpenSSL default order for HIGH is problematic because it orders 3DES higher than AES128. This is wrong because 3DES offers less security than AES128, and it is also much slower. +3DES reorders it after all other HIGH and MEDIUM ciphers. !aNULL Disables anonymous cipher suites that do no authentication. Such cipher suites are vulnerable to man-in-the-middle attacks and therefore should not be used. Available cipher suite details will vary across OpenSSL versions. Use the command openssl ciphers -v 'HIGH:MEDIUM:+3DES:!aNULL' to see actual details for the currently installed OpenSSL version. Note that this list is filtered at run time based on the server key type. ssl_prefer_server_ciphers (bool) ssl_prefer_server_ciphers configuration parameter Specifies whether to use the server's SSL cipher preferences, rather than the client's. The default is true. Older PostgreSQL versions do not have this setting and always use the client's preferences. This setting is mainly for backward compatibility with those versions. Using the server's preferences is usually better because it is more likely that the server is appropriately configured. ssl_ecdh_curve (string) ssl_ecdh_curve configuration parameter Specifies the name of the curve to use in ECDH key exchange. It needs to be supported by all clients that connect. It does not need to be same curve as used by server's Elliptic Curve key. The default is prime256v1. OpenSSL names for most common curves: prime256v1 (NIST P-256), secp384r1 (NIST P-384), secp521r1 (NIST P-521). The full list of available curves can be shown with the command openssl ecparam -list_curves. Not all of them are usable in TLS though. password_encryption (boolean) password_encryption configuration parameter When a password is specified in or without writing either ENCRYPTED or UNENCRYPTED, this parameter determines whether the password is to be encrypted. The default is on (encrypt the password). krb_server_keyfile (string) krb_server_keyfile configuration parameter Sets the location of the Kerberos server key file. See for details. This parameter can only be set in the postgresql.conf file or on the server command line. krb_caseins_users (boolean) krb_caseins_users configuration parameter Sets whether GSSAPI user names should be treated case-insensitively. The default is off (case sensitive). This parameter can only be set in the postgresql.conf file or on the server command line. db_user_namespace (boolean) db_user_namespace configuration parameter This parameter enables per-database user names. It is off by default. This parameter can only be set in the postgresql.conf file or on the server command line. If this is on, you should create users as username@dbname. When username is passed by a connecting client, @ and the database name are appended to the user name and that database-specific user name is looked up by the server. Note that when you create users with names containing @ within the SQL environment, you will need to quote the user name. With this parameter enabled, you can still create ordinary global users. Simply append @ when specifying the user name in the client, e.g. joe@. The @ will be stripped off before the user name is looked up by the server. db_user_namespace causes the client's and server's user name representation to differ. Authentication checks are always done with the server's user name so authentication methods must be configured for the server's user name, not the client's. Because md5 uses the user name as salt on both the client and server, md5 cannot be used with db_user_namespace. This feature is intended as a temporary measure until a complete solution is found. At that time, this option will be removed. shared_buffers (integer) shared_buffers configuration parameter Sets the amount of memory the database server uses for shared memory buffers. The default is typically 128 megabytes (128MB), but might be less if your kernel settings will not support it (as determined during initdb). This setting must be at least 128 kilobytes. (Non-default values of BLCKSZ change the minimum.) However, settings significantly higher than the minimum are usually needed for good performance. This parameter can only be set at server start. If you have a dedicated database server with 1GB or more of RAM, a reasonable starting value for shared_buffers is 25% of the memory in your system. There are some workloads where even large settings for shared_buffers are effective, but because PostgreSQL also relies on the operating system cache, it is unlikely that an allocation of more than 40% of RAM to shared_buffers will work better than a smaller amount. Larger settings for shared_buffers usually require a corresponding increase in max_wal_size, in order to spread out the process of writing large quantities of new or changed data over a longer period of time. On systems with less than 1GB of RAM, a smaller percentage of RAM is appropriate, so as to leave adequate space for the operating system. Also, on Windows, large values for shared_buffers aren't as effective. You may find better results keeping the setting relatively low and using the operating system cache more instead. The useful range for shared_buffers on Windows systems is generally from 64MB to 512MB. huge_pages (enum) huge_pages configuration parameter Enables/disables the use of huge memory pages. Valid values are try (the default), on, and off. At present, this feature is supported only on Linux. The setting is ignored on other systems when set to try. The use of huge pages results in smaller page tables and less CPU time spent on memory management, increasing performance. For more details, see . With huge_pages set to try, the server will try to use huge pages, but fall back to using normal allocation if that fails. With on, failure to use huge pages will prevent the server from starting up. With off, huge pages will not be used. temp_buffers (integer) temp_buffers configuration parameter Sets the maximum number of temporary buffers used by each database session. These are session-local buffers used only for access to temporary tables. The default is eight megabytes (8MB). The setting can be changed within individual sessions, but only before the first use of temporary tables within the session; subsequent attempts to change the value will have no effect on that session. A session will allocate temporary buffers as needed up to the limit given by temp_buffers. The cost of setting a large value in sessions that do not actually need many temporary buffers is only a buffer descriptor, or about 64 bytes, per increment in temp_buffers. However if a buffer is actually used an additional 8192 bytes will be consumed for it (or in general, BLCKSZ bytes). max_prepared_transactions (integer) max_prepared_transactions configuration parameter Sets the maximum number of transactions that can be in the prepared state simultaneously (see ). Setting this parameter to zero (which is the default) disables the prepared-transaction feature. This parameter can only be set at server start. If you are not planning to use prepared transactions, this parameter should be set to zero to prevent accidental creation of prepared transactions. If you are using prepared transactions, you will probably want max_prepared_transactions to be at least as large as , so that every session can have a prepared transaction pending. When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server. work_mem (integer) work_mem configuration parameter Specifies the amount of memory to be used by internal sort operations and hash tables before writing to temporary disk files. The value defaults to four megabytes (4MB). Note that for a complex query, several sort or hash operations might be running in parallel; each operation will be allowed to use as much memory as this value specifies before it starts to write data into temporary files. Also, several running sessions could be doing such operations concurrently. Therefore, the total memory used could be many times the value of work_mem; it is necessary to keep this fact in mind when choosing the value. Sort operations are used for ORDER BY, DISTINCT, and merge joins. Hash tables are used in hash joins, hash-based aggregation, and hash-based processing of IN subqueries. maintenance_work_mem (integer) maintenance_work_mem configuration parameter Specifies the maximum amount of memory to be used by maintenance operations, such as VACUUM, CREATE INDEX, and ALTER TABLE ADD FOREIGN KEY. It defaults to 64 megabytes (64MB). Since only one of these operations can be executed at a time by a database session, and an installation normally doesn't have many of them running concurrently, it's safe to set this value significantly larger than work_mem. Larger settings might improve performance for vacuuming and for restoring database dumps. Note that when autovacuum runs, up to times this memory may be allocated, so be careful not to set the default value too high. It may be useful to control for this by separately setting . replacement_sort_tuples (integer) replacement_sort_tuples configuration parameter When the number of tuples to be sorted is smaller than this number, a sort will produce its first output run using replacement selection rather than quicksort. This may be useful in memory-constrained environments where tuples that are input into larger sort operations have a strong physical-to-logical correlation. Note that this does not include input tuples with an inverse correlation. It is possible for the replacement selection algorithm to generate one long run that requires no merging, where use of the default strategy would result in many runs that must be merged to produce a final sorted output. This may allow sort operations to complete sooner. The default is 150,000 tuples. Note that higher values are typically not much more effective, and may be counter-productive, since the priority queue is sensitive to the size of available CPU cache, whereas the default strategy sorts runs using a cache oblivious algorithm. This property allows the default sort strategy to automatically and transparently make effective use of available CPU cache. Setting maintenance_work_mem to its default value usually prevents utility command external sorts (e.g., sorts used by CREATE INDEX to build B-Tree indexes) from ever using replacement selection sort, unless the input tuples are quite wide. autovacuum_work_mem (integer) autovacuum_work_mem configuration parameter Specifies the maximum amount of memory to be used by each autovacuum worker process. It defaults to -1, indicating that the value of should be used instead. The setting has no effect on the behavior of VACUUM when run in other contexts. max_stack_depth (integer) max_stack_depth configuration parameter Specifies the maximum safe depth of the server's execution stack. The ideal setting for this parameter is the actual stack size limit enforced by the kernel (as set by ulimit -s or local equivalent), less a safety margin of a megabyte or so. The safety margin is needed because the stack depth is not checked in every routine in the server, but only in key potentially-recursive routines such as expression evaluation. The default setting is two megabytes (2MB), which is conservatively small and unlikely to risk crashes. However, it might be too small to allow execution of complex functions. Only superusers can change this setting. Setting max_stack_depth higher than the actual kernel limit will mean that a runaway recursive function can crash an individual backend process. On platforms where PostgreSQL can determine the kernel limit, the server will not allow this variable to be set to an unsafe value. However, not all platforms provide the information, so caution is recommended in selecting a value. dynamic_shared_memory_type (enum) dynamic_shared_memory_type configuration parameter Specifies the dynamic shared memory implementation that the server should use. Possible values are posix (for POSIX shared memory allocated using shm_open), sysv (for System V shared memory allocated via shmget), windows (for Windows shared memory), mmap (to simulate shared memory using memory-mapped files stored in the data directory), and none (to disable this feature). Not all values are supported on all platforms; the first supported option is the default for that platform. The use of the mmap option, which is not the default on any platform, is generally discouraged because the operating system may write modified pages back to disk repeatedly, increasing system I/O load; however, it may be useful for debugging, when the pg_dynshmem directory is stored on a RAM disk, or when other shared memory facilities are not available. temp_file_limit (integer) temp_file_limit configuration parameter Specifies the maximum amount of disk space that a session can use for temporary files, such as sort and hash temporary files, or the storage file for a held cursor. A transaction attempting to exceed this limit will be canceled. The value is specified in kilobytes, and -1 (the default) means no limit. Only superusers can change this setting. This setting constrains the total space used at any instant by all temporary files used by a given PostgreSQL session. It should be noted that disk space used for explicit temporary tables, as opposed to temporary files used behind-the-scenes in query execution, does not count against this limit. max_files_per_process (integer) max_files_per_process configuration parameter Sets the maximum number of simultaneously open files allowed to each server subprocess. The default is one thousand files. If the kernel is enforcing a safe per-process limit, you don't need to worry about this setting. But on some platforms (notably, most BSD systems), the kernel will allow individual processes to open many more files than the system can actually support if many processes all try to open that many files. If you find yourself seeing Too many open files failures, try reducing this setting. This parameter can only be set at server start. During the execution of and commands, the system maintains an internal counter that keeps track of the estimated cost of the various I/O operations that are performed. When the accumulated cost reaches a limit (specified by vacuum_cost_limit), the process performing the operation will sleep for a short period of time, as specified by vacuum_cost_delay. Then it will reset the counter and continue execution. The intent of this feature is to allow administrators to reduce the I/O impact of these commands on concurrent database activity. There are many situations where it is not important that maintenance commands like VACUUM and ANALYZE finish quickly; however, it is usually very important that these commands do not significantly interfere with the ability of the system to perform other database operations. Cost-based vacuum delay provides a way for administrators to achieve this. This feature is disabled by default for manually issued VACUUM commands. To enable it, set the vacuum_cost_delay variable to a nonzero value. vacuum_cost_delay (integer) vacuum_cost_delay configuration parameter The length of time, in milliseconds, that the process will sleep when the cost limit has been exceeded. The default value is zero, which disables the cost-based vacuum delay feature. Positive values enable cost-based vacuuming. Note that on many systems, the effective resolution of sleep delays is 10 milliseconds; setting vacuum_cost_delay to a value that is not a multiple of 10 might have the same results as setting it to the next higher multiple of 10. When using cost-based vacuuming, appropriate values for vacuum_cost_delay are usually quite small, perhaps 10 or 20 milliseconds. Adjusting vacuum's resource consumption is best done by changing the other vacuum cost parameters. vacuum_cost_page_hit (integer) vacuum_cost_page_hit configuration parameter The estimated cost for vacuuming a buffer found in the shared buffer cache. It represents the cost to lock the buffer pool, lookup the shared hash table and scan the content of the page. The default value is one. vacuum_cost_page_miss (integer) vacuum_cost_page_miss configuration parameter The estimated cost for vacuuming a buffer that has to be read from disk. This represents the effort to lock the buffer pool, lookup the shared hash table, read the desired block in from the disk and scan its content. The default value is 10. vacuum_cost_page_dirty (integer) vacuum_cost_page_dirty configuration parameter The estimated cost charged when vacuum modifies a block that was previously clean. It represents the extra I/O required to flush the dirty block out to disk again. The default value is 20. vacuum_cost_limit (integer) vacuum_cost_limit configuration parameter The accumulated cost that will cause the vacuuming process to sleep. The default value is 200. There are certain operations that hold critical locks and should therefore complete as quickly as possible. Cost-based vacuum delays do not occur during such operations. Therefore it is possible that the cost accumulates far higher than the specified limit. To avoid uselessly long delays in such cases, the actual delay is calculated as vacuum_cost_delay * accumulated_balance / vacuum_cost_limit with a maximum of vacuum_cost_delay * 4. There is a separate server process called the background writer, whose function is to issue writes of dirty (new or modified) shared buffers. It writes shared buffers so server processes handling user queries seldom or never need to wait for a write to occur. However, the background writer does cause a net overall increase in I/O load, because while a repeatedly-dirtied page might otherwise be written only once per checkpoint interval, the background writer might write it several times as it is dirtied in the same interval. The parameters discussed in this subsection can be used to tune the behavior for local needs. bgwriter_delay (integer) bgwriter_delay configuration parameter Specifies the delay between activity rounds for the background writer. In each round the writer issues writes for some number of dirty buffers (controllable by the following parameters). It then sleeps for bgwriter_delay milliseconds, and repeats. When there are no dirty buffers in the buffer pool, though, it goes into a longer sleep regardless of bgwriter_delay. The default value is 200 milliseconds (200ms). Note that on many systems, the effective resolution of sleep delays is 10 milliseconds; setting bgwriter_delay to a value that is not a multiple of 10 might have the same results as setting it to the next higher multiple of 10. This parameter can only be set in the postgresql.conf file or on the server command line. bgwriter_lru_maxpages (integer) bgwriter_lru_maxpages configuration parameter In each round, no more than this many buffers will be written by the background writer. Setting this to zero disables background writing. (Note that checkpoints, which are managed by a separate, dedicated auxiliary process, are unaffected.) The default value is 100 buffers. This parameter can only be set in the postgresql.conf file or on the server command line. bgwriter_lru_multiplier (floating point) bgwriter_lru_multiplier configuration parameter The number of dirty buffers written in each round is based on the number of new buffers that have been needed by server processes during recent rounds. The average recent need is multiplied by bgwriter_lru_multiplier to arrive at an estimate of the number of buffers that will be needed during the next round. Dirty buffers are written until there are that many clean, reusable buffers available. (However, no more than bgwriter_lru_maxpages buffers will be written per round.) Thus, a setting of 1.0 represents a just in time policy of writing exactly the number of buffers predicted to be needed. Larger values provide some cushion against spikes in demand, while smaller values intentionally leave writes to be done by server processes. The default is 2.0. This parameter can only be set in the postgresql.conf file or on the server command line. bgwriter_flush_after (integer) bgwriter_flush_after configuration parameter Whenever more than bgwriter_flush_after bytes have been written by the bgwriter, attempt to force the OS to issue these writes to the underlying storage. Doing so will limit the amount of dirty data in the kernel's page cache, reducing the likelihood of stalls when an fsync is issued at the end of a checkpoint, or when the OS writes data back in larger batches in the background. Often that will result in greatly reduced transaction latency, but there also are some cases, especially with workloads that are bigger than , but smaller than the OS's page cache, where performance might degrade. This setting may have no effect on some platforms. The valid range is between 0, which disables controlled writeback, and 2MB. The default is 512Kb on Linux, 0 elsewhere. (Non-default values of BLCKSZ change the default and maximum.) This parameter can only be set in the postgresql.conf file or on the server command line. Smaller values of bgwriter_lru_maxpages and bgwriter_lru_multiplier reduce the extra I/O load caused by the background writer, but make it more likely that server processes will have to issue writes for themselves, delaying interactive queries. effective_io_concurrency (integer) effective_io_concurrency configuration parameter Sets the number of concurrent disk I/O operations that PostgreSQL expects can be executed simultaneously. Raising this value will increase the number of I/O operations that any individual PostgreSQL session attempts to initiate in parallel. The allowed range is 1 to 1000, or zero to disable issuance of asynchronous I/O requests. Currently, this setting only affects bitmap heap scans. A good starting point for this setting is the number of separate drives comprising a RAID 0 stripe or RAID 1 mirror being used for the database. (For RAID 5 the parity drive should not be counted.) However, if the database is often busy with multiple queries issued in concurrent sessions, lower values may be sufficient to keep the disk array busy. A value higher than needed to keep the disks busy will only result in extra CPU overhead. For more exotic systems, such as memory-based storage or a RAID array that is limited by bus bandwidth, the correct value might be the number of I/O paths available. Some experimentation may be needed to find the best value. Asynchronous I/O depends on an effective posix_fadvise function, which some operating systems lack. If the function is not present then setting this parameter to anything but zero will result in an error. On some operating systems (e.g., Solaris), the function is present but does not actually do anything. The default is 1 on supported systems, otherwise 0. This value can be overridden for tables in a particular tablespace by setting the tablespace parameter of the same name (see ). max_worker_processes (integer) max_worker_processes configuration parameter Sets the maximum number of background processes that the system can support. This parameter can only be set at server start. When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server. max_parallel_degree (integer) max_parallel_degree configuration parameter Sets the maximum number of workers that can be started for an individual parallel operation. Parallel workers are taken from the pool of processes established by . Note that the requested number of workers may not actually be available at runtime. If this occurs, the plan will run with fewer workers than expected, which may be inefficient. The default value is 2. Setting this value to 0 disables parallel query execution. backend_flush_after (integer) backend_flush_after configuration parameter Whenever more than backend_flush_after bytes have been written by a single backend, attempt to force the OS to issue these writes to the underlying storage. Doing so will limit the amount of dirty data in the kernel's page cache, reducing the likelihood of stalls when an fsync is issued at the end of a checkpoint, or when the OS writes data back in larger batches in the background. Often that will result in greatly reduced transaction latency, but there also are some cases, especially with workloads that are bigger than , but smaller than the OS's page cache, where performance might degrade. This setting may have no effect on some platforms. The valid range is between 0, which disables controlled writeback, and 2MB. The default is 128Kb on Linux, 0 elsewhere. (Non-default values of BLCKSZ change the default and maximum.) old_snapshot_threshold (integer) old_snapshot_threshold configuration parameter Sets the minimum time that a snapshot can be used without risk of a snapshot too old error occurring when using the snapshot. This parameter can only be set at server start. Beyond the threshold, old data may be vacuumed away. This can help prevent bloat in the face of snapshots which remain in use for a long time. To prevent incorrect results due to cleanup of data which would otherwise be visible to the snapshot, an error is generated when the snapshot is older than this threshold and the snapshot is used to read a page which has been modified since the snapshot was built. A value of -1 disables this feature, and is the default. Useful values for production work probably range from a small number of hours to a few days. The setting will be coerced to a granularity of minutes, and small numbers (such as 0 or 1min) are only allowed because they may sometimes be useful for testing. While a setting as high as 60d is allowed, please note that in many workloads extreme bloat or transaction ID wraparound may occur in much shorter time frames. This setting does not attempt to guarantee that an error will be generated under any particular circumstances. In fact, if the correct results can be generated from (for example) a cursor which has materialized a result set, no error will be generated even if the underlying rows in the referenced table have been vacuumed away. Some tables cannot safely be vacuumed early, and so will not be affected by this setting. Examples include system catalogs and any table which has a hash index. For such tables this setting will neither reduce bloat nor create a possibility of a snapshot too old error on scanning. For additional information on tuning these settings, see . wal_level (enum) wal_level configuration parameter wal_level determines how much information is written to the WAL. The default value is minimal, which writes only the information needed to recover from a crash or immediate shutdown. replica adds logging required for WAL archiving as well as information required to run read-only queries on a standby server. Finally, logical adds information necessary to support logical decoding. Each level includes the information logged at all lower levels. This parameter can only be set at server start. In minimal level, WAL-logging of some bulk operations can be safely skipped, which can make those operations much faster (see ). Operations in which this optimization can be applied include: CREATE TABLE AS CREATE INDEX CLUSTER COPY into tables that were created or truncated in the same transaction But minimal WAL does not contain enough information to reconstruct the data from a base backup and the WAL logs, so replica or higher must be used to enable WAL archiving () and streaming replication. In logical level, the same information is logged as with replica, plus information needed to allow extracting logical change sets from the WAL. Using a level of logical will increase the WAL volume, particularly if many tables are configured for REPLICA IDENTITY FULL and many UPDATE and DELETE statements are executed. In releases prior to 9.6, this parameter also allowed the values archive and hot_standby. These are still accepted but mapped to replica. fsync (boolean) fsync configuration parameter If this parameter is on, the PostgreSQL server will try to make sure that updates are physically written to disk, by issuing fsync() system calls or various equivalent methods (see ). This ensures that the database cluster can recover to a consistent state after an operating system or hardware crash. While turning off fsync is often a performance benefit, this can result in unrecoverable data corruption in the event of a power failure or system crash. Thus it is only advisable to turn off fsync if you can easily recreate your entire database from external data. Examples of safe circumstances for turning off fsync include the initial loading of a new database cluster from a backup file, using a database cluster for processing a batch of data after which the database will be thrown away and recreated, or for a read-only database clone which gets recreated frequently and is not used for failover. High quality hardware alone is not a sufficient justification for turning off fsync. For reliable recovery when changing fsync off to on, it is necessary to force all modified buffers in the kernel to durable storage. This can be done while the cluster is shutdown or while fsync is on by running initdb --sync-only, running sync, unmounting the file system, or rebooting the server. In many situations, turning off for noncritical transactions can provide much of the potential performance benefit of turning off fsync, without the attendant risks of data corruption. fsync can only be set in the postgresql.conf file or on the server command line. If you turn this parameter off, also consider turning off . synchronous_commit (enum) synchronous_commit configuration parameter Specifies whether transaction commit will wait for WAL records to be written to disk before the command returns a success indication to the client. Valid values are on, remote_apply, remote_write, local, and off. The default, and safe, setting is on. When off, there can be a delay between when success is reported to the client and when the transaction is really guaranteed to be safe against a server crash. (The maximum delay is three times .) Unlike , setting this parameter to off does not create any risk of database inconsistency: an operating system or database crash might result in some recent allegedly-committed transactions being lost, but the database state will be just the same as if those transactions had been aborted cleanly. So, turning synchronous_commit off can be a useful alternative when performance is more important than exact certainty about the durability of a transaction. For more discussion see . If is non-empty, this parameter also controls whether or not transaction commits will wait for their WAL records to be replicated to the standby server(s). When set to on, commits will wait until replies from the current synchronous standby(s) indicate they have received the commit record of the transaction and flushed it to disk. This ensures the transaction will not be lost unless both the primary and all synchronous standbys suffer corruption of their database storage. When set to remote_apply, commits will wait until replies from the current synchronous standby(s) indicate they have received the commit record of the transaction and applied it, so that it has become visible to queries on the standby(s). When set to remote_write, commits will wait until replies from the current synchronous standby(s) indicate they have received the commit record of the transaction and written it out to their operating system. This setting is sufficient to ensure data preservation even if a standby instance of PostgreSQL were to crash, but not if the standby suffers an operating-system-level crash, since the data has not necessarily reached stable storage on the standby. Finally, the setting local causes commits to wait for local flush to disk, but not for replication. This is not usually desirable when synchronous replication is in use, but is provided for completeness. If synchronous_standby_names is empty, the settings on, remote_apply, remote_write and local all provide the same synchronization level: transaction commits only wait for local flush to disk. This parameter can be changed at any time; the behavior for any one transaction is determined by the setting in effect when it commits. It is therefore possible, and useful, to have some transactions commit synchronously and others asynchronously. For example, to make a single multistatement transaction commit asynchronously when the default is the opposite, issue SET LOCAL synchronous_commit TO OFF within the transaction. wal_sync_method (enum) wal_sync_method configuration parameter Method used for forcing WAL updates out to disk. If fsync is off then this setting is irrelevant, since WAL file updates will not be forced out at all. Possible values are: open_datasync (write WAL files with open() option O_DSYNC) fdatasync (call fdatasync() at each commit) fsync (call fsync() at each commit) fsync_writethrough (call fsync() at each commit, forcing write-through of any disk write cache) open_sync (write WAL files with open() option O_SYNC) The open_* options also use O_DIRECT if available. Not all of these choices are available on all platforms. The default is the first method in the above list that is supported by the platform, except that fdatasync is the default on Linux. The default is not necessarily ideal; it might be necessary to change this setting or other aspects of your system configuration in order to create a crash-safe configuration or achieve optimal performance. These aspects are discussed in . This parameter can only be set in the postgresql.conf file or on the server command line. full_page_writes (boolean) full_page_writes configuration parameter When this parameter is on, the PostgreSQL server writes the entire content of each disk page to WAL during the first modification of that page after a checkpoint. This is needed because a page write that is in process during an operating system crash might be only partially completed, leading to an on-disk page that contains a mix of old and new data. The row-level change data normally stored in WAL will not be enough to completely restore such a page during post-crash recovery. Storing the full page image guarantees that the page can be correctly restored, but at the price of increasing the amount of data that must be written to WAL. (Because WAL replay always starts from a checkpoint, it is sufficient to do this during the first change of each page after a checkpoint. Therefore, one way to reduce the cost of full-page writes is to increase the checkpoint interval parameters.) Turning this parameter off speeds normal operation, but might lead to either unrecoverable data corruption, or silent data corruption, after a system failure. The risks are similar to turning off fsync, though smaller, and it should be turned off only based on the same circumstances recommended for that parameter. Turning off this parameter does not affect use of WAL archiving for point-in-time recovery (PITR) (see ). This parameter can only be set in the postgresql.conf file or on the server command line. The default is on. wal_log_hints (boolean) wal_log_hints configuration parameter When this parameter is on, the PostgreSQL server writes the entire content of each disk page to WAL during the first modification of that page after a checkpoint, even for non-critical modifications of so-called hint bits. If data checksums are enabled, hint bit updates are always WAL-logged and this setting is ignored. You can use this setting to test how much extra WAL-logging would occur if your database had data checksums enabled. This parameter can only be set at server start. The default value is off. wal_compression (boolean) wal_compression configuration parameter When this parameter is on, the PostgreSQL server compresses a full page image written to WAL when is on or during a base backup. A compressed page image will be decompressed during WAL replay. The default value is off. Only superusers can change this setting. Turning this parameter on can reduce the WAL volume without increasing the risk of unrecoverable data corruption, but at the cost of some extra CPU spent on the compression during WAL logging and on the decompression during WAL replay. wal_buffers (integer) wal_buffers configuration parameter The amount of shared memory used for WAL data that has not yet been written to disk. The default setting of -1 selects a size equal to 1/32nd (about 3%) of , but not less than 64kB nor more than the size of one WAL segment, typically 16MB. This value can be set manually if the automatic choice is too large or too small, but any positive value less than 32kB will be treated as 32kB. This parameter can only be set at server start. The contents of the WAL buffers are written out to disk at every transaction commit, so extremely large values are unlikely to provide a significant benefit. However, setting this value to at least a few megabytes can improve write performance on a busy server where many clients are committing at once. The auto-tuning selected by the default setting of -1 should give reasonable results in most cases. wal_writer_delay (integer) wal_writer_delay configuration parameter Specifies how often the WAL writer flushes WAL. After flushing WAL it sleeps for wal_writer_delay milliseconds, unless woken up by an asynchronously committing transaction. In case the last flush happened less than wal_writer_delay milliseconds ago and less than wal_writer_flush_after bytes of WAL have been produced since, WAL is only written to the OS, not flushed to disk. The default value is 200 milliseconds (200ms). Note that on many systems, the effective resolution of sleep delays is 10 milliseconds; setting wal_writer_delay to a value that is not a multiple of 10 might have the same results as setting it to the next higher multiple of 10. This parameter can only be set in the postgresql.conf file or on the server command line. wal_writer_flush_after (integer) wal_writer_flush_after configuration parameter Specifies how often the WAL writer flushes WAL. In case the last flush happened less than wal_writer_delay milliseconds ago and less than wal_writer_flush_after bytes of WAL have been produced since, WAL is only written to the OS, not flushed to disk. If wal_writer_flush_after is set to 0 WAL is flushed everytime the WAL writer has written WAL. The default is 1MB. This parameter can only be set in the postgresql.conf file or on the server command line. commit_delay (integer) commit_delay configuration parameter commit_delay adds a time delay, measured in microseconds, before a WAL flush is initiated. This can improve group commit throughput by allowing a larger number of transactions to commit via a single WAL flush, if system load is high enough that additional transactions become ready to commit within the given interval. However, it also increases latency by up to commit_delay microseconds for each WAL flush. Because the delay is just wasted if no other transactions become ready to commit, a delay is only performed if at least commit_siblings other transactions are active when a flush is about to be initiated. Also, no delays are performed if fsync is disabled. The default commit_delay is zero (no delay). Only superusers can change this setting. In PostgreSQL releases prior to 9.3, commit_delay behaved differently and was much less effective: it affected only commits, rather than all WAL flushes, and waited for the entire configured delay even if the WAL flush was completed sooner. Beginning in PostgreSQL 9.3, the first process that becomes ready to flush waits for the configured interval, while subsequent processes wait only until the leader completes the flush operation. commit_siblings (integer) commit_siblings configuration parameter Minimum number of concurrent open transactions to require before performing the commit_delay delay. A larger value makes it more probable that at least one other transaction will become ready to commit during the delay interval. The default is five transactions. checkpoint_timeout (integer) checkpoint_timeout configuration parameter Maximum time between automatic WAL checkpoints, in seconds. The valid range is between 30 seconds and one hour. The default is five minutes (5min). Increasing this parameter can increase the amount of time needed for crash recovery. This parameter can only be set in the postgresql.conf file or on the server command line. checkpoint_completion_target (floating point) checkpoint_completion_target configuration parameter Specifies the target of checkpoint completion, as a fraction of total time between checkpoints. The default is 0.5. This parameter can only be set in the postgresql.conf file or on the server command line. checkpoint_flush_after (integer) checkpoint_flush_after configuration parameter Whenever more than checkpoint_flush_after bytes have been written while performing a checkpoint, attempt to force the OS to issue these writes to the underlying storage. Doing so will limit the amount of dirty data in the kernel's page cache, reducing the likelihood of stalls when an fsync is issued at the end of the checkpoint, or when the OS writes data back in larger batches in the background. Often that will result in greatly reduced transaction latency, but there also are some cases, especially with workloads that are bigger than , but smaller than the OS's page cache, where performance might degrade. This setting may have no effect on some platforms. The valid range is between 0, which disables controlled writeback, and 2MB. The default is 256Kb on Linux, 0 elsewhere. (Non-default values of BLCKSZ change the default and maximum.) This parameter can only be set in the postgresql.conf file or on the server command line. checkpoint_warning (integer) checkpoint_warning configuration parameter Write a message to the server log if checkpoints caused by the filling of checkpoint segment files happen closer together than this many seconds (which suggests that max_wal_size ought to be raised). The default is 30 seconds (30s). Zero disables the warning. No warnings will be generated if checkpoint_timeout is less than checkpoint_warning. This parameter can only be set in the postgresql.conf file or on the server command line. max_wal_size (integer) max_wal_size configuration parameter Maximum size to let the WAL grow to between automatic WAL checkpoints. This is a soft limit; WAL size can exceed max_wal_size under special circumstances, like under heavy load, a failing archive_command, or a high wal_keep_segments setting. The default is 1 GB. Increasing this parameter can increase the amount of time needed for crash recovery. This parameter can only be set in the postgresql.conf file or on the server command line. min_wal_size (integer) min_wal_size configuration parameter As long as WAL disk usage stays below this setting, old WAL files are always recycled for future use at a checkpoint, rather than removed. This can be used to ensure that enough WAL space is reserved to handle spikes in WAL usage, for example when running large batch jobs. The default is 80 MB. This parameter can only be set in the postgresql.conf file or on the server command line. archive_mode (enum) archive_mode configuration parameter When archive_mode is enabled, completed WAL segments are sent to archive storage by setting . In addition to off, to disable, there are two modes: on, and always. During normal operation, there is no difference between the two modes, but when set to always the WAL archiver is enabled also during archive recovery or standby mode. In always mode, all files restored from the archive or streamed with streaming replication will be archived (again). See for details. archive_mode and archive_command are separate variables so that archive_command can be changed without leaving archiving mode. This parameter can only be set at server start. archive_mode cannot be enabled when wal_level is set to minimal. archive_command (string) archive_command configuration parameter The local shell command to execute to archive a completed WAL file segment. Any %p in the string is replaced by the path name of the file to archive, and any %f is replaced by only the file name. (The path name is relative to the working directory of the server, i.e., the cluster's data directory.) Use %% to embed an actual % character in the command. It is important for the command to return a zero exit status only if it succeeds. For more information see . This parameter can only be set in the postgresql.conf file or on the server command line. It is ignored unless archive_mode was enabled at server start. If archive_command is an empty string (the default) while archive_mode is enabled, WAL archiving is temporarily disabled, but the server continues to accumulate WAL segment files in the expectation that a command will soon be provided. Setting archive_command to a command that does nothing but return true, e.g. /bin/true (REM on Windows), effectively disables archiving, but also breaks the chain of WAL files needed for archive recovery, so it should only be used in unusual circumstances. archive_timeout (integer) archive_timeout configuration parameter The is only invoked for completed WAL segments. Hence, if your server generates little WAL traffic (or has slack periods where it does so), there could be a long delay between the completion of a transaction and its safe recording in archive storage. To limit how old unarchived data can be, you can set archive_timeout to force the server to switch to a new WAL segment file periodically. When this parameter is greater than zero, the server will switch to a new segment file whenever this many seconds have elapsed since the last segment file switch, and there has been any database activity, including a single checkpoint. (Increasing checkpoint_timeout will reduce unnecessary checkpoints on an idle system.) Note that archived files that are closed early due to a forced switch are still the same length as completely full files. Therefore, it is unwise to use a very short archive_timeout — it will bloat your archive storage. archive_timeout settings of a minute or so are usually reasonable. You should consider using streaming replication, instead of archiving, if you want data to be copied off the master server more quickly than that. This parameter can only be set in the postgresql.conf file or on the server command line. These settings control the behavior of the built-in streaming replication feature (see ). Servers will be either a Master or a Standby server. Masters can send data, while Standby(s) are always receivers of replicated data. When cascading replication (see ) is used, Standby server(s) can also be senders, as well as receivers. Parameters are mainly for Sending and Standby servers, though some parameters have meaning only on the Master server. Settings may vary across the cluster without problems if that is required. These parameters can be set on any server that is to send replication data to one or more standby servers. The master is always a sending server, so these parameters must always be set on the master. The role and meaning of these parameters does not change after a standby becomes the master. max_wal_senders (integer) max_wal_senders configuration parameter Specifies the maximum number of concurrent connections from standby servers or streaming base backup clients (i.e., the maximum number of simultaneously running WAL sender processes). The default is zero, meaning replication is disabled. WAL sender processes count towards the total number of connections, so the parameter cannot be set higher than . Abrupt streaming client disconnection might cause an orphaned connection slot until a timeout is reached, so this parameter should be set slightly higher than the maximum number of expected clients so disconnected clients can immediately reconnect. This parameter can only be set at server start. wal_level must be set to replica or higher to allow connections from standby servers. max_replication_slots (integer) max_replication_slots configuration parameter Specifies the maximum number of replication slots (see ) that the server can support. The default is zero. This parameter can only be set at server start. wal_level must be set to replica or higher to allow replication slots to be used. Setting it to a lower value than the number of currently existing replication slots will prevent the server from starting. wal_keep_segments (integer) wal_keep_segments configuration parameter Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is normally 16 megabytes. If a standby server connected to the sending server falls behind by more than wal_keep_segments segments, the sending server might remove a WAL segment still needed by the standby, in which case the replication connection will be terminated. Downstream connections will also eventually fail as a result. (However, the standby server can recover by fetching the segment from archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint. If wal_keep_segments is zero (the default), the system doesn't keep any extra segments for standby purposes, so the number of old WAL segments available to standby servers is a function of the location of the previous checkpoint and status of WAL archiving. This parameter can only be set in the postgresql.conf file or on the server command line. wal_sender_timeout (integer) wal_sender_timeout configuration parameter Terminate replication connections that are inactive longer than the specified number of milliseconds. This is useful for the sending server to detect a standby crash or network outage. A value of zero disables the timeout mechanism. This parameter can only be set in the postgresql.conf file or on the server command line. The default value is 60 seconds. track_commit_timestamp (bool) track_commit_timestamp configuration parameter Record commit time of transactions. This parameter can only be set in postgresql.conf file or on the server command line. The default value is off. These parameters can be set on the master/primary server that is to send replication data to one or more standby servers. Note that in addition to these parameters, must be set appropriately on the master server, and optionally WAL archiving can be enabled as well (see ). The values of these parameters on standby servers are irrelevant, although you may wish to set them there in preparation for the possibility of a standby becoming the master. synchronous_standby_names (string) synchronous_standby_names configuration parameter Specifies a list of standby servers that can support synchronous replication, as described in . There will be one or more active synchronous standbys; transactions waiting for commit will be allowed to proceed after these standby servers confirm receipt of their data. The synchronous standbys will be those whose names appear earlier in this list, and that are both currently connected and streaming data in real-time (as shown by a state of streaming in the pg_stat_replication view). Other standby servers appearing later in this list represent potential synchronous standbys. If any of the current synchronous standbys disconnects for whatever reason, it will be replaced immediately with the next-highest-priority standby. Specifying more than one standby name can allow very high availability. This parameter specifies a list of standby servers using either of the following syntaxes: num_sync ( standby_name [, ...] ) standby_name [, ...] where num_sync is the number of synchronous standbys that transactions need to wait for replies from, and standby_name is the name of a standby server. For example, a setting of 3 (s1, s2, s3, s4) makes transaction commits wait until their WAL records are received by three higher-priority standbys chosen from standby servers s1, s2, s3 and s4. The second syntax was used before PostgreSQL version 9.6 and is still supported. It's the same as the first syntax with num_sync equal to 1. For example, 1 (s1, s2) and s1, s2 have the same meaning: either s1 or s2 is chosen as a synchronous standby. The name of a standby server for this purpose is the application_name setting of the standby, as set in the primary_conninfo of the standby's WAL receiver. There is no mechanism to enforce uniqueness. In case of duplicates one of the matching standbys will be considered as higher priority, though exactly which one is indeterminate. The special entry * matches any application_name, including the default application name of walreceiver. Each standby_name should have the form of a valid SQL identifier, unless it is *. You can use double-quoting if necessary. But note that standby_names are compared to standby application names case-insensitively, whether double-quoted or not. If no synchronous standby names are specified here, then synchronous replication is not enabled and transaction commits will not wait for replication. This is the default configuration. Even when synchronous replication is enabled, individual transactions can be configured not to wait for replication by setting the parameter to local or off. This parameter can only be set in the postgresql.conf file or on the server command line. vacuum_defer_cleanup_age (integer) vacuum_defer_cleanup_age configuration parameter Specifies the number of transactions by which VACUUM and HOT updates will defer cleanup of dead row versions. The default is zero transactions, meaning that dead row versions can be removed as soon as possible, that is, as soon as they are no longer visible to any open transaction. You may wish to set this to a non-zero value on a primary server that is supporting hot standby servers, as described in . This allows more time for queries on the standby to complete without incurring conflicts due to early cleanup of rows. However, since the value is measured in terms of number of write transactions occurring on the primary server, it is difficult to predict just how much additional grace time will be made available to standby queries. This parameter can only be set in the postgresql.conf file or on the server command line. You should also consider setting hot_standby_feedback on standby server(s) as an alternative to using this parameter. This does not prevent cleanup of dead rows which have reached the age specified by old_snapshot_threshold. These settings control the behavior of a standby server that is to receive replication data. Their values on the master server are irrelevant. hot_standby (boolean) hot_standby configuration parameter Specifies whether or not you can connect and run queries during recovery, as described in . The default value is off. This parameter can only be set at server start. It only has effect during archive recovery or in standby mode. max_standby_archive_delay (integer) max_standby_archive_delay configuration parameter When Hot Standby is active, this parameter determines how long the standby server should wait before canceling standby queries that conflict with about-to-be-applied WAL entries, as described in . max_standby_archive_delay applies when WAL data is being read from WAL archive (and is therefore not current). The default is 30 seconds. Units are milliseconds if not specified. A value of -1 allows the standby to wait forever for conflicting queries to complete. This parameter can only be set in the postgresql.conf file or on the server command line. Note that max_standby_archive_delay is not the same as the maximum length of time a query can run before cancellation; rather it is the maximum total time allowed to apply any one WAL segment's data. Thus, if one query has resulted in significant delay earlier in the WAL segment, subsequent conflicting queries will have much less grace time. max_standby_streaming_delay (integer) max_standby_streaming_delay configuration parameter When Hot Standby is active, this parameter determines how long the standby server should wait before canceling standby queries that conflict with about-to-be-applied WAL entries, as described in . max_standby_streaming_delay applies when WAL data is being received via streaming replication. The default is 30 seconds. Units are milliseconds if not specified. A value of -1 allows the standby to wait forever for conflicting queries to complete. This parameter can only be set in the postgresql.conf file or on the server command line. Note that max_standby_streaming_delay is not the same as the maximum length of time a query can run before cancellation; rather it is the maximum total time allowed to apply WAL data once it has been received from the primary server. Thus, if one query has resulted in significant delay, subsequent conflicting queries will have much less grace time until the standby server has caught up again. wal_receiver_status_interval (integer) wal_receiver_status_interval configuration parameter Specifies the minimum frequency for the WAL receiver process on the standby to send information about replication progress to the primary or upstream standby, where it can be seen using the pg_stat_replication view. The standby will report the last transaction log position it has written, the last position it has flushed to disk, and the last position it has applied. This parameter's value is the maximum interval, in seconds, between reports. Updates are sent each time the write or flush positions change, or at least as often as specified by this parameter. Thus, the apply position may lag slightly behind the true position. Setting this parameter to zero disables status updates completely. This parameter can only be set in the postgresql.conf file or on the server command line. The default value is 10 seconds. hot_standby_feedback (boolean) hot_standby_feedback configuration parameter Specifies whether or not a hot standby will send feedback to the primary or upstream standby about queries currently executing on the standby. This parameter can be used to eliminate query cancels caused by cleanup records, but can cause database bloat on the primary for some workloads. Feedback messages will not be sent more frequently than once per wal_receiver_status_interval. The default value is off. This parameter can only be set in the postgresql.conf file or on the server command line. If cascaded replication is in use the feedback is passed upstream until it eventually reaches the primary. Standbys make no other use of feedback they receive other than to pass upstream. This setting does not override the behavior of old_snapshot_threshold on the primary; a snapshot on the standby which exceeds the primary's age threshold can become invalid, resulting in cancellation of transactions on the standby. This is because old_snapshot_threshold is intended to provide an absolute limit on the time which dead rows can contribute to bloat, which would otherwise be violated because of the configuration of a standby. wal_receiver_timeout (integer) wal_receiver_timeout configuration parameter Terminate replication connections that are inactive longer than the specified number of milliseconds. This is useful for the receiving standby server to detect a primary node crash or network outage. A value of zero disables the timeout mechanism. This parameter can only be set in the postgresql.conf file or on the server command line. The default value is 60 seconds. wal_retrieve_retry_interval (integer) wal_retrieve_retry_interval configuration parameter Specify how long the standby server should wait when WAL data is not available from any sources (streaming replication, local pg_xlog or WAL archive) before retrying to retrieve WAL data. This parameter can only be set in the postgresql.conf file or on the server command line. The default value is 5 seconds. Units are milliseconds if not specified. This parameter is useful in configurations where a node in recovery needs to control the amount of time to wait for new WAL data to be available. For example, in archive recovery, it is possible to make the recovery more responsive in the detection of a new WAL log file by reducing the value of this parameter. On a system with low WAL activity, increasing it reduces the amount of requests necessary to access WAL archives, something useful for example in cloud environments where the amount of times an infrastructure is accessed is taken into account. These configuration parameters provide a crude method of influencing the query plans chosen by the query optimizer. If the default plan chosen by the optimizer for a particular query is not optimal, a temporary solution is to use one of these configuration parameters to force the optimizer to choose a different plan. Better ways to improve the quality of the plans chosen by the optimizer include adjusting the planer cost constants (see ), running manually, increasing the value of the configuration parameter, and increasing the amount of statistics collected for specific columns using ALTER TABLE SET STATISTICS. enable_bitmapscan (boolean) bitmap scan enable_bitmapscan configuration parameter Enables or disables the query planner's use of bitmap-scan plan types. The default is on. enable_hashagg (boolean) enable_hashagg configuration parameter Enables or disables the query planner's use of hashed aggregation plan types. The default is on. enable_hashjoin (boolean) enable_hashjoin configuration parameter Enables or disables the query planner's use of hash-join plan types. The default is on. enable_indexscan (boolean) index scan enable_indexscan configuration parameter Enables or disables the query planner's use of index-scan plan types. The default is on. enable_indexonlyscan (boolean) enable_indexonlyscan configuration parameter Enables or disables the query planner's use of index-only-scan plan types (see ). The default is on. enable_material (boolean) enable_material configuration parameter Enables or disables the query planner's use of materialization. It is impossible to suppress materialization entirely, but turning this variable off prevents the planner from inserting materialize nodes except in cases where it is required for correctness. The default is on. enable_mergejoin (boolean) enable_mergejoin configuration parameter Enables or disables the query planner's use of merge-join plan types. The default is on. enable_nestloop (boolean) enable_nestloop configuration parameter Enables or disables the query planner's use of nested-loop join plans. It is impossible to suppress nested-loop joins entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on. enable_seqscan (boolean) sequential scan enable_seqscan configuration parameter Enables or disables the query planner's use of sequential scan plan types. It is impossible to suppress sequential scans entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on. enable_sort (boolean) enable_sort configuration parameter Enables or disables the query planner's use of explicit sort steps. It is impossible to suppress explicit sorts entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on. enable_tidscan (boolean) enable_tidscan configuration parameter Enables or disables the query planner's use of TID scan plan types. The default is on. The cost variables described in this section are measured on an arbitrary scale. Only their relative values matter, hence scaling them all up or down by the same factor will result in no change in the planner's choices. By default, these cost variables are based on the cost of sequential page fetches; that is, seq_page_cost is conventionally set to 1.0 and the other cost variables are set with reference to that. But you can use a different scale if you prefer, such as actual execution times in milliseconds on a particular machine. Unfortunately, there is no well-defined method for determining ideal values for the cost variables. They are best treated as averages over the entire mix of queries that a particular installation will receive. This means that changing them on the basis of just a few experiments is very risky. seq_page_cost (floating point) seq_page_cost configuration parameter Sets the planner's estimate of the cost of a disk page fetch that is part of a series of sequential fetches. The default is 1.0. This value can be overridden for tables and indexes in a particular tablespace by setting the tablespace parameter of the same name (see ). random_page_cost (floating point) random_page_cost configuration parameter Sets the planner's estimate of the cost of a non-sequentially-fetched disk page. The default is 4.0. This value can be overridden for tables and indexes in a particular tablespace by setting the tablespace parameter of the same name (see ). Reducing this value relative to seq_page_cost will cause the system to prefer index scans; raising it will make index scans look relatively more expensive. You can raise or lower both values together to change the importance of disk I/O costs relative to CPU costs, which are described by the following parameters. Random access to mechanical disk storage is normally much more expensive than four times sequential access. However, a lower default is used (4.0) because the majority of random accesses to disk, such as indexed reads, are assumed to be in cache. The default value can be thought of as modeling random access as 40 times slower than sequential, while expecting 90% of random reads to be cached. If you believe a 90% cache rate is an incorrect assumption for your workload, you can increase random_page_cost to better reflect the true cost of random storage reads. Correspondingly, if your data is likely to be completely in cache, such as when the database is smaller than the total server memory, decreasing random_page_cost can be appropriate. Storage that has a low random read cost relative to sequential, e.g. solid-state drives, might also be better modeled with a lower value for random_page_cost. Although the system will let you set random_page_cost to less than seq_page_cost, it is not physically sensible to do so. However, setting them equal makes sense if the database is entirely cached in RAM, since in that case there is no penalty for touching pages out of sequence. Also, in a heavily-cached database you should lower both values relative to the CPU parameters, since the cost of fetching a page already in RAM is much smaller than it would normally be. cpu_tuple_cost (floating point) cpu_tuple_cost configuration parameter Sets the planner's estimate of the cost of processing each row during a query. The default is 0.01. cpu_index_tuple_cost (floating point) cpu_index_tuple_cost configuration parameter Sets the planner's estimate of the cost of processing each index entry during an index scan. The default is 0.005. cpu_operator_cost (floating point) cpu_operator_cost configuration parameter Sets the planner's estimate of the cost of processing each operator or function executed during a query. The default is 0.0025. parallel_setup_cost (floating point) parallel_setup_cost configuration parameter Sets the planner's estimate of the cost of launching parallel worker processes. The default is 1000. parallel_tuple_cost (floating point) parallel_tuple_cost configuration parameter Sets the planner's estimate of the cost of transferring one tuple from a parallel worker process to another process. The default is 0.1. effective_cache_size (integer) effective_cache_size configuration parameter Sets the planner's assumption about the effective size of the disk cache that is available to a single query. This is factored into estimates of the cost of using an index; a higher value makes it more likely index scans will be used, a lower value makes it more likely sequential scans will be used. When setting this parameter you should consider both PostgreSQL's shared buffers and the portion of the kernel's disk cache that will be used for PostgreSQL data files. Also, take into account the expected number of concurrent queries on different tables, since they will have to share the available space. This parameter has no effect on the size of shared memory allocated by PostgreSQL, nor does it reserve kernel disk cache; it is used only for estimation purposes. The system also does not assume data remains in the disk cache between queries. The default is 4 gigabytes (4GB). The genetic query optimizer (GEQO) is an algorithm that does query planning using heuristic searching. This reduces planning time for complex queries (those joining many relations), at the cost of producing plans that are sometimes inferior to those found by the normal exhaustive-search algorithm. For more information see . geqo (boolean) genetic query optimization GEQO genetic query optimization geqo configuration parameter Enables or disables genetic query optimization. This is on by default. It is usually best not to turn it off in production; the geqo_threshold variable provides more granular control of GEQO. geqo_threshold (integer) geqo_threshold configuration parameter Use genetic query optimization to plan queries with at least this many FROM items involved. (Note that a FULL OUTER JOIN construct counts as only one FROM item.) The default is 12. For simpler queries it is usually best to use the regular, exhaustive-search planner, but for queries with many tables the exhaustive search takes too long, often longer than the penalty of executing a suboptimal plan. Thus, a threshold on the size of the query is a convenient way to manage use of GEQO. geqo_effort (integer) geqo_effort configuration parameter Controls the trade-off between planning time and query plan quality in GEQO. This variable must be an integer in the range from 1 to 10. The default value is five. Larger values increase the time spent doing query planning, but also increase the likelihood that an efficient query plan will be chosen. geqo_effort doesn't actually do anything directly; it is only used to compute the default values for the other variables that influence GEQO behavior (described below). If you prefer, you can set the other parameters by hand instead. geqo_pool_size (integer) geqo_pool_size configuration parameter Controls the pool size used by GEQO, that is the number of individuals in the genetic population. It must be at least two, and useful values are typically 100 to 1000. If it is set to zero (the default setting) then a suitable value is chosen based on geqo_effort and the number of tables in the query. geqo_generations (integer) geqo_generations configuration parameter Controls the number of generations used by GEQO, that is the number of iterations of the algorithm. It must be at least one, and useful values are in the same range as the pool size. If it is set to zero (the default setting) then a suitable value is chosen based on geqo_pool_size. geqo_selection_bias (floating point) geqo_selection_bias configuration parameter Controls the selection bias used by GEQO. The selection bias is the selective pressure within the population. Values can be from 1.50 to 2.00; the latter is the default. geqo_seed (floating point) geqo_seed configuration parameter Controls the initial value of the random number generator used by GEQO to select random paths through the join order search space. The value can range from zero (the default) to one. Varying the value changes the set of join paths explored, and may result in a better or worse best path being found. default_statistics_target (integer) default_statistics_target configuration parameter Sets the default statistics target for table columns without a column-specific target set via ALTER TABLE SET STATISTICS. Larger values increase the time needed to do ANALYZE, but might improve the quality of the planner's estimates. The default is 100. For more information on the use of statistics by the PostgreSQL query planner, refer to . constraint_exclusion (enum) constraint exclusion constraint_exclusion configuration parameter Controls the query planner's use of table constraints to optimize queries. The allowed values of constraint_exclusion are on (examine constraints for all tables), off (never examine constraints), and partition (examine constraints only for inheritance child tables and UNION ALL subqueries). partition is the default setting. It is often used with inheritance and partitioned tables to improve performance. When this parameter allows it for a particular table, the planner compares query conditions with the table's CHECK constraints, and omits scanning tables for which the conditions contradict the constraints. For example: CREATE TABLE parent(key integer, ...); CREATE TABLE child1000(check (key between 1000 and 1999)) INHERITS(parent); CREATE TABLE child2000(check (key between 2000 and 2999)) INHERITS(parent); ... SELECT * FROM parent WHERE key = 2400; With constraint exclusion enabled, this SELECT will not scan child1000 at all, improving performance. Currently, constraint exclusion is enabled by default only for cases that are often used to implement table partitioning. Turning it on for all tables imposes extra planning overhead that is quite noticeable on simple queries, and most often will yield no benefit for simple queries. If you have no partitioned tables you might prefer to turn it off entirely. Refer to for more information on using constraint exclusion and partitioning. cursor_tuple_fraction (floating point) cursor_tuple_fraction configuration parameter Sets the planner's estimate of the fraction of a cursor's rows that will be retrieved. The default is 0.1. Smaller values of this setting bias the planner towards using fast start plans for cursors, which will retrieve the first few rows quickly while perhaps taking a long time to fetch all rows. Larger values put more emphasis on the total estimated time. At the maximum setting of 1.0, cursors are planned exactly like regular queries, considering only the total estimated time and not how soon the first rows might be delivered. from_collapse_limit (integer) from_collapse_limit configuration parameter The planner will merge sub-queries into upper queries if the resulting FROM list would have no more than this many items. Smaller values reduce planning time but might yield inferior query plans. The default is eight. For more information see . Setting this value to or more may trigger use of the GEQO planner, resulting in non-optimal plans. See . join_collapse_limit (integer) join_collapse_limit configuration parameter The planner will rewrite explicit JOIN constructs (except FULL JOINs) into lists of FROM items whenever a list of no more than this many items would result. Smaller values reduce planning time but might yield inferior query plans. By default, this variable is set the same as from_collapse_limit, which is appropriate for most uses. Setting it to 1 prevents any reordering of explicit JOINs. Thus, the explicit join order specified in the query will be the actual order in which the relations are joined. Because the query planner does not always choose the optimal join order, advanced users can elect to temporarily set this variable to 1, and then specify the join order they desire explicitly. For more information see . Setting this value to or more may trigger use of the GEQO planner, resulting in non-optimal plans. See . force_parallel_mode (enum) force_parallel_mode configuration parameter Allows the use of parallel queries for testing purposes even in cases where no performance benefit is expected. The allowed values of force_parallel_mode are off (use parallel mode only when it is expected to improve performance), on (force parallel query for all queries for which it is thought to be safe), and regress (like on, but with additional behavior changes as explained below). More specifically, setting this value to on will add a Gather node to the top of any query plan for which this appears to be safe, so that the query runs inside of a parallel worker. Even when a parallel worker is not available or cannot be used, operations such as starting a subtransaction that would be prohibited in a parallel query context will be prohibited unless the planner believes that this will cause the query to fail. If failures or unexpected results occur when this option is set, some functions used by the query may need to be marked PARALLEL UNSAFE (or, possibly, PARALLEL RESTRICTED). Setting this value to regress has all of the same effects as setting it to on plus some additional effects that are intended to facilitate automated regression testing. Normally, messages from a parallel worker include a context line indicating that, but a setting of regress suppresses this line so that the output is the same as in non-parallel execution. Also, the Gather nodes added to plans by this setting are hidden in EXPLAIN output so that the output matches what would be obtained if this setting were turned off. server log where to log log_destination (string) log_destination configuration parameter PostgreSQL supports several methods for logging server messages, including stderr, csvlog and syslog. On Windows, eventlog is also supported. Set this parameter to a list of desired log destinations separated by commas. The default is to log to stderr only. This parameter can only be set in the postgresql.conf file or on the server command line. If csvlog is included in log_destination, log entries are output in comma separated value (CSV) format, which is convenient for loading logs into programs. See for details. must be enabled to generate CSV-format log output. On most Unix systems, you will need to alter the configuration of your system's syslog daemon in order to make use of the syslog option for log_destination. PostgreSQL can log to syslog facilities LOCAL0 through LOCAL7 (see ), but the default syslog configuration on most platforms will discard all such messages. You will need to add something like: local0.* /var/log/postgresql to the syslog daemon's configuration file to make it work. On Windows, when you use the eventlog option for log_destination, you should register an event source and its library with the operating system so that the Windows Event Viewer can display event log messages cleanly. See for details. logging_collector (boolean) logging_collector configuration parameter This parameter enables the logging collector, which is a background process that captures log messages sent to stderr and redirects them into log files. This approach is often more useful than logging to syslog, since some types of messages might not appear in syslog output. (One common example is dynamic-linker failure messages; another is error messages produced by scripts such as archive_command.) This parameter can only be set at server start. It is possible to log to stderr without using the logging collector; the log messages will just go to wherever the server's stderr is directed. However, that method is only suitable for low log volumes, since it provides no convenient way to rotate log files. Also, on some platforms not using the logging collector can result in lost or garbled log output, because multiple processes writing concurrently to the same log file can overwrite each other's output. The logging collector is designed to never lose messages. This means that in case of extremely high load, server processes could be blocked while trying to send additional log messages when the collector has fallen behind. In contrast, syslog prefers to drop messages if it cannot write them, which means it may fail to log some messages in such cases but it will not block the rest of the system. log_directory (string) log_directory configuration parameter When logging_collector is enabled, this parameter determines the directory in which log files will be created. It can be specified as an absolute path, or relative to the cluster data directory. This parameter can only be set in the postgresql.conf file or on the server command line. The default is pg_log. log_filename (string) log_filename configuration parameter When logging_collector is enabled, this parameter sets the file names of the created log files. The value is treated as a strftime pattern, so %-escapes can be used to specify time-varying file names. (Note that if there are any time-zone-dependent %-escapes, the computation is done in the zone specified by .) The supported %-escapes are similar to those listed in the Open Group's strftime specification. Note that the system's strftime is not used directly, so platform-specific (nonstandard) extensions do not work. The default is postgresql-%Y-%m-%d_%H%M%S.log. If you specify a file name without escapes, you should plan to use a log rotation utility to avoid eventually filling the entire disk. In releases prior to 8.4, if no % escapes were present, PostgreSQL would append the epoch of the new log file's creation time, but this is no longer the case. If CSV-format output is enabled in log_destination, .csv will be appended to the timestamped log file name to create the file name for CSV-format output. (If log_filename ends in .log, the suffix is replaced instead.) This parameter can only be set in the postgresql.conf file or on the server command line. log_file_mode (integer) log_file_mode configuration parameter On Unix systems this parameter sets the permissions for log files when logging_collector is enabled. (On Microsoft Windows this parameter is ignored.) The parameter value is expected to be a numeric mode specified in the format accepted by the chmod and umask system calls. (To use the customary octal format the number must start with a 0 (zero).) The default permissions are 0600, meaning only the server owner can read or write the log files. The other commonly useful setting is 0640, allowing members of the owner's group to read the files. Note however that to make use of such a setting, you'll need to alter to store the files somewhere outside the cluster data directory. In any case, it's unwise to make the log files world-readable, since they might contain sensitive data. This parameter can only be set in the postgresql.conf file or on the server command line. log_rotation_age (integer) log_rotation_age configuration parameter When logging_collector is enabled, this parameter determines the maximum lifetime of an individual log file. After this many minutes have elapsed, a new log file will be created. Set to zero to disable time-based creation of new log files. This parameter can only be set in the postgresql.conf file or on the server command line. log_rotation_size (integer) log_rotation_size configuration parameter When logging_collector is enabled, this parameter determines the maximum size of an individual log file. After this many kilobytes have been emitted into a log file, a new log file will be created. Set to zero to disable size-based creation of new log files. This parameter can only be set in the postgresql.conf file or on the server command line. log_truncate_on_rotation (boolean) log_truncate_on_rotation configuration parameter When logging_collector is enabled, this parameter will cause PostgreSQL to truncate (overwrite), rather than append to, any existing log file of the same name. However, truncation will occur only when a new file is being opened due to time-based rotation, not during server startup or size-based rotation. When off, pre-existing files will be appended to in all cases. For example, using this setting in combination with a log_filename like postgresql-%H.log would result in generating twenty-four hourly log files and then cyclically overwriting them. This parameter can only be set in the postgresql.conf file or on the server command line. Example: To keep 7 days of logs, one log file per day named server_log.Mon, server_log.Tue, etc, and automatically overwrite last week's log with this week's log, set log_filename to server_log.%a, log_truncate_on_rotation to on, and log_rotation_age to 1440. Example: To keep 24 hours of logs, one log file per hour, but also rotate sooner if the log file size exceeds 1GB, set log_filename to server_log.%H%M, log_truncate_on_rotation to on, log_rotation_age to 60, and log_rotation_size to 1000000. Including %M in log_filename allows any size-driven rotations that might occur to select a file name different from the hour's initial file name. syslog_facility (enum) syslog_facility configuration parameter When logging to syslog is enabled, this parameter determines the syslog facility to be used. You can choose from LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7; the default is LOCAL0. See also the documentation of your system's syslog daemon. This parameter can only be set in the postgresql.conf file or on the server command line. syslog_ident (string) syslog_ident configuration parameter When logging to syslog is enabled, this parameter determines the program name used to identify PostgreSQL messages in syslog logs. The default is postgres. This parameter can only be set in the postgresql.conf file or on the server command line. syslog_sequence_numbers (boolean) syslog_sequence_numbers configuration parameter When logging to syslog and this is on (the default), then each message will be prefixed by an increasing sequence number (such as [2]). This circumvents the --- last message repeated N times --- suppression that many syslog implementations perform by default. In more modern syslog implementations, repeat message suppression can be configured (for example, $RepeatedMsgReduction in rsyslog), so this might not be necessary. Also, you could turn this off if you actually want to suppress repeated messages. This parameter can only be set in the postgresql.conf file or on the server command line. syslog_split_messages (boolean) syslog_split_messages configuration parameter When logging to syslog is enabled, this parameter determines how messages are delivered to syslog. When on (the default), messages are split by lines, and long lines are split so that they will fit into 1024 bytes, which is a typical size limit for traditional syslog implementations. When off, PostgreSQL server log messages are delivered to the syslog service as is, and it is up to the syslog service to cope with the potentially bulky messages. If syslog is ultimately logging to a text file, then the effect will be the same either way, and it is best to leave the setting on, since most syslog implementations either cannot handle large messages or would need to be specially configured to handle them. But if syslog is ultimately writing into some other medium, it might be necessary or more useful to keep messages logically together. This parameter can only be set in the postgresql.conf file or on the server command line. event_source (string) event_source configuration parameter When logging to event log is enabled, this parameter determines the program name used to identify PostgreSQL messages in the log. The default is PostgreSQL. This parameter can only be set in the postgresql.conf file or on the server command line. client_min_messages (enum) client_min_messages configuration parameter Controls which message levels are sent to the client. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, LOG, NOTICE, WARNING, ERROR, FATAL, and PANIC. Each level includes all the levels that follow it. The later the level, the fewer messages are sent. The default is NOTICE. Note that LOG has a different rank here than in log_min_messages. log_min_messages (enum) log_min_messages configuration parameter Controls which message levels are written to the server log. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, INFO, NOTICE, WARNING, ERROR, LOG, FATAL, and PANIC. Each level includes all the levels that follow it. The later the level, the fewer messages are sent to the log. The default is WARNING. Note that LOG has a different rank here than in client_min_messages. Only superusers can change this setting. log_min_error_statement (enum) log_min_error_statement configuration parameter Controls which SQL statements that cause an error condition are recorded in the server log. The current SQL statement is included in the log entry for any message of the specified severity or higher. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, INFO, NOTICE, WARNING, ERROR, LOG, FATAL, and PANIC. The default is ERROR, which means statements causing errors, log messages, fatal errors, or panics will be logged. To effectively turn off logging of failing statements, set this parameter to PANIC. Only superusers can change this setting. log_min_duration_statement (integer) log_min_duration_statement configuration parameter Causes the duration of each completed statement to be logged if the statement ran for at least the specified number of milliseconds. Setting this to zero prints all statement durations. Minus-one (the default) disables logging statement durations. For example, if you set it to 250ms then all SQL statements that run 250ms or longer will be logged. Enabling this parameter can be helpful in tracking down unoptimized queries in your applications. Only superusers can change this setting. For clients using extended query protocol, durations of the Parse, Bind, and Execute steps are logged independently. When using this option together with , the text of statements that are logged because of log_statement will not be repeated in the duration log message. If you are not using syslog, it is recommended that you log the PID or session ID using so that you can link the statement message to the later duration message using the process ID or session ID. explains the message severity levels used by PostgreSQL. If logging output is sent to syslog or Windows' eventlog, the severity levels are translated as shown in the table. Severity Usage syslog eventlog DEBUG1..DEBUG5 Provides successively-more-detailed information for use by developers. DEBUG INFORMATION INFO Provides information implicitly requested by the user, e.g., output from VACUUM VERBOSE. INFO INFORMATION NOTICE Provides information that might be helpful to users, e.g., notice of truncation of long identifiers. NOTICE INFORMATION WARNING Provides warnings of likely problems, e.g., COMMIT outside a transaction block. NOTICE WARNING ERROR Reports an error that caused the current command to abort. WARNING ERROR LOG Reports information of interest to administrators, e.g., checkpoint activity. INFO INFORMATION FATAL Reports an error that caused the current session to abort. ERR ERROR PANIC Reports an error that caused all database sessions to abort. CRIT ERROR application_name (string) application_name configuration parameter The application_name can be any string of less than NAMEDATALEN characters (64 characters in a standard build). It is typically set by an application upon connection to the server. The name will be displayed in the pg_stat_activity view and included in CSV log entries. It can also be included in regular log entries via the parameter. Only printable ASCII characters may be used in the application_name value. Other characters will be replaced with question marks (?). debug_print_parse (boolean) debug_print_parse configuration parameter debug_print_rewritten (boolean) debug_print_rewritten configuration parameter debug_print_plan (boolean) debug_print_plan configuration parameter These parameters enable various debugging output to be emitted. When set, they print the resulting parse tree, the query rewriter output, or the execution plan for each executed query. These messages are emitted at LOG message level, so by default they will appear in the server log but will not be sent to the client. You can change that by adjusting and/or . These parameters are off by default. debug_pretty_print (boolean) debug_pretty_print configuration parameter When set, debug_pretty_print indents the messages produced by debug_print_parse, debug_print_rewritten, or debug_print_plan. This results in more readable but much longer output than the compact format used when it is off. It is on by default. log_checkpoints (boolean) log_checkpoints configuration parameter Causes checkpoints and restartpoints to be logged in the server log. Some statistics are included in the log messages, including the number of buffers written and the time spent writing them. This parameter can only be set in the postgresql.conf file or on the server command line. The default is off. log_connections (boolean) log_connections configuration parameter Causes each attempted connection to the server to be logged, as well as successful completion of client authentication. Only superusers can change this parameter at session start, and it cannot be changed at all within a session. The default is off. Some client programs, like psql, attempt to connect twice while determining if a password is required, so duplicate connection received messages do not necessarily indicate a problem. log_disconnections (boolean) log_disconnections configuration parameter Causes session terminations to be logged. The log output provides information similar to log_connections, plus the duration of the session. Only superusers can change this parameter at session start, and it cannot be changed at all within a session. The default is off. log_duration (boolean) log_duration configuration parameter Causes the duration of every completed statement to be logged. The default is off. Only superusers can change this setting. For clients using extended query protocol, durations of the Parse, Bind, and Execute steps are logged independently. The difference between setting this option and setting to zero is that exceeding log_min_duration_statement forces the text of the query to be logged, but this option doesn't. Thus, if log_duration is on and log_min_duration_statement has a positive value, all durations are logged but the query text is included only for statements exceeding the threshold. This behavior can be useful for gathering statistics in high-load installations. log_error_verbosity (enum) log_error_verbosity configuration parameter Controls the amount of detail written in the server log for each message that is logged. Valid values are TERSE, DEFAULT, and VERBOSE, each adding more fields to displayed messages. TERSE excludes the logging of DETAIL, HINT, QUERY, and CONTEXT error information. VERBOSE output includes the SQLSTATE error code (see also ) and the source code file name, function name, and line number that generated the error. Only superusers can change this setting. log_hostname (boolean) log_hostname configuration parameter By default, connection log messages only show the IP address of the connecting host. Turning this parameter on causes logging of the host name as well. Note that depending on your host name resolution setup this might impose a non-negligible performance penalty. This parameter can only be set in the postgresql.conf file or on the server command line. log_line_prefix (string) log_line_prefix configuration parameter This is a printf-style string that is output at the beginning of each log line. % characters begin escape sequences that are replaced with status information as outlined below. Unrecognized escapes are ignored. Other characters are copied straight to the log line. Some escapes are only recognized by session processes, and will be treated as empty by background processes such as the main server process. Status information may be aligned either left or right by specifying a numeric literal after the % and before the option. A negative value will cause the status information to be padded on the right with spaces to give it a minimum width, whereas a positive value will pad on the left. Padding can be useful to aid human readability in log files. This parameter can only be set in the postgresql.conf file or on the server command line. The default is an empty string. Escape Effect Session only %a Application name yes %u User name yes %d Database name yes %r Remote host name or IP address, and remote port yes %h Remote host name or IP address yes %p Process ID no %t Time stamp without milliseconds no %m Time stamp with milliseconds no %n Time stamp with milliseconds (as a Unix epoch) no %i Command tag: type of session's current command yes %e SQLSTATE error code no %c Session ID: see below no %l Number of the log line for each session or process, starting at 1 no %s Process start time stamp no %v Virtual transaction ID (backendID/localXID) no %x Transaction ID (0 if none is assigned) no %q Produces no output, but tells non-session processes to stop at this point in the string; ignored by session processes no %% Literal % no The %c escape prints a quasi-unique session identifier, consisting of two 4-byte hexadecimal numbers (without leading zeros) separated by a dot. The numbers are the process start time and the process ID, so %c can also be used as a space saving way of printing those items. For example, to generate the session identifier from pg_stat_activity, use this query: SELECT to_hex(trunc(EXTRACT(EPOCH FROM backend_start))::integer) || '.' || to_hex(pid) FROM pg_stat_activity; If you set a nonempty value for log_line_prefix, you should usually make its last character be a space, to provide visual separation from the rest of the log line. A punctuation character can be used too. Syslog produces its own time stamp and process ID information, so you probably do not want to include those escapes if you are logging to syslog. log_lock_waits (boolean) log_lock_waits configuration parameter Controls whether a log message is produced when a session waits longer than to acquire a lock. This is useful in determining if lock waits are causing poor performance. The default is off. log_statement (enum) log_statement configuration parameter Controls which SQL statements are logged. Valid values are none (off), ddl, mod, and all (all statements). ddl logs all data definition statements, such as CREATE, ALTER, and DROP statements. mod logs all ddl statements, plus data-modifying statements such as INSERT, UPDATE, DELETE, TRUNCATE, and COPY FROM. PREPARE, EXECUTE, and EXPLAIN ANALYZE statements are also logged if their contained command is of an appropriate type. For clients using extended query protocol, logging occurs when an Execute message is received, and values of the Bind parameters are included (with any embedded single-quote marks doubled). The default is none. Only superusers can change this setting. Statements that contain simple syntax errors are not logged even by the log_statement = all setting, because the log message is emitted only after basic parsing has been done to determine the statement type. In the case of extended query protocol, this setting likewise does not log statements that fail before the Execute phase (i.e., during parse analysis or planning). Set log_min_error_statement to ERROR (or lower) to log such statements. log_replication_commands (boolean) log_replication_commands configuration parameter Causes each replication command to be logged in the server log. See for more information about replication command. The default value is off. Only superusers can change this setting. log_temp_files (integer) log_temp_files configuration parameter Controls logging of temporary file names and sizes. Temporary files can be created for sorts, hashes, and temporary query results. A log entry is made for each temporary file when it is deleted. A value of zero logs all temporary file information, while positive values log only files whose size is greater than or equal to the specified number of kilobytes. The default setting is -1, which disables such logging. Only superusers can change this setting. log_timezone (string) log_timezone configuration parameter Sets the time zone used for timestamps written in the server log. Unlike , this value is cluster-wide, so that all sessions will report timestamps consistently. The built-in default is GMT, but that is typically overridden in postgresql.conf; initdb will install a setting there corresponding to its system environment. See for more information. This parameter can only be set in the postgresql.conf file or on the server command line. Including csvlog in the log_destination list provides a convenient way to import log files into a database table. This option emits log lines in comma-separated-values (CSV) format, with these columns: time stamp with milliseconds, user name, database name, process ID, client host:port number, session ID, per-session line number, command tag, session start time, virtual transaction ID, regular transaction ID, error severity, SQLSTATE code, error message, error message detail, hint, internal query that led to the error (if any), character count of the error position therein, error context, user query that led to the error (if any and enabled by log_min_error_statement), character count of the error position therein, location of the error in the PostgreSQL source code (if log_error_verbosity is set to verbose), and application name. Here is a sample table definition for storing CSV-format log output: CREATE TABLE postgres_log ( log_time timestamp(3) with time zone, user_name text, database_name text, process_id integer, connection_from text, session_id text, session_line_num bigint, command_tag text, session_start_time timestamp with time zone, virtual_transaction_id text, transaction_id bigint, error_severity text, sql_state_code text, message text, detail text, hint text, internal_query text, internal_query_pos integer, context text, query text, query_pos integer, location text, application_name text, PRIMARY KEY (session_id, session_line_num) ); To import a log file into this table, use the COPY FROM command: COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv; There are a few things you need to do to simplify importing CSV log files: Set log_filename and log_rotation_age to provide a consistent, predictable naming scheme for your log files. This lets you predict what the file name will be and know when an individual log file is complete and therefore ready to be imported. Set log_rotation_size to 0 to disable size-based log rotation, as it makes the log file name difficult to predict. Set log_truncate_on_rotation to on so that old log data isn't mixed with the new in the same file. The table definition above includes a primary key specification. This is useful to protect against accidentally importing the same information twice. The COPY command commits all of the data it imports at one time, so any error will cause the entire import to fail. If you import a partial log file and later import the file again when it is complete, the primary key violation will cause the import to fail. Wait until the log is complete and closed before importing. This procedure will also protect against accidentally importing a partial line that hasn't been completely written, which would also cause COPY to fail. These settings control how the process title as seen by ps is modified. See for details. cluster_name (string) cluster_name configuration parameter Sets the cluster name that appears in the process title for all processes in this cluster. The name can be any string of less than NAMEDATALEN characters (64 characters in a standard build). Only printable ASCII characters may be used in the cluster_name value. Other characters will be replaced with question marks (?). No name is shown if this parameter is set to the empty string '' (which is the default). This parameter can only be set at server start. The process title is typically viewed using programs like ps or, on Windows, Process Explorer. update_process_title (boolean) update_process_title configuration parameter Enables updating of the process title every time a new SQL command is received by the server. The process title is typically viewed by the ps command, or in Windows by using the Process Explorer. Only superusers can change this setting. These parameters control server-wide statistics collection features. When statistics collection is enabled, the data that is produced can be accessed via the pg_stat and pg_statio family of system views. Refer to for more information. track_activities (boolean) track_activities configuration parameter Enables the collection of information on the currently executing command of each session, along with the time when that command began execution. This parameter is on by default. Note that even when enabled, this information is not visible to all users, only to superusers and the user owning the session being reported on, so it should not represent a security risk. Only superusers can change this setting. track_activity_query_size (integer) track_activity_query_size configuration parameter Specifies the number of bytes reserved to track the currently executing command for each active session, for the pg_stat_activity.query field. The default value is 1024. This parameter can only be set at server start. track_counts (boolean) track_counts configuration parameter Enables collection of statistics on database activity. This parameter is on by default, because the autovacuum daemon needs the collected information. Only superusers can change this setting. track_io_timing (boolean) track_io_timing configuration parameter Enables timing of database I/O calls. This parameter is off by default, because it will repeatedly query the operating system for the current time, which may cause significant overhead on some platforms. You can use the tool to measure the overhead of timing on your system. I/O timing information is displayed in , in the output of when the BUFFERS option is used, and by . Only superusers can change this setting. track_functions (enum) track_functions configuration parameter Enables tracking of function call counts and time used. Specify pl to track only procedural-language functions, all to also track SQL and C language functions. The default is none, which disables function statistics tracking. Only superusers can change this setting. SQL-language functions that are simple enough to be inlined into the calling query will not be tracked, regardless of this setting. stats_temp_directory (string) stats_temp_directory configuration parameter Sets the directory to store temporary statistics data in. This can be a path relative to the data directory or an absolute path. The default is pg_stat_tmp. Pointing this at a RAM-based file system will decrease physical I/O requirements and can lead to improved performance. This parameter can only be set in the postgresql.conf file or on the server command line. log_statement_stats (boolean) log_statement_stats configuration parameter log_parser_stats (boolean) log_parser_stats configuration parameter log_planner_stats (boolean) log_planner_stats configuration parameter log_executor_stats (boolean) log_executor_stats configuration parameter For each query, output performance statistics of the respective module to the server log. This is a crude profiling instrument, similar to the Unix getrusage() operating system facility. log_statement_stats reports total statement statistics, while the others report per-module statistics. log_statement_stats cannot be enabled together with any of the per-module options. All of these options are disabled by default. Only superusers can change these settings. autovacuum configuration parameters These settings control the behavior of the autovacuum feature. Refer to for more information. Note that many of these settings can be overridden on a per-table basis; see . autovacuum (boolean) autovacuum configuration parameter Controls whether the server should run the autovacuum launcher daemon. This is on by default; however, must also be enabled for autovacuum to work. This parameter can only be set in the postgresql.conf file or on the server command line; however, autovacuuming can be disabled for individual tables by changing table storage parameters. Note that even when this parameter is disabled, the system will launch autovacuum processes if necessary to prevent transaction ID wraparound. See for more information. log_autovacuum_min_duration (integer) log_autovacuum_min_duration configuration parameter Causes each action executed by autovacuum to be logged if it ran for at least the specified number of milliseconds. Setting this to zero logs all autovacuum actions. Minus-one (the default) disables logging autovacuum actions. For example, if you set this to 250ms then all automatic vacuums and analyzes that run 250ms or longer will be logged. In addition, when this parameter is set to any value other than -1, a message will be logged if an autovacuum action is skipped due to the existence of a conflicting lock. Enabling this parameter can be helpful in tracking autovacuum activity. This parameter can only be set in the postgresql.conf file or on the server command line; but the setting can be overridden for individual tables by changing table storage parameters. autovacuum_max_workers (integer) autovacuum_max_workers configuration parameter Specifies the maximum number of autovacuum processes (other than the autovacuum launcher) that may be running at any one time. The default is three. This parameter can only be set at server start. autovacuum_naptime (integer) autovacuum_naptime configuration parameter Specifies the minimum delay between autovacuum runs on any given database. In each round the daemon examines the database and issues VACUUM and ANALYZE commands as needed for tables in that database. The delay is measured in seconds, and the default is one minute (1min). This parameter can only be set in the postgresql.conf file or on the server command line. autovacuum_vacuum_threshold (integer) autovacuum_vacuum_threshold configuration parameter Specifies the minimum number of updated or deleted tuples needed to trigger a VACUUM in any one table. The default is 50 tuples. This parameter can only be set in the postgresql.conf file or on the server command line; but the setting can be overridden for individual tables by changing table storage parameters. autovacuum_analyze_threshold (integer) autovacuum_analyze_threshold configuration parameter Specifies the minimum number of inserted, updated or deleted tuples needed to trigger an ANALYZE in any one table. The default is 50 tuples. This parameter can only be set in the postgresql.conf file or on the server command line; but the setting can be overridden for individual tables by changing table storage parameters. autovacuum_vacuum_scale_factor (floating point) autovacuum_vacuum_scale_factor configuration parameter Specifies a fraction of the table size to add to autovacuum_vacuum_threshold when deciding whether to trigger a VACUUM. The default is 0.2 (20% of table size). This parameter can only be set in the postgresql.conf file or on the server command line; but the setting can be overridden for individual tables by changing table storage parameters. autovacuum_analyze_scale_factor (floating point) autovacuum_analyze_scale_factor configuration parameter Specifies a fraction of the table size to add to autovacuum_analyze_threshold when deciding whether to trigger an ANALYZE. The default is 0.1 (10% of table size). This parameter can only be set in the postgresql.conf file or on the server command line; but the setting can be overridden for individual tables by changing table storage parameters. autovacuum_freeze_max_age (integer) autovacuum_freeze_max_age configuration parameter Specifies the maximum age (in transactions) that a table's pg_class.relfrozenxid field can attain before a VACUUM operation is forced to prevent transaction ID wraparound within the table. Note that the system will launch autovacuum processes to prevent wraparound even when autovacuum is otherwise disabled. Vacuum also allows removal of old files from the pg_clog subdirectory, which is why the default is a relatively low 200 million transactions. This parameter can only be set at server start, but the setting can be reduced for individual tables by changing table storage parameters. For more information see . autovacuum_multixact_freeze_max_age (integer) autovacuum_multixact_freeze_max_age configuration parameter Specifies the maximum age (in multixacts) that a table's pg_class.relminmxid field can attain before a VACUUM operation is forced to prevent multixact ID wraparound within the table. Note that the system will launch autovacuum processes to prevent wraparound even when autovacuum is otherwise disabled. Vacuuming multixacts also allows removal of old files from the pg_multixact/members and pg_multixact/offsets subdirectories, which is why the default is a relatively low 400 million multixacts. This parameter can only be set at server start, but the setting can be reduced for individual tables by changing table storage parameters. For more information see . autovacuum_vacuum_cost_delay (integer) autovacuum_vacuum_cost_delay configuration parameter Specifies the cost delay value that will be used in automatic VACUUM operations. If -1 is specified, the regular value will be used. The default value is 20 milliseconds. This parameter can only be set in the postgresql.conf file or on the server command line; but the setting can be overridden for individual tables by changing table storage parameters. autovacuum_vacuum_cost_limit (integer) autovacuum_vacuum_cost_limit configuration parameter Specifies the cost limit value that will be used in automatic VACUUM operations. If -1 is specified (which is the default), the regular value will be used. Note that the value is distributed proportionally among the running autovacuum workers, if there is more than one, so that the sum of the limits for each worker does not exceed the value of this variable. This parameter can only be set in the postgresql.conf file or on the server command line; but the setting can be overridden for individual tables by changing table storage parameters. search_path (string) search_path configuration parameter pathfor schemas This variable specifies the order in which schemas are searched when an object (table, data type, function, etc.) is referenced by a simple name with no schema specified. When there are objects of identical names in different schemas, the one found first in the search path is used. An object that is not in any of the schemas in the search path can only be referenced by specifying its containing schema with a qualified (dotted) name. The value for search_path must be a comma-separated list of schema names. Any name that is not an existing schema, or is a schema for which the user does not have USAGE permission, is silently ignored. If one of the list items is the special name $user, then the schema having the name returned by SESSION_USER is substituted, if there is such a schema and the user has USAGE permission for it. (If not, $user is ignored.) The system catalog schema, pg_catalog, is always searched, whether it is mentioned in the path or not. If it is mentioned in the path then it will be searched in the specified order. If pg_catalog is not in the path then it will be searched before searching any of the path items. Likewise, the current session's temporary-table schema, pg_temp_nnn, is always searched if it exists. It can be explicitly listed in the path by using the alias pg_temppg_temp. If it is not listed in the path then it is searched first (even before pg_catalog). However, the temporary schema is only searched for relation (table, view, sequence, etc) and data type names. It is never searched for function or operator names. When objects are created without specifying a particular target schema, they will be placed in the first valid schema named in search_path. An error is reported if the search path is empty. The default value for this parameter is "$user", public. This setting supports shared use of a database (where no users have private schemas, and all share use of public), private per-user schemas, and combinations of these. Other effects can be obtained by altering the default search path setting, either globally or per-user. The current effective value of the search path can be examined via the SQL function current_schemas (see ). This is not quite the same as examining the value of search_path, since current_schemas shows how the items appearing in search_path were resolved. For more information on schema handling, see . row_security (boolean) row_security configuration parameter This variable controls whether to raise an error in lieu of applying a row security policy. When set to on, policies apply normally. When set to off, queries fail which would otherwise apply at least one policy. The default is on. Change to off where limited row visibility could cause incorrect results; for example, pg_dump makes that change by default. This variable has no effect on roles which bypass every row security policy, to wit, superusers and roles with the BYPASSRLS attribute. For more information on row security policies, see . default_tablespace (string) default_tablespace configuration parameter tablespacedefault This variable specifies the default tablespace in which to create objects (tables and indexes) when a CREATE command does not explicitly specify a tablespace. The value is either the name of a tablespace, or an empty string to specify using the default tablespace of the current database. If the value does not match the name of any existing tablespace, PostgreSQL will automatically use the default tablespace of the current database. If a nondefault tablespace is specified, the user must have CREATE privilege for it, or creation attempts will fail. This variable is not used for temporary tables; for them, is consulted instead. This variable is also not used when creating databases. By default, a new database inherits its tablespace setting from the template database it is copied from. For more information on tablespaces, see . temp_tablespaces (string) temp_tablespaces configuration parameter tablespacetemporary This variable specifies tablespaces in which to create temporary objects (temp tables and indexes on temp tables) when a CREATE command does not explicitly specify a tablespace. Temporary files for purposes such as sorting large data sets are also created in these tablespaces. The value is a list of names of tablespaces. When there is more than one name in the list, PostgreSQL chooses a random member of the list each time a temporary object is to be created; except that within a transaction, successively created temporary objects are placed in successive tablespaces from the list. If the selected element of the list is an empty string, PostgreSQL will automatically use the default tablespace of the current database instead. When temp_tablespaces is set interactively, specifying a nonexistent tablespace is an error, as is specifying a tablespace for which the user does not have CREATE privilege. However, when using a previously set value, nonexistent tablespaces are ignored, as are tablespaces for which the user lacks CREATE privilege. In particular, this rule applies when using a value set in postgresql.conf. The default value is an empty string, which results in all temporary objects being created in the default tablespace of the current database. See also . check_function_bodies (boolean) check_function_bodies configuration parameter This parameter is normally on. When set to off, it disables validation of the function body string during . Disabling validation avoids side effects of the validation process and avoids false positives due to problems such as forward references. Set this parameter to off before loading functions on behalf of other users; pg_dump does so automatically. default_transaction_isolation (enum) transaction isolation level setting default default_transaction_isolation configuration parameter Each SQL transaction has an isolation level, which can be either read uncommitted, read committed, repeatable read, or serializable. This parameter controls the default isolation level of each new transaction. The default is read committed. Consult and for more information. default_transaction_read_only (boolean) read-only transaction setting default default_transaction_read_only configuration parameter A read-only SQL transaction cannot alter non-temporary tables. This parameter controls the default read-only status of each new transaction. The default is off (read/write). Consult for more information. default_transaction_deferrable (boolean) deferrable transaction setting default default_transaction_deferrable configuration parameter When running at the serializable isolation level, a deferrable read-only SQL transaction may be delayed before it is allowed to proceed. However, once it begins executing it does not incur any of the overhead required to ensure serializability; so serialization code will have no reason to force it to abort because of concurrent updates, making this option suitable for long-running read-only transactions. This parameter controls the default deferrable status of each new transaction. It currently has no effect on read-write transactions or those operating at isolation levels lower than serializable. The default is off. Consult for more information. session_replication_role (enum) session_replication_role configuration parameter Controls firing of replication-related triggers and rules for the current session. Setting this variable requires superuser privilege and results in discarding any previously cached query plans. Possible values are origin (the default), replica and local. See for more information. statement_timeout (integer) statement_timeout configuration parameter Abort any statement that takes more than the specified number of milliseconds, starting from the time the command arrives at the server from the client. If log_min_error_statement is set to ERROR or lower, the statement that timed out will also be logged. A value of zero (the default) turns this off. Setting statement_timeout in postgresql.conf is not recommended because it would affect all sessions. lock_timeout (integer) lock_timeout configuration parameter Abort any statement that waits longer than the specified number of milliseconds while attempting to acquire a lock on a table, index, row, or other database object. The time limit applies separately to each lock acquisition attempt. The limit applies both to explicit locking requests (such as LOCK TABLE, or SELECT FOR UPDATE without NOWAIT) and to implicitly-acquired locks. If log_min_error_statement is set to ERROR or lower, the statement that timed out will be logged. A value of zero (the default) turns this off. Unlike statement_timeout, this timeout can only occur while waiting for locks. Note that if statement_timeout is nonzero, it is rather pointless to set lock_timeout to the same or larger value, since the statement timeout would always trigger first. Setting lock_timeout in postgresql.conf is not recommended because it would affect all sessions. idle_in_transaction_session_timeout (integer) idle_in_transaction_session_timeout configuration parameter Terminate any session with an open transaction that has been idle for longer than the specified duration in milliseconds. This allows any locks held by that session to be released and the connection slot to be reused; it also allows tuples visible only to this transaction to be vacuumed. See for more details about this. The default value of 0 disables this feature. vacuum_freeze_table_age (integer) vacuum_freeze_table_age configuration parameter VACUUM performs an aggressive scan if the table's pg_class.relfrozenxid field has reached the age specified by this setting. An aggressive scan differs from a regular VACUUM in that it visits every page that might contain unfrozen XIDs or MXIDs, not just those that might contain dead tuples. The default is 150 million transactions. Although users can set this value anywhere from zero to two billions, VACUUM will silently limit the effective value to 95% of , so that a periodical manual VACUUM has a chance to run before an anti-wraparound autovacuum is launched for the table. For more information see . vacuum_freeze_min_age (integer) vacuum_freeze_min_age configuration parameter Specifies the cutoff age (in transactions) that VACUUM should use to decide whether to freeze row versions while scanning a table. The default is 50 million transactions. Although users can set this value anywhere from zero to one billion, VACUUM will silently limit the effective value to half the value of , so that there is not an unreasonably short time between forced autovacuums. For more information see . vacuum_multixact_freeze_table_age (integer) vacuum_multixact_freeze_table_age configuration parameter VACUUM performs an aggressive scan if the table's pg_class.relminmxid field has reached the age specified by this setting. An aggressive scan differs from a regular VACUUM in that it visits every page that might contain unfrozen XIDs or MXIDs, not just those that might contain dead tuples. The default is 150 million multixacts. Although users can set this value anywhere from zero to two billions, VACUUM will silently limit the effective value to 95% of , so that a periodical manual VACUUM has a chance to run before an anti-wraparound is launched for the table. For more information see . vacuum_multixact_freeze_min_age (integer) vacuum_multixact_freeze_min_age configuration parameter Specifies the cutoff age (in multixacts) that VACUUM should use to decide whether to replace multixact IDs with a newer transaction ID or multixact ID while scanning a table. The default is 5 million multixacts. Although users can set this value anywhere from zero to one billion, VACUUM will silently limit the effective value to half the value of , so that there is not an unreasonably short time between forced autovacuums. For more information see . bytea_output (enum) bytea_output configuration parameter Sets the output format for values of type bytea. Valid values are hex (the default) and escape (the traditional PostgreSQL format). See for more information. The bytea type always accepts both formats on input, regardless of this setting. xmlbinary (enum) xmlbinary configuration parameter Sets how binary values are to be encoded in XML. This applies for example when bytea values are converted to XML by the functions xmlelement or xmlforest. Possible values are base64 and hex, which are both defined in the XML Schema standard. The default is base64. For further information about XML-related functions, see . The actual choice here is mostly a matter of taste, constrained only by possible restrictions in client applications. Both methods support all possible values, although the hex encoding will be somewhat larger than the base64 encoding. xmloption (enum) xmloption configuration parameter SET XML OPTION XML option Sets whether DOCUMENT or CONTENT is implicit when converting between XML and character string values. See for a description of this. Valid values are DOCUMENT and CONTENT. The default is CONTENT. According to the SQL standard, the command to set this option is SET XML OPTION { DOCUMENT | CONTENT }; This syntax is also available in PostgreSQL. gin_pending_list_limit (integer) gin_pending_list_limit configuration parameter Sets the maximum size of the GIN pending list which is used when fastupdate is enabled. If the list grows larger than this maximum size, it is cleaned up by moving the entries in it to the main GIN data structure in bulk. The default is four megabytes (4MB). This setting can be overridden for individual GIN indexes by changing index storage parameters. See and for more information. DateStyle (string) DateStyle configuration parameter Sets the display format for date and time values, as well as the rules for interpreting ambiguous date input values. For historical reasons, this variable contains two independent components: the output format specification (ISO, Postgres, SQL, or German) and the input/output specification for year/month/day ordering (DMY, MDY, or YMD). These can be set separately or together. The keywords Euro and European are synonyms for DMY; the keywords US, NonEuro, and NonEuropean are synonyms for MDY. See for more information. The built-in default is ISO, MDY, but initdb will initialize the configuration file with a setting that corresponds to the behavior of the chosen lc_time locale. IntervalStyle (enum) IntervalStyle configuration parameter Sets the display format for interval values. The value sql_standard will produce output matching SQL standard interval literals. The value postgres (which is the default) will produce output matching PostgreSQL releases prior to 8.4 when the parameter was set to ISO. The value postgres_verbose will produce output matching PostgreSQL releases prior to 8.4 when the DateStyle parameter was set to non-ISO output. The value iso_8601 will produce output matching the time interval format with designators defined in section 4.4.3.2 of ISO 8601. The IntervalStyle parameter also affects the interpretation of ambiguous interval input. See for more information. TimeZone (string) TimeZone configuration parameter time zone Sets the time zone for displaying and interpreting time stamps. The built-in default is GMT, but that is typically overridden in postgresql.conf; initdb will install a setting there corresponding to its system environment. See for more information. timezone_abbreviations (string) timezone_abbreviations configuration parameter time zone names Sets the collection of time zone abbreviations that will be accepted by the server for datetime input. The default is 'Default', which is a collection that works in most of the world; there are also 'Australia' and 'India', and other collections can be defined for a particular installation. See for more information. extra_float_digits (integer) significant digits floating-point display extra_float_digits configuration parameter This parameter adjusts the number of digits displayed for floating-point values, including float4, float8, and geometric data types. The parameter value is added to the standard number of digits (FLT_DIG or DBL_DIG as appropriate). The value can be set as high as 3, to include partially-significant digits; this is especially useful for dumping float data that needs to be restored exactly. Or it can be set negative to suppress unwanted digits. See also . client_encoding (string) client_encoding configuration parameter character set Sets the client-side encoding (character set). The default is to use the database encoding. The character sets supported by the PostgreSQL server are described in . lc_messages (string) lc_messages configuration parameter Sets the language in which messages are displayed. Acceptable values are system-dependent; see for more information. If this variable is set to the empty string (which is the default) then the value is inherited from the execution environment of the server in a system-dependent way. On some systems, this locale category does not exist. Setting this variable will still work, but there will be no effect. Also, there is a chance that no translated messages for the desired language exist. In that case you will continue to see the English messages. Only superusers can change this setting, because it affects the messages sent to the server log as well as to the client, and an improper value might obscure the readability of the server logs. lc_monetary (string) lc_monetary configuration parameter Sets the locale to use for formatting monetary amounts, for example with the to_char family of functions. Acceptable values are system-dependent; see for more information. If this variable is set to the empty string (which is the default) then the value is inherited from the execution environment of the server in a system-dependent way. lc_numeric (string) lc_numeric configuration parameter Sets the locale to use for formatting numbers, for example with the to_char family of functions. Acceptable values are system-dependent; see for more information. If this variable is set to the empty string (which is the default) then the value is inherited from the execution environment of the server in a system-dependent way. lc_time (string) lc_time configuration parameter Sets the locale to use for formatting dates and times, for example with the to_char family of functions. Acceptable values are system-dependent; see for more information. If this variable is set to the empty string (which is the default) then the value is inherited from the execution environment of the server in a system-dependent way. default_text_search_config (string) default_text_search_config configuration parameter Selects the text search configuration that is used by those variants of the text search functions that do not have an explicit argument specifying the configuration. See for further information. The built-in default is pg_catalog.simple, but initdb will initialize the configuration file with a setting that corresponds to the chosen lc_ctype locale, if a configuration matching that locale can be identified. Several settings are available for preloading shared libraries into the server, in order to load additional functionality or achieve performance benefits. For example, a setting of '$libdir/mylib' would cause mylib.so (or on some platforms, mylib.sl) to be preloaded from the installation's standard library directory. The differences between the settings are when they take effect and what privileges are required to change them. PostgreSQL procedural language libraries can be preloaded in this way, typically by using the syntax '$libdir/plXXX' where XXX is pgsql, perl, tcl, or python. For each parameter, if more than one library is to be loaded, separate their names with commas. All library names are converted to lower case unless double-quoted. Only shared libraries specifically intended to be used with PostgreSQL can be loaded this way. Every PostgreSQL-supported library has a magic block that is checked to guarantee compatibility. For this reason, non-PostgreSQL libraries cannot be loaded in this way. You might be able to use operating-system facilities such as LD_PRELOAD for that. In general, refer to the documentation of a specific module for the recommended way to load that module. local_preload_libraries (string) local_preload_libraries configuration parameter $libdir/plugins This variable specifies one or more shared libraries that are to be preloaded at connection start. The parameter value only takes effect at the start of the connection. Subsequent changes have no effect. If a specified library is not found, the connection attempt will fail. This option can be set by any user. Because of that, the libraries that can be loaded are restricted to those appearing in the plugins subdirectory of the installation's standard library directory. (It is the database administrator's responsibility to ensure that only safe libraries are installed there.) Entries in local_preload_libraries can specify this directory explicitly, for example $libdir/plugins/mylib, or just specify the library name — mylib would have the same effect as $libdir/plugins/mylib. The intent of this feature is to allow unprivileged users to load debugging or performance-measurement libraries into specific sessions without requiring an explicit LOAD command. To that end, it would be typical to set this parameter using the PGOPTIONS environment variable on the client or by using ALTER ROLE SET. However, unless a module is specifically designed to be used in this way by non-superusers, this is usually not the right setting to use. Look at instead. session_preload_libraries (string) session_preload_libraries configuration parameter This variable specifies one or more shared libraries that are to be preloaded at connection start. Only superusers can change this setting. The parameter value only takes effect at the start of the connection. Subsequent changes have no effect. If a specified library is not found, the connection attempt will fail. The intent of this feature is to allow debugging or performance-measurement libraries to be loaded into specific sessions without an explicit LOAD command being given. For example, could be enabled for all sessions under a given user name by setting this parameter with ALTER ROLE SET. Also, this parameter can be changed without restarting the server (but changes only take effect when a new session is started), so it is easier to add new modules this way, even if they should apply to all sessions. Unlike , there is no large performance advantage to loading a library at session start rather than when it is first used. There is some advantage, however, when connection pooling is used. shared_preload_libraries (string) shared_preload_libraries configuration parameter This variable specifies one or more shared libraries to be preloaded at server start. This parameter can only be set at server start. If a specified library is not found, the server will fail to start. Some libraries need to perform certain operations that can only take place at postmaster start, such as allocating shared memory, reserving light-weight locks, or starting background workers. Those libraries must be loaded at server start through this parameter. See the documentation of each library for details. Other libraries can also be preloaded. By preloading a shared library, the library startup time is avoided when the library is first used. However, the time to start each new server process might increase slightly, even if that process never uses the library. So this parameter is recommended only for libraries that will be used in most sessions. Also, changing this parameter requires a server restart, so this is not the right setting to use for short-term debugging tasks, say. Use for that instead. On Windows hosts, preloading a library at server start will not reduce the time required to start each new server process; each server process will re-load all preload libraries. However, shared_preload_libraries is still useful on Windows hosts for libraries that need to perform operations at postmaster start time. dynamic_library_path (string) dynamic_library_path configuration parameter dynamic loading If a dynamically loadable module needs to be opened and the file name specified in the CREATE FUNCTION or LOAD command does not have a directory component (i.e., the name does not contain a slash), the system will search this path for the required file. The value for dynamic_library_path must be a list of absolute directory paths separated by colons (or semi-colons on Windows). If a list element starts with the special string $libdir, the compiled-in PostgreSQL package library directory is substituted for $libdir; this is where the modules provided by the standard PostgreSQL distribution are installed. (Use pg_config --pkglibdir to find out the name of this directory.) For example: dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir' or, in a Windows environment: dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir' The default value for this parameter is '$libdir'. If the value is set to an empty string, the automatic path search is turned off. This parameter can be changed at run time by superusers, but a setting done that way will only persist until the end of the client connection, so this method should be reserved for development purposes. The recommended way to set this parameter is in the postgresql.conf configuration file. gin_fuzzy_search_limit (integer) gin_fuzzy_search_limit configuration parameter Soft upper limit of the size of the set returned by GIN index scans. For more information see . deadlock_timeout (integer) deadlock timeout during timeout deadlock deadlock_timeout configuration parameter This is the amount of time, in milliseconds, to wait on a lock before checking to see if there is a deadlock condition. The check for deadlock is relatively expensive, so the server doesn't run it every time it waits for a lock. We optimistically assume that deadlocks are not common in production applications and just wait on the lock for a while before checking for a deadlock. Increasing this value reduces the amount of time wasted in needless deadlock checks, but slows down reporting of real deadlock errors. The default is one second (1s), which is probably about the smallest value you would want in practice. On a heavily loaded server you might want to raise it. Ideally the setting should exceed your typical transaction time, so as to improve the odds that a lock will be released before the waiter decides to check for deadlock. Only superusers can change this setting. When is set, this parameter also determines the length of time to wait before a log message is issued about the lock wait. If you are trying to investigate locking delays you might want to set a shorter than normal deadlock_timeout. max_locks_per_transaction (integer) max_locks_per_transaction configuration parameter The shared lock table tracks locks on max_locks_per_transaction * ( + ) objects (e.g., tables); hence, no more than this many distinct objects can be locked at any one time. This parameter controls the average number of object locks allocated for each transaction; individual transactions can lock more objects as long as the locks of all transactions fit in the lock table. This is not the number of rows that can be locked; that value is unlimited. The default, 64, has historically proven sufficient, but you might need to raise this value if you have queries that touch many different tables in a single transaction, e.g. query of a parent table with many children. This parameter can only be set at server start. When running a standby server, you must set this parameter to the same or higher value than on the master server. Otherwise, queries will not be allowed in the standby server. max_pred_locks_per_transaction (integer) max_pred_locks_per_transaction configuration parameter The shared predicate lock table tracks locks on max_pred_locks_per_transaction * ( + ) objects (e.g., tables); hence, no more than this many distinct objects can be locked at any one time. This parameter controls the average number of object locks allocated for each transaction; individual transactions can lock more objects as long as the locks of all transactions fit in the lock table. This is not the number of rows that can be locked; that value is unlimited. The default, 64, has generally been sufficient in testing, but you might need to raise this value if you have clients that touch many different tables in a single serializable transaction. This parameter can only be set at server start. array_nulls (boolean) array_nulls configuration parameter This controls whether the array input parser recognizes unquoted NULL as specifying a null array element. By default, this is on, allowing array values containing null values to be entered. However, PostgreSQL versions before 8.2 did not support null values in arrays, and therefore would treat NULL as specifying a normal array element with the string value NULL. For backward compatibility with applications that require the old behavior, this variable can be turned off. Note that it is possible to create array values containing null values even when this variable is off. backslash_quote (enum) stringsbackslash quotes backslash_quote configuration parameter This controls whether a quote mark can be represented by \' in a string literal. The preferred, SQL-standard way to represent a quote mark is by doubling it ('') but PostgreSQL has historically also accepted \'. However, use of \' creates security risks because in some client character set encodings, there are multibyte characters in which the last byte is numerically equivalent to ASCII \. If client-side code does escaping incorrectly then a SQL-injection attack is possible. This risk can be prevented by making the server reject queries in which a quote mark appears to be escaped by a backslash. The allowed values of backslash_quote are on (allow \' always), off (reject always), and safe_encoding (allow only if client encoding does not allow ASCII \ within a multibyte character). safe_encoding is the default setting. Note that in a standard-conforming string literal, \ just means \ anyway. This parameter only affects the handling of non-standard-conforming literals, including escape string syntax (E'...'). default_with_oids (boolean) default_with_oids configuration parameter This controls whether CREATE TABLE and CREATE TABLE AS include an OID column in newly-created tables, if neither WITH OIDS nor WITHOUT OIDS is specified. It also determines whether OIDs will be included in tables created by SELECT INTO. The parameter is off by default; in PostgreSQL 8.0 and earlier, it was on by default. The use of OIDs in user tables is considered deprecated, so most installations should leave this variable disabled. Applications that require OIDs for a particular table should specify WITH OIDS when creating the table. This variable can be enabled for compatibility with old applications that do not follow this behavior. escape_string_warning (boolean) stringsescape warning escape_string_warning configuration parameter When on, a warning is issued if a backslash (\) appears in an ordinary string literal ('...' syntax) and standard_conforming_strings is off. The default is on. Applications that wish to use backslash as escape should be modified to use escape string syntax (E'...'), because the default behavior of ordinary strings is now to treat backslash as an ordinary character, per SQL standard. This variable can be enabled to help locate code that needs to be changed. lo_compat_privileges (boolean) lo_compat_privileges configuration parameter In PostgreSQL releases prior to 9.0, large objects did not have access privileges and were, therefore, always readable and writable by all users. Setting this variable to on disables the new privilege checks, for compatibility with prior releases. The default is off. Only superusers can change this setting. Setting this variable does not disable all security checks related to large objects — only those for which the default behavior has changed in PostgreSQL 9.0. For example, lo_import() and lo_export() need superuser privileges regardless of this setting. operator_precedence_warning (boolean) operator_precedence_warning configuration parameter When on, the parser will emit a warning for any construct that might have changed meanings since PostgreSQL 9.4 as a result of changes in operator precedence. This is useful for auditing applications to see if precedence changes have broken anything; but it is not meant to be kept turned on in production, since it will warn about some perfectly valid, standard-compliant SQL code. The default is off. See for more information. quote_all_identifiers (boolean) quote_all_identifiers configuration parameter When the database generates SQL, force all identifiers to be quoted, even if they are not (currently) keywords. This will affect the output of EXPLAIN as well as the results of functions like pg_get_viewdef. See also the --quote-all-identifiers option of and . sql_inheritance (boolean) sql_inheritance configuration parameter inheritance This setting controls whether undecorated table references are considered to include inheritance child tables. The default is on, which means child tables are included (thus, a * suffix is assumed by default). If turned off, child tables are not included (thus, an ONLY prefix is assumed). The SQL standard requires child tables to be included, so the off setting is not spec-compliant, but it is provided for compatibility with PostgreSQL releases prior to 7.1. See for more information. Turning sql_inheritance off is deprecated, because that behavior has been found to be error-prone as well as contrary to SQL standard. Discussions of inheritance behavior elsewhere in this manual generally assume that it is on. standard_conforming_strings (boolean) stringsstandard conforming standard_conforming_strings configuration parameter This controls whether ordinary string literals ('...') treat backslashes literally, as specified in the SQL standard. Beginning in PostgreSQL 9.1, the default is on (prior releases defaulted to off). Applications can check this parameter to determine how string literals will be processed. The presence of this parameter can also be taken as an indication that the escape string syntax (E'...') is supported. Escape string syntax () should be used if an application desires backslashes to be treated as escape characters. synchronize_seqscans (boolean) synchronize_seqscans configuration parameter This allows sequential scans of large tables to synchronize with each other, so that concurrent scans read the same block at about the same time and hence share the I/O workload. When this is enabled, a scan might start in the middle of the table and then wrap around the end to cover all rows, so as to synchronize with the activity of scans already in progress. This can result in unpredictable changes in the row ordering returned by queries that have no ORDER BY clause. Setting this parameter to off ensures the pre-8.3 behavior in which a sequential scan always starts from the beginning of the table. The default is on. transform_null_equals (boolean) IS NULL transform_null_equals configuration parameter When on, expressions of the form expr = NULL (or NULL = expr) are treated as expr IS NULL, that is, they return true if expr evaluates to the null value, and false otherwise. The correct SQL-spec-compliant behavior of expr = NULL is to always return null (unknown). Therefore this parameter defaults to off. However, filtered forms in Microsoft Access generate queries that appear to use expr = NULL to test for null values, so if you use that interface to access the database you might want to turn this option on. Since expressions of the form expr = NULL always return the null value (using the SQL standard interpretation), they are not very useful and do not appear often in normal applications so this option does little harm in practice. But new users are frequently confused about the semantics of expressions involving null values, so this option is off by default. Note that this option only affects the exact form = NULL, not other comparison operators or other expressions that are computationally equivalent to some expression involving the equals operator (such as IN). Thus, this option is not a general fix for bad programming. Refer to for related information. exit_on_error (boolean) exit_on_error configuration parameter If true, any error will terminate the current session. By default, this is set to false, so that only FATAL errors will terminate the session. restart_after_crash (boolean) restart_after_crash configuration parameter When set to true, which is the default, PostgreSQL will automatically reinitialize after a backend crash. Leaving this value set to true is normally the best way to maximize the availability of the database. However, in some circumstances, such as when PostgreSQL is being invoked by clusterware, it may be useful to disable the restart so that the clusterware can gain control and take any actions it deems appropriate. The following parameters are read-only, and are determined when PostgreSQL is compiled or when it is installed. As such, they have been excluded from the sample postgresql.conf file. These options report various aspects of PostgreSQL behavior that might be of interest to certain applications, particularly administrative front-ends. block_size (integer) block_size configuration parameter Reports the size of a disk block. It is determined by the value of BLCKSZ when building the server. The default value is 8192 bytes. The meaning of some configuration variables (such as ) is influenced by block_size. See for information. data_checksums (boolean) data_checksums configuration parameter Reports whether data checksums are enabled for this cluster. See for more information. debug_assertions (boolean) debug_assertions configuration parameter Reports whether PostgreSQL has been built with assertions enabled. That is the case if the macro USE_ASSERT_CHECKING is defined when PostgreSQL is built (accomplished e.g. by the configure option --enable-cassert). By default PostgreSQL is built without assertions. integer_datetimes (boolean) integer_datetimes configuration parameter Reports whether PostgreSQL was built with support for 64-bit-integer dates and times. This can be disabled by configuring with --disable-integer-datetimes when building PostgreSQL. The default value is on. lc_collate (string) lc_collate configuration parameter Reports the locale in which sorting of textual data is done. See for more information. This value is determined when a database is created. lc_ctype (string) lc_ctype configuration parameter Reports the locale that determines character classifications. See for more information. This value is determined when a database is created. Ordinarily this will be the same as lc_collate, but for special applications it might be set differently. max_function_args (integer) max_function_args configuration parameter Reports the maximum number of function arguments. It is determined by the value of FUNC_MAX_ARGS when building the server. The default value is 100 arguments. max_identifier_length (integer) max_identifier_length configuration parameter Reports the maximum identifier length. It is determined as one less than the value of NAMEDATALEN when building the server. The default value of NAMEDATALEN is 64; therefore the default max_identifier_length is 63 bytes, which can be less than 63 characters when using multibyte encodings. max_index_keys (integer) max_index_keys configuration parameter Reports the maximum number of index keys. It is determined by the value of INDEX_MAX_KEYS when building the server. The default value is 32 keys. segment_size (integer) segment_size configuration parameter Reports the number of blocks (pages) that can be stored within a file segment. It is determined by the value of RELSEG_SIZE when building the server. The maximum size of a segment file in bytes is equal to segment_size multiplied by block_size; by default this is 1GB. server_encoding (string) server_encoding configuration parameter character set Reports the database encoding (character set). It is determined when the database is created. Ordinarily, clients need only be concerned with the value of . server_version (string) server_version configuration parameter Reports the version number of the server. It is determined by the value of PG_VERSION when building the server. server_version_num (integer) server_version_num configuration parameter Reports the version number of the server as an integer. It is determined by the value of PG_VERSION_NUM when building the server. wal_block_size (integer) wal_block_size configuration parameter Reports the size of a WAL disk block. It is determined by the value of XLOG_BLCKSZ when building the server. The default value is 8192 bytes. wal_segment_size (integer) wal_segment_size configuration parameter Reports the number of blocks (pages) in a WAL segment file. The total size of a WAL segment file in bytes is equal to wal_segment_size multiplied by wal_block_size; by default this is 16MB. See for more information. This feature was designed to allow parameters not normally known to PostgreSQL to be added by add-on modules (such as procedural languages). This allows extension modules to be configured in the standard ways. Custom options have two-part names: an extension name, then a dot, then the parameter name proper, much like qualified names in SQL. An example is plpgsql.variable_conflict. Because custom options may need to be set in processes that have not loaded the relevant extension module, PostgreSQL will accept a setting for any two-part parameter name. Such variables are treated as placeholders and have no function until the module that defines them is loaded. When an extension module is loaded, it will add its variable definitions, convert any placeholder values according to those definitions, and issue warnings for any unrecognized placeholders that begin with its extension name. The following parameters are intended for work on the PostgreSQL source code, and in some cases to assist with recovery of severely damaged databases. There should be no reason to use them on a production database. As such, they have been excluded from the sample postgresql.conf file. Note that many of these parameters require special source compilation flags to work at all. allow_system_table_mods (boolean) allow_system_table_mods configuration parameter Allows modification of the structure of system tables. This is used by initdb. This parameter can only be set at server start. ignore_system_indexes (boolean) ignore_system_indexes configuration parameter Ignore system indexes when reading system tables (but still update the indexes when modifying the tables). This is useful when recovering from damaged system indexes. This parameter cannot be changed after session start. post_auth_delay (integer) post_auth_delay configuration parameter If nonzero, a delay of this many seconds occurs when a new server process is started, after it conducts the authentication procedure. This is intended to give developers an opportunity to attach to the server process with a debugger. This parameter cannot be changed after session start. pre_auth_delay (integer) pre_auth_delay configuration parameter If nonzero, a delay of this many seconds occurs just after a new server process is forked, before it conducts the authentication procedure. This is intended to give developers an opportunity to attach to the server process with a debugger to trace down misbehavior in authentication. This parameter can only be set in the postgresql.conf file or on the server command line. trace_notify (boolean) trace_notify configuration parameter Generates a great amount of debugging output for the LISTEN and NOTIFY commands. or must be DEBUG1 or lower to send this output to the client or server logs, respectively. trace_recovery_messages (enum) trace_recovery_messages configuration parameter Enables logging of recovery-related debugging output that otherwise would not be logged. This parameter allows the user to override the normal setting of , but only for specific messages. This is intended for use in debugging Hot Standby. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, and LOG. The default, LOG, does not affect logging decisions at all. The other values cause recovery-related debug messages of that priority or higher to be logged as though they had LOG priority; for common settings of log_min_messages this results in unconditionally sending them to the server log. This parameter can only be set in the postgresql.conf file or on the server command line. trace_sort (boolean) trace_sort configuration parameter If on, emit information about resource usage during sort operations. This parameter is only available if the TRACE_SORT macro was defined when PostgreSQL was compiled. (However, TRACE_SORT is currently defined by default.) trace_locks (boolean) trace_locks configuration parameter If on, emit information about lock usage. Information dumped includes the type of lock operation, the type of lock and the unique identifier of the object being locked or unlocked. Also included are bit masks for the lock types already granted on this object as well as for the lock types awaited on this object. For each lock type a count of the number of granted locks and waiting locks is also dumped as well as the totals. An example of the log file output is shown here: LOG: LockAcquire: new: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0 wait(0) type(AccessShareLock) LOG: GrantLock: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(2) req(1,0,0,0,0,0,0)=1 grant(1,0,0,0,0,0,0)=1 wait(0) type(AccessShareLock) LOG: UnGrantLock: updated: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0 wait(0) type(AccessShareLock) LOG: CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1) grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0 wait(0) type(INVALID) Details of the structure being dumped may be found in src/include/storage/lock.h. This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled. trace_lwlocks (boolean) trace_lwlocks configuration parameter If on, emit information about lightweight lock usage. Lightweight locks are intended primarily to provide mutual exclusion of access to shared-memory data structures. This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled. trace_userlocks (boolean) trace_userlocks configuration parameter If on, emit information about user lock usage. Output is the same as for trace_locks, only for advisory locks. This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled. trace_lock_oidmin (integer) trace_lock_oidmin configuration parameter If set, do not trace locks for tables below this OID. (use to avoid output on system tables) This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled. trace_lock_table (integer) trace_lock_table configuration parameter Unconditionally trace locks on this table (OID). This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled. debug_deadlocks (boolean) debug_deadlocks configuration parameter If set, dumps information about all current locks when a deadlock timeout occurs. This parameter is only available if the LOCK_DEBUG macro was defined when PostgreSQL was compiled. log_btree_build_stats (boolean) log_btree_build_stats configuration parameter If set, logs system resource usage statistics (memory and CPU) on various B-tree operations. This parameter is only available if the BTREE_BUILD_STATS macro was defined when PostgreSQL was compiled. wal_debug (boolean) wal_debug configuration parameter If on, emit WAL-related debugging output. This parameter is only available if the WAL_DEBUG macro was defined when PostgreSQL was compiled. ignore_checksum_failure (boolean) ignore_checksum_failure configuration parameter Only has effect if are enabled. Detection of a checksum failure during a read normally causes PostgreSQL to report an error, aborting the current transaction. Setting ignore_checksum_failure to on causes the system to ignore the failure (but still report a warning), and continue processing. This behavior may cause crashes, propagate or hide corruption, or other serious problems. However, it may allow you to get past the error and retrieve undamaged tuples that might still be present in the table if the block header is still sane. If the header is corrupt an error will be reported even if this option is enabled. The default setting is off, and it can only be changed by a superuser. zero_damaged_pages (boolean) zero_damaged_pages configuration parameter Detection of a damaged page header normally causes PostgreSQL to report an error, aborting the current transaction. Setting zero_damaged_pages to on causes the system to instead report a warning, zero out the damaged page in memory, and continue processing. This behavior will destroy data, namely all the rows on the damaged page. However, it does allow you to get past the error and retrieve rows from any undamaged pages that might be present in the table. It is useful for recovering data if corruption has occurred due to a hardware or software error. You should generally not set this on until you have given up hope of recovering data from the damaged pages of a table. Zeroed-out pages are not forced to disk so it is recommended to recreate the table or the index before turning this parameter off again. The default setting is off, and it can only be changed by a superuser. For convenience there are also single letter command-line option switches available for some parameters. They are described in . Some of these options exist for historical reasons, and their presence as a single-letter option does not necessarily indicate an endorsement to use the option heavily. Short Option Equivalent -B x shared_buffers = x -d x log_min_messages = DEBUGx -e datestyle = euro -fb, -fh, -fi, -fm, -fn, -fo, -fs, -ft enable_bitmapscan = off, enable_hashjoin = off, enable_indexscan = off, enable_mergejoin = off, enable_nestloop = off, enable_indexonlyscan = off, enable_seqscan = off, enable_tidscan = off -F fsync = off -h x listen_addresses = x -i listen_addresses = '*' -k x unix_socket_directories = x -l ssl = on -N x max_connections = x -O allow_system_table_mods = on -p x port = x -P ignore_system_indexes = on -s log_statement_stats = on -S x work_mem = x -tpa, -tpl, -te log_parser_stats = on, log_planner_stats = on, log_executor_stats = on -W x post_auth_delay = x