This chapter discusses how to set up and run the database server and the interactions with the operating system. postgres user As with any other server daemon that is connected to the world at large, it is advisable to run Postgres under a separate user account. This user account should only own the data itself that is being managed by the server, and should not be shared with other daemons. (Thus, using the user nobody is a bad idea.) It is not advisable to install the executables as owned by this user account because that runs the risk of user-defined functions gone astray or any other exploits compromising the executable programs. To add a user account to your system, look for a command useradd or adduser. The user name postgres is often used but by no means required. database cluster data area database cluster Before you can do anything, you must initialize a database storage area on disk. We call this a database cluster. (SQL speaks of a catalog cluster instead.) A database cluster is a collection of databases that will be accessible through a single instance of a running database server. After initialization, a database cluster will contain one database named template1. As the name suggests, this will be used as a template for any subsequently created database; it should not be used for actual work. In file system terms, a database cluster will be a single directory under which all data will be stored. We call this the data directory or data area. It is completely up to you where you choose to store your data, there is no default, although locations such as /usr/local/pgsql/data or /var/lib/pgsql/data are popular. To initialize a database cluster, use the command initdb, which is installed with PostgreSQL. The desired file system location of your database system is indicated by the -D option, for example > initdb -D /usr/local/pgsql/data Note that you must execute this command while being logged in to the Postgres user account, which is described in the previous section. PGDATA As an alternative to the -D option, you can set the environment variable PGDATA. initdb will attempt to create the directory you specify if it does not already exist. It is likely that it won't have the permission to do so (if you followed our advice and created an unprivileged account). In that case you should create the directory yourself (as root) and transfer ownership of it to the Postgres user account. Here is how this might work: root# mkdir /usr/local/pgsql/data root# chown postgres /usr/local/pgsql/data root# su postgres postgres> initdb -D /usr/local/pgsql/data initdb will refuse to run if the data directory looks like it belongs to an already initialized installation. Because the data directory contains all the data stored in the database it is essential that it be well secured from unauthorized access. initdb therefore revokes access permissions from everyone but the Postgres user account. One surprise you might encounter while running initdb is a notice similar to this one: NOTICE: Initializing database with en_US collation order. This locale setting will prevent use of index optimization for LIKE and regexp searches. If you are concerned about speed of such queries, you may wish to set LC_COLLATE to "C" and re-initdb. For more information see the Administrator's Guide. This notice is intended to warn you that the currently selected locale will cause indexes to be sorted in an order that prevents them from being used for LIKE and regular-expression searches. If you need good performance of such searches, you should set your current locale to "C" and re-run initdb. On most systems, setting the current locale is done by changing the value of the environment variable LC_ALL or LANG. The sort order used within a particular database cluster is set by initdb and cannot be changed later, short of dumping all data, re-initdb, reload data. So it's important to make this choice correctly now. postmaster Before anyone can access the database you must start the database server. The database server is called postmaster. The postmaster must know where to find the data it is supposed to work on. This is done with the -D option. Thus, the simplest way to start the server is, for example, > postmaster -D /usr/local/pgsql/data which will leave the server running in the foreground. This must again be done while logged in to the Postgres user account. Without a -D, the server will try to use the data directory in the environment variable PGDATA; if neither of these works it will fail. To start the postmaster in the background, use the usual shell syntax: > postmaster -D /usr/local/pgsql/data > logfile 2>&1 & It is an extremely good idea to keep the server output around somewhere, as indicated here. It will help both for auditing purposes and to diagnose problems. TCP/IP The postmaster also takes a number of other command line options. For more information see the reference page and below under runtime configuration. In particular, in order for the postmaster to accept TCP/IP connections (rather than just Unix domain socket ones), you must also specify the -i option. pg_ctl This shell syntax can get tedious quickly. Therefore the shell script wrapper pg_ctl is provided that encapsulates some of the tasks. E.g., pg_ctl start -l logfile will start the server in the background and put the output into the named log file. The -D option has the same meaning as when invoking postmaster directly. pg_ctl also implements a symmetric stop operation. Normally, you will want to start the database server when the computer boots up. This is not required; the PostgreSQL server can be run successfully from non-privileged accounts without root intervention. Different systems have different conventions for starting up daemons at boot time, so you are advised to familiarize yourself with them. Many systems have a file /etc/rc.local or /etc/rc.d/rc.local which is almost certainly no bad place to put such a command. Whatever you do, the server must be run by the Postgres user account and not by root or any other user. Therefore you probably always want to form your command lines along the lines of su -c '...' postgres, for example: su -c 'pg_ctl start -D /usr/local/pgsql/data -l serverlog' postgres Here are a few more operating system specific suggestions. (Always replace the proper installation directory and the user name you chose.) For FreeBSD, take a look at the file contrib/start-scripts/freebsd in the PostgreSQL source distribution. On OpenBSD, add the following lines to the file /etc/rc.local: if [ -x /usr/local/pgsql/bin/pg_ctl -a -x /usr/local/pgsql/bin/postmaster ]; then su - -c '/usr/local/pgsql/bin/pg_ctl start -l /var/postgresql/log -s' postgres echo -n ' postgresql' fi On Linux systems either add /usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data to /etc/rc.d/rc.local or look into the file contrib/start-scripts/linux in the PostgreSQL source distribution to integrate the start and shutdown into the run level system. On NetBSD, either use the FreeBSD or Linux start scripts, depending on preference, as an example and place the file at /usr/local/etc/rc.d/postgresql. On Solaris, create a file called /etc/init.d/postgresql to contain the following single line: su - postgres -c "/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data" Then, create a symlink to it in /etc/rc3.d as S99postgresql. While the postmaster is running, its PID is in the file postmaster.pid in the data directory. This is used as an interlock against multiple postmasters running in the same data directory, and can also be used for shutting down the postmaster. There are several common reasons for the postmaster to fail to start up. Check the postmaster's log file, or start it by hand (without redirecting standard output or standard error) to see what complaint messages appear. Some of the possible error messages are reasonably self-explanatory, but here are some that are not. FATAL: StreamServerPort: bind() failed: Address already in use Is another postmaster already running on that port? This usually means just what it suggests: you accidentally started a second postmaster on the same port where one is already running. However, if the kernel error message is not Address already in use or some variant of that wording, there may be a different problem. For example, trying to start a postmaster on a reserved port number may draw something like > postmaster -i -p 666 FATAL: StreamServerPort: bind() failed: Permission denied Is another postmaster already running on that port? A message like IpcMemoryCreate: shmget(key=5440001, size=83918612, 01600) failed: Invalid argument FATAL 1: ShmemCreate: cannot create region probably means that your kernel's limit on the size of shared memory areas is smaller than the buffer area that Postgres is trying to create (83918612 bytes in this example). Or it could mean that you don't have System-V-style shared memory support configured into your kernel at all. As a temporary workaround, you can try starting the postmaster with a smaller-than-normal number of buffers (-B switch). You will eventually want to reconfigure your kernel to increase the allowed shared memory size, however. You may see this message when trying to start multiple postmasters on the same machine, if their total space requests exceed the kernel limit. An error like IpcSemaphoreCreate: semget(key=5440026, num=16, 01600) failed: No space left on device does not mean that you've run out of disk space; it means that your kernel's limit on the number of System V semaphores is smaller than the number Postgres wants to create. As above, you may be able to work around the problem by starting the postmaster with a reduced number of backend processes (-N switch), but you'll eventually want to increase the kernel limit. If you get an illegal system call error, then it is likely that shared memory or semaphores are not supported at all in your kernel. In that case your only option is to re-configure the kernel to turn on these features. Details about configuring System V IPC facilities are given in . Although the possible error conditions on the client side are both virtually infinite and application dependent, a few of them might be directly related to how the server was started up. Conditions other than those shown below should be documented with the respective client application. psql: could not connect to server: Connection refused Is the server running on host server.joe.com and accepting TCP/IP connections on port 5432? This is the generic I couldn't find a server to talk to failure. It looks like the above when TCP/IP communication is attempted. A common mistake is to forget the -i to the postmaster to allow TCP/IP connections. Alternatively, you'll get this when attempting Unix-socket communication to a local postmaster: psql: could not connect to server: Connection refused Is the server running locally and accepting connections on Unix domain socket "/tmp/.s.PGSQL.5432"? The last line is useful in verifying that the client is trying to connect where it is supposed to. If there is in fact no postmaster running there, the kernel error message will typically be either Connection refused or No such file or directory, as illustrated. (It is particularly important to realize that Connection refused in this context does not mean that the postmaster got your connection request and rejected it -- that case will produce a different message, as shown in .) Other error messages such as Connection timed out may indicate more fundamental problems, like lack of network connectivity. configuration server There are a lot of configuration parameters that affect the behavior of the database system in some way or other. Here we describe how to set them and the following subsections will discuss each of them. All parameter names are case-insensitive. Every parameter takes a value of one of the four types boolean, integer, floating point, string as described below. Boolean values are ON, OFF, TRUE, FALSE, YES, NO, 1, 0 (case-insensitive) or any non-ambiguous prefix of these. One way to set these options is to create a file postgresql.conf in the data directory (e.g., /usr/local/pgsql/data). An example of what this file could look like is: # This is a comment log_connections = yes syslog = 2 As you see, options are one per line. The equal sign between name and value is optional. White space is insignificant, blank lines are ignored. Hash marks (#) introduce comments anywhere. SIGHUP The configuration file is reread whenever the postmaster receives a SIGHUP signal. This signal is also propagated to all running backend processes, so that running sessions get the new default. Alternatively, you can send the signal to only one backend process directly. A second way to set these configuration parameters is to give them as a command line option to the postmaster, such as postmaster -c log_connections=yes -c syslog=2 which would have the same effect as the previous example. Command-line options override any conflicting settings in postgresql.conf. Occasionally it is also useful to give a command line option to one particular backend session only. The environment variable PGOPTIONS can be used for this purpose on the client side: env PGOPTIONS='-c geqo=off' psql (This works for any client application, not just psql.) Note that this won't work for options that are necessarily fixed once the server is started, such as the port number. Finally, some options can be changed in individual SQL sessions with the SET command, for example => SET ENABLE_SEQSCAN TO OFF; See the SQL command language reference for details on the syntax. CPU_INDEX_TUPLE_COST (floating point) Sets the query optimizer's estimate of the cost of processing each index tuple during an index scan. This is measured as a fraction of the cost of a sequential page fetch. CPU_OPERATOR_COST (floating point) Sets the optimizer's estimate of the cost of processing each operator in a WHERE clause. This is measured as a fraction of the cost of a sequential page fetch. CPU_TUPLE_COST (floating point) Sets the query optimizer's estimate of the cost of processing each tuple during a query. This is measured as a fraction of the cost of a sequential page fetch. EFFECTIVE_CACHE_SIZE (floating point) Sets the optimizer's assumption about the effective size of the disk cache (that is, the portion of the kernel's disk cache that will be used for Postgres data files). This is measured in disk pages, which are normally 8kB apiece. ENABLE_HASHJOIN (boolean) Enables or disables the query planner's use of hash-join plan types. The default is on. This is mostly useful to debug the query planner. index scan ENABLE_INDEXSCAN (boolean) Enables or disables the query planner's use of index scan plan types. The default is on. This is mostly useful to debug the query planner. ENABLE_MERGEJOIN (boolean) Enables or disables the query planner's use of merge-join plan types. The default is on. This is mostly useful to debug the query planner. ENABLE_NESTLOOP (boolean) Enables or disables the query planner's use of nested-loop join plans. It's not possible to suppress nested-loop joins entirely, but turning this variable off discourages the planner from using one if there is any other method available. The default is on. This is mostly useful to debug the query planner. sequential scan ENABLE_SEQSCAN (boolean) Enables or disables the query planner's use of sequential scan plan types. It's not possible to suppress sequential scans entirely, but turning this variable off discourages the planner from using one if there is any other method available. The default is on. This is mostly useful to debug the query planner. ENABLE_SORT (boolean) Enables or disables the query planner's use of explicit sort steps. It's not possible to suppress explicit sorts entirely, but turning this variable off discourages the planner from using one if there is any other method available. The default is on. This is mostly useful to debug the query planner. ENABLE_TIDSCAN (boolean) Enables or disables the query planner's use of TID scan plan types. The default is on. This is mostly useful to debug the query planner. genetic query optimization GEQO genetic query optimization GEQO (boolean) Enables or disables genetic query optimization, which is an algorithm that attempts to do query planning without exhaustive search. This is on by default. See also the various other GEQO_ settings. GEQO_EFFORT (integer) GEQO_GENERATIONS (integer) GEQO_POOL_SIZE (integer) GEQO_RANDOM_SEED (integer) GEQO_SELECTION_BIAS (floating point) Various tuning parameters for the genetic query optimization algorithm: The pool size is the number of individuals in one population. Valid values are between 128 and 1024. If it is set to 0 (the default) a pool size of 2^(QS+1), where QS is the number of FROM items in the query, is taken. The effort is used to calculate a default for generations. Valid values are between 1 and 80, 40 being the default. Generations specifies the number of iterations in the algorithm. The number must be a positive integer. If 0 is specified then Effort * Log2(PoolSize) is used. The run time of the algorithm is roughly proportional to the sum of pool size and generations. The selection bias is the selective pressure within the population. Values can be from 1.50 to 2.00; the latter is the default. The random seed can be set to get reproduceable results from the algorithm. If it is set to -1 then the algorithm behaves non-deterministically. GEQO_THRESHOLD (integer) Use genetic query optimization to plan queries with at least this many FROM items involved. (Note that a JOIN construct counts as only one FROM item.) The default is 11. For simpler queries it is usually best to use the deterministic, exhaustive planner. KSQO (boolean) The Key Set Query Optimizer (KSQO) causes the query planner to convert queries whose WHERE clause contains many OR'ed AND clauses (such as WHERE (a=1 AND b=2) OR (a=2 AND b=3) ...) into a UNION query. This method can be faster than the default implementation, but it doesn't necessarily give exactly the same results, since UNION implicitly adds a SELECT DISTINCT clause to eliminate identical output rows. KSQO is commonly used when working with products like Microsoft Access, which tend to generate queries of this form. The KSQO algorithm used to be absolutely essential for queries with many OR'ed AND clauses, but in Postgres 7.0 and later the standard planner handles these queries fairly successfully. Hence the default is OFF. RANDOM_PAGE_COST (floating point) Sets the query optimizer's estimate of the cost of a nonsequentially fetched disk page. This is measured as a multiple of the cost of a sequential page fetch. Unfortunately, there is no well-defined method of determining ideal values for the family of COST variables that were just described. You are encouraged to experiment and share your findings. DEBUG_ASSERTIONS (boolean) Turns on various assertion checks. This is a debugging aid. If you are experiencing strange problems or crashes you might want to turn this on, as it might expose programming mistakes. To use this option, the macro USE_ASSERT_CHECKING must be defined when Postgres is built (see the configure option --enable-cassert). Note that DEBUG_ASSERTIONS defaults to ON if Postgres has been built this way. DEBUG_LEVEL (integer) The higher this value is set, the more debugging output of various sorts is generated in the server log during operation. This option is 0 by default, which means no debugging output. Values up to about 4 currently make sense. DEBUG_PRINT_QUERY (boolean) DEBUG_PRINT_PARSE (boolean) DEBUG_PRINT_REWRITTEN (boolean) DEBUG_PRINT_PLAN (boolean) DEBUG_PRETTY_PRINT (boolean) These flags enable various debugging output to be sent to the server log. For each executed query, prints either the query text, the resulting parse tree, the query rewriter output, or the execution plan. DEBUG_PRETTY_PRINT indents these displays to produce a more readable but much longer output format. Setting DEBUG_LEVEL above zero implicitly turns on some of these flags. HOSTNAME_LOOKUP (boolean) By default, connection logs only show the IP address of the connecting host. If you want it to show the host name you can turn this on, but depending on your host name resolution setup it might impose a non-negligible performance penalty. This option can only be set at server start. LOG_CONNECTIONS (boolean) Prints a line informing about each successful connection to the server log. This is off by default, although it is probably very useful. This option can only be set at server start. LOG_PID (boolean) Prefixes each server log message with the process id of the backend process. This is useful to sort out which messages pertain to which connection. The default is off. LOG_TIMESTAMP (boolean) Prefixes each server log message with a timestamp. The default is off. SHOW_QUERY_STATS (boolean) SHOW_PARSER_STATS (boolean) SHOW_PLANNER_STATS (boolean) SHOW_EXECUTOR_STATS (boolean) For each query, write performance statistics of the respective module to the server log. This is a crude profiling instrument. SHOW_SOURCE_PORT (boolean) Shows the outgoing port number of the connecting host in the connection log messages. You could trace back the port number to find out what user initiated the connection. Other than that it's pretty useless and therefore off by default. This option can only be set at server start. SYSLOG (integer) Postgres allows the use of syslog for logging. If this option is set to 1, messages go both to syslog and the standard output. A setting of 2 sends output only to syslog. (Some messages will still go to the standard output/error.) The default is 0, which means syslog is off. This option must be set at server start. To use syslog, the build of Postgres must be configured with the --enable-syslog option. SYSLOG_FACILITY (string) This option determines the syslog facility to be used when syslog is enabled. You may choose from LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7; the default is LOCAL0. See also the documentation of your system's syslog. SYSLOG_IDENT (string) If logging to syslog is enabled, this option determines the program name used to identify PostgreSQL messages in syslog log messages. The default is postgres. TRACE_NOTIFY (boolean) Generates a great amount of debugging output for the LISTEN and NOTIFY commands. AUSTRALIAN_TIMEZONES (bool) If set to true, CST, EST, and SAT are interpreted as Australian timezones rather than as North American Central/Eastern Timezones and Saturday. The default is false. deadlock timeout DEADLOCK_TIMEOUT (integer) This is the amount of time, in milliseconds, to wait on a lock before checking to see if there is a deadlock condition or not. The check for deadlock is relatively slow, so we don't want to run it every time we wait for a lock. We (optimistically?) assume that deadlocks are not common in production applications, and just wait on the lock for awhile before starting to ask questions about whether it can ever get unlocked. Increasing this value reduces the amount of time wasted in needless deadlock checks, but slows down reporting of real deadlock errors. The default is 1000 (i.e., one second), 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 the lock will be released before the waiter decides to check for deadlock. This option can only be set at server start. transaction isolation level DEFAUL_TRANSACTION_ISOLATION (string) Each SQL transaction has an isolation level, which can be either read committed or serializable. This parameter controls what the isolation level of each new transaction is set to. The default is read committed. Consult the PostgreSQL User's Guide and the command SET TRANSACTION for more information. DYNAMIC_LIBRARY_PATH (string) If a dynamically loadable module needs to be opened and the specified name does not have a directory component (i.e., the name does not contain a slash), the system will search this path for the specified file. (The name that is used is the name specified in the CREATE FUNCTION or LOAD command.) The value for dynamic_library_path has to be a colon-separated list of absolute directory names. If a directory name starts with the special value $libdir, the compiled-in PostgreSQL library directory, which is where the modules provided by the PostgreSQL distribution are installed, is substituted. An example value: dynamic_library_path = '/usr/local/lib:/home/my_project/lib:$libdir:$libdir/contrib' The default value for this parameter is $libdir. If the value is set to the empty string, the automatic path search is turned off. This parameter can be changed at run time by superusers, but note that a setting done that way will only persist till 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. fsync FSYNC (boolean) If this option is on, the Postgres backend will use the fsync() system call in several places to make sure that updates are physically written to disk and do not hang around in the kernel buffer cache. This increases the chance by a large amount that a database installation will still be usable after an operating system or hardware crash. (Crashes of the database server itself do not affect this consideration.) However, this operation slows down Postgres, because at all those points it has to block and wait for the operating system to flush the buffers. Without fsync, the operating system is allowed to do its best in buffering, sorting, and delaying writes, which can make for a considerable perfomance increase. However, if the system crashes, the results of the last few committed transactions may be lost in part or whole; in the worst case, unrecoverable data corruption may occur. This option is the subject of an eternal debate in the Postgres user and developer communities. Some always leave it off, some turn it off only for bulk loads, where there is a clear restart point if something goes wrong, some leave it on just to be on the safe side. Because it is the safe side, on is also the default. If you trust your operating system, your hardware, and your utility company (or better your UPS), you might want to disable fsync. It should be noted that the performance penalty from doing fsyncs is considerably less in Postgres version 7.1 than it was in prior releases. If you previously suppressed fsyncs because of performance problems, you may wish to reconsider your choice. This option can only be set at server start or in the postgresql.conf file. KRB_SERVER_KEYFILE (string) Sets the location of the Kerberos server key file. See for details. MAX_CONNECTIONS (integer) Determines how many concurrent connections the database server will allow. The default is 32. There is also a compiled-in hard upper limit on this value, which is typically 1024 (both numbers can be altered when compiling the server). This parameter can only be set at server start. MAX_EXPR_DEPTH (integer) Sets the maximum expression nesting depth that the parser will accept. The default value is high enough for any normal query, but you can raise it if you need to. (But if you raise it too high, you run the risk of backend crashes due to stack overflow.) MAX_FSM_RELATIONS (integer) Sets the maximum number of relations (tables) for which free space will be tracked in the shared free-space map. The default is 100. This option can only be set at server start. MAX_FSM_PAGES (integer) Sets the maximum number of disk pages for which free space will be tracked in the shared free-space map. The default is 10000. This option can only be set at server start. MAX_LOCKS_PER_XACT (integer) The shared lock table is sized on the assumption that at most max_locks_per_xact * max_connections distinct objects will need to be locked at any one time. The default, 64, has historically proven sufficient, but you might need to raise this value if you have clients that touch many different tables in a single transaction. This option can only be set at server start. PORT (integer) The TCP port the server listens on; 5432 by default. This option can only be set at server start. SHARED_BUFFERS (integer) Sets the number of shared memory buffers the database server will use. The default is 64. Each buffer is typically 8192 bytes. This option can only be set at server start. SILENT_MODE (bool) Runs postmaster silently. If this option is set, postmaster will automatically run in background and any controlling ttys are disassociated, thus no messages are written to stdout or stderr (same effect as postmaster's -S option). Unless some logging system such as syslog is enabled, using this option is discouraged since it makes it impossible to see error messages. SORT_MEM (integer) Specifies the amount of memory to be used by internal sorts and hashes before resorting to temporary disk files. The value is specified in kilobytes, and defaults to 512 kilobytes. Note that for a complex query, several sorts and/or hashes might be running in parallel, and each one will be allowed to use as much memory as this value specifies before it starts to put data into temporary files. And don't forget that each running backend could be doing one or more sorts. So the total memory space needed could be many times the value of SORT_MEM. SQL_INHERITANCE (bool) This controls the inheritance semantics, in particular whether subtables are included into the consideration of various commands by default. This was not the case in versions prior to 7.1. If you need the old behaviour you can set this variable to off, but in the long run you are encouraged to change your applications to use the ONLY keyword to exclude subtables. See the SQL language reference and the User's Guide for more information about inheritance. SSL SSL (boolean) Enables SSL connections. Please read before using this. The default is off. TCPIP_SOCKET (boolean) If this is true, then the server will accept TCP/IP connections. Otherwise only local Unix domain socket connections are accepted. It is off by default. This option can only be set at server start. UNIX_SOCKET_DIRECTORY (string) Specifies the directory of the Unix-domain socket on which the postmaster is to listen for connections from client applications. The default is normally /tmp, but can be changed at build time. UNIX_SOCKET_GROUP (string) Sets the group owner of the Unix domain socket. (The owning user of the socket is always the user that starts the postmaster.) In combination with the option UNIX_SOCKET_PERMISSIONS this can be used as an additional access control mechanism for this socket type. By default this is the empty string, which uses the default group for the current user. This option can only be set at server start. UNIX_SOCKET_PERMISSIONS (integer) Sets the access permissions of the Unix domain socket. Unix domain sockets use the usual Unix file system permission set. The option value is expected to be an numeric mode specification in the form 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 would be 0770 (only user and group, see also under UNIX_SOCKET_GROUP) and 0700 (only user). (Note that actually for a Unix socket, only write permission matters and there is no point in setting or revoking read or execute permissions.) This access control mechanism is independent from the one described in . This option can only be set at server start. VIRTUAL_HOST (string) Specifies the TCP/IP hostname or address on which the postmaster is to listen for connections from client applications. Defaults to listening on all configured addresses (including localhost). See also for details on WAL tuning. CHECKPOINT_SEGMENTS (integer) Maximum distance between automatic WAL checkpoints, in logfile segments (each segment is normally 16 megabytes). This option can only be set at server start or in the postgresql.conf file. CHECKPOINT_TIMEOUT (integer) Maximum time between automatic WAL checkpoints, in seconds. This option can only be set at server start or in the postgresql.conf file. COMMIT_DELAY (integer) Time delay between writing a commit record to the WAL buffer and flushing the buffer out to disk, in microseconds. A nonzero delay allows multiple transactions to be committed with only one fsync, if system load is high enough that additional transactions become ready to commit within the given interval. But the delay is just wasted time if no other transactions become ready to commit. Therefore, the delay is only performed if at least COMMIT_SIBLINGS other transactions are active at the instant that a backend has written its commit record. COMMIT_SIBLINGS (integer) 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. WAL_BUFFERS (integer) Number of disk-page buffers in shared memory for WAL log. This option can only be set at server start. WAL_DEBUG (integer) If non-zero, turn on WAL-related debugging output on standard error. WAL_FILES (integer) Number of log files that are created in advance at checkpoint time. This option can only be set at server start or in the postgresql.conf file. WAL_SYNC_METHOD (string) Method used for forcing WAL updates out to disk. Possible values are FSYNC (call fsync() at each commit), FDATASYNC (call fdatasync() at each commit), OPEN_SYNC (write WAL files with open() option O_SYNC), or OPEN_DATASYNC (write WAL files with open() option O_DSYNC). Not all of these choices are available on all platforms. This option can only be set at server start or in the postgresql.conf file. For convenience there are also single letter option switches available for many parameters. They are described in the following table. Short option Equivalent Remark -B x shared_buffers = x -d x debug_level = x -F fsync = off -h x virtual_host = x -i tcpip_socket = on -k x unix_socket_directory = x -l ssl = on -N x max_connections = x -p x port = x -fi, -fh, -fm, -fn, -fs, -ft enable_indexscan=off, enable_hashjoin=off, enable_mergejoin=off, enable_nestloop=off, enable_seqscan=off, enable_tidscan=off * -S x sort_mem = x * -s show_query_stats = on * -tpa, -tpl, -te show_parser_stats=on, show_planner_stats=on, show_executor_stats=on * For historical reasons, options marked * must be passed to the individual backend process via the -o postmaster option, for example, > postmaster -o '-S 1024 -s' or via PGOPTIONS from the client side, as explained above. A large Postgres installation can quickly hit various operating system resource limits. (On some systems, the factory defaults are so low that you don't even need a really large installation.) If you have encountered this kind of problem then keep reading. shared memory semaphores Shared memory and semaphores are collectively referred to as System V IPC (together with message queues, which are not relevant for Postgres). Almost all modern operating systems provide these features, but not all of them have them turned on or sufficiently sized by default, especially systems with BSD heritage. (For the QNX and BeOS ports, Postgres provides its own replacement implementation of these facilities.) The complete lack of these facilities is usually manifested by an Illegal system call error upon postmaster start. In that case there's nothing left to do but to reconfigure your kernel -- Postgres won't work without them. When Postgres exceeds one of the various hard limits of the IPC resources then the postmaster will refuse to start up and should leave a marginally instructive error message about which problem was encountered and what needs to be done about it. (See also .) The relevant kernel parameters are named consistently across different systems; gives an overview. The methods to set them, however, vary; suggestions for some platforms are given below. Be warned that it is often necessary to reboot your machine at least, possibly even recompile the kernel, to change these settings. Unix-like operating systems enforce various kinds of resource limits that might interfere with the operation of your Postgres server. Of importance are especially the limits on the number of processes per user, the number of open files per process, and the amount of memory available to a process. Each of these have a hard and a soft limit. The soft limit is what actually counts but it can be changed by the user up to the hard limit. The hard limit can only be changed by the root user. The system call setrlimit is responsible for setting these parameters. The shell's built-in command ulimit (Bourne shells) or limit (csh) is used to control the resource limits from the command line. On BSD-derived systems the file /etc/login.conf controls what values the various resource limits are set to upon login. See login.conf 5 for details. The relevant parameters are maxproc, openfiles, and datasize. For example: default:\ ... :datasize-cur=256M:\ :maxproc-cur=256:\ :openfiles-cur=256:\ ... (-cur is the soft limit. Append -max to set the hard limit.) Kernels generally also have an implementation-dependent system-wide limit on some resources. On Linux /proc/sys/fs/file-max determines the maximum number of files that the kernel will allocate. It can be changed by writing a different number into the file or by adding an assignment in /etc/sysctl.conf. The maximum limit of files per process is fixed at the time the kernel is compiled; see /usr/src/linux/Documentation/proc.txt for more information. The Postgres server uses one process per connection so you should provide for at least as many processes as allowed connections, in addition to what you need for the rest of your system. This is usually not a problem but if you run several servers on one machine things might get tight. The factory default limit on open files is often set to socially friendly values that allow many users to coexist on a machine without using an inappropriate fraction of the system resources. If you run many servers on a machine this is perhaps what you want, but on dedicated servers you may want to raise this limit. Depending on your needs, there are several ways to shut down the database server when your work is done. The differentiation is done by what signal you send to the server process. SIGTERM After receiving SIGTERM, the postmaster disallows new connections, but lets existing backends end their work normally. It shuts down only after all of the backends terminate by client request. This is the Smart Shutdown. SIGINT The postmaster disallows new connections and sends all existing backends SIGTERM, which will cause them to abort their current transactions and exit promptly. It then waits for the backends to exit and finally shuts down the data base. This is the Fast Shutdown. SIGQUIT This is the Immediate Shutdown which will cause the postmaster to send a SIGQUIT to all backends and exit immediately (without properly shutting down the database system). The backends likewise exit immediately upon receiving SIGQUIT. This will lead to recovery (by replaying the WAL log) upon next start-up. This is recommended only in emergencies. It is best not to use SIGKILL to shut down the postmaster. This will prevent the postmaster from releasing shared memory and semaphores, which you may then have to do by hand. The PID of the postmaster process can be found using the ps program, or from the file postmaster.pid in the data directory. So for example, to do a fast shutdown: > kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid` The program pg_ctl is a shell script that provides a more convenient interface for shutting down the postmaster. SSL PostgreSQL has native support for connections over SSL to encrypt client/server communications for increased security. This requires OpenSSL to be installed on both client and server systems and support enabled at build-time (see ). With SSL support compiled in, the PostgreSQL server can be started with the argument -l (ell) to enable SSL connections. When starting in SSL mode, the postmaster will look for the files server.key and server.crt in the data directory. These files should contain the server private key and certificate respectively. These files must be set up correctly before an SSL-enabled server can start. If the private key is protected with a passphrase, the postmaster will prompt for the passphrase and will not start until it has been entered. The postmaster will listen for both standard and SSL connections on the same TCP/IP port, and will negotiate with any connecting client whether or not to use SSL. See about how to force on the server side the use of SSL for certain connections. For details on how to create your server private key and certificate, refer to the OpenSSL documentation. A simple self-signed certificate can be used to get started for testing, but a certificate signed by a CA (either one of the global CAs or a local one) should be used in production so the client can verify the servers identity. To create a quick self-signed certificate, use the following OpenSSL command: openssl req -new -text -out cert.req Fill out the information that openssl asks for. Make sure that you enter the local host name as Common Name; the challenge password can be left blank. The script will generate a key that is passphrase protected; it will not accept a pass phrase that is less than four characters long. To remove the passphrase (as you must if you want automatic start-up of the postmaster), run the commands openssl rsa -in privkey.pem -out cert.pem Enter the old passphrase to unlock the existing key. Now do openssl req -x509 -in cert.req -text -key cert.pem -out cert.cert cp cert.pem $PGDATA/server.key cp cert.cert $PGDATA/server.crt to turn the certificate into a self-signed certificate and to copy the key and certificate to where the postmaster will look for them. ssh Idea taken from an email by Gene Selkov, Jr. (selkovjr@mcs.anl.gov) written on 1999-09-08 in response to a question from Eric Marsden. One can use ssh to encrypt the network connection between clients and a Postgres server. Done properly, this should lead to an adequately secure network connection. First make sure that an ssh server is running properly on the same machine as Postgres and that you can log in using ssh as some user. Then you can establish a secure tunnel with a command like this from the client machine: > ssh -L 3333:foo.com:5432 joe@foo.com The first number in the -L argument, 3333, is the port number of your end of the tunnel; it can be chosen freely. The second number, 5432, is the remote end of the tunnel -- the port number your backend is using. The name or the address in between the port numbers is the host with the database server you are going to connect to. In order to connect to the database server using this tunnel, you connect to port 3333 on the local machine: psql -h localhost -p 3333 template1 To the database server it will then look as though you are really user joe@foo.com and it will use whatever authentication procedure was set up for this user. In order for the tunnel setup to succeed you must be allowed to connect via ssh as joe@foo.com, just as if you had attempted to use ssh to set up a terminal session. Several other products exist that can provide secure tunnels using a procedure similar in concept to the one just described.