Server Configurationconfigurationof 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 set configuration parameters. The subsequent sections
discuss each parameter in detail.
Setting Parameters
All parameter names are case-insensitive. Every parameter takes a
value of one of four types: Boolean, integer, floating point,
or string. Boolean values may be written as ON,
OFF, TRUE,
FALSE, YES,
NO, 1, 0
(all case-insensitive) or any unambiguous prefix of these.
One way to set these parameters is to edit the file
postgresql.conf>postgresql.conf>>,
which is normally kept in the data directory. (initdb>
installs a default copy there.) An example of what this file might look
like is:
# This is a comment
log_connections = yes
log_destination = 'syslog'
search_path = '"$user", public'
One parameter is specified per line. The equal sign between name and
value is optional. Whitespace is insignificant and blank lines are
ignored. Hash marks (#) introduce comments
anywhere. 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.
SIGHUP
The configuration file is reread whenever the
postmaster process receives a
SIGHUP> signal (which is most easily sent by means
of pg_ctl reload>). The postmaster
also propagates this signal to all currently running server
processes so that existing sessions also get the new
value. 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.
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 log_destination='syslog'
Command-line options override any conflicting settings in
postgresql.conf. Note that this means you won't
be able to change the value on-the-fly by editing
postgresql.conf, so while the command-line
method may be convenient, it can cost you flexibility later.
Occasionally it is useful to give a command line option to
one particular 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 libpq>-based client application, not
just psql.) Note that this won't work for
parameters that are fixed when the server is started or that must be
specified in postgresql.conf.
Furthermore, it is possible to assign a set of parameter settings to
a user or a database. Whenever a session is started, the default
settings for the user and database involved are loaded. The
commands
and ,
respectively, are used to configure these settings. Per-database
settings override anything received from the
postmaster command-line or the configuration
file, and in turn are overridden by per-user settings; both are
overridden by per-session settings.
Some parameters can be changed in individual SQL
sessions with the
command, for example:
SET ENABLE_SEQSCAN TO OFF;
If SET> is allowed, it overrides all other sources of
values for the parameter. Some parameters cannot be changed via
SET: for example, if they control behavior that
cannot be changed without restarting the entire
PostgreSQL server. Also, some parameters can
be modified via SET or ALTER> by superusers,
but not by ordinary users.
The
command allows inspection of the current values of all parameters.
The virtual table pg_settings
(described in ) also allows
displaying and updating session run-time parameters. It is equivalent
to SHOW> and SET>, but can be more convenient
to use because it can be joined with other tables, or selected from using
any desired selection condition.
File Locations
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 postmaster 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
ident> authentication
(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
postmaster> 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 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 postmaster's
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 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
postmaster 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 or PGDATA.
When setting any of these parameters, a relative path will be interpreted
with respect to the directory in which the postmaster
is started.
Connections and AuthenticationConnection Settingslisten_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.
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 loopback> connections to be made.
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, but may be less
if your kernel settings will not support it (as determined
during initdb>). This parameter can only be
set at server start.
Increasing this parameter may cause PostgreSQL>
to request more System V> shared
memory or semaphores than your operating system's default configuration
allows. See for information on how to
adjust those parameters, if necessary.
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.
The default value is 2. The value must be less than the value of
max_connections. This parameter can only be
set at server start.
unix_socket_directory (string)unix_socket_directory> configuration parameter
Specifies the directory of the Unix-domain socket on which the
server is to listen for
connections from client applications. The default is normally
/tmp, but can be changed at build time.
This parameter can only be set at server start.
unix_socket_group (string)unix_socket_group> configuration parameter
Sets the owning group of the Unix-domain socket. (The owning
user of the socket 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 selects the default
group for the current user. This parameter can only be set at
server start.
unix_socket_permissions (integer)unix_socket_permissions> configuration parameter
Sets the access permissions of the Unix-domain socket. Unix-domain
sockets use the usual Unix file system permission set.
The parameter value is expected to be a 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 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 and 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.
bonjour_name (string)bonjour_name> configuration parameter
Specifies the Bonjour broadcast
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
On systems that support the TCP_KEEPIDLE socket option, specifies the
number of seconds between sending keepalives on an otherwise idle
connection. A value of 0 uses the system default. If TCP_KEEPIDLE is
not supported, this parameter must be 0. This parameter is ignored for
connections made via a Unix-domain socket.
tcp_keepalives_interval (integer)tcp_keepalives_interval> configuration parameter
On systems that support the TCP_KEEPINTVL socket option, specifies how
long, in seconds, to wait for a response to a keepalive before
retransmitting. A value of 0 uses the system default. If TCP_KEEPINTVL
is not supported, this parameter must be 0. This parameter is ignored
for connections made via a Unix-domain socket.
tcp_keepalives_count (integer)tcp_keepalives_count> configuration parameter
On systems that support the TCP_KEEPCNT socket option, specifies how
many keepalives may be lost before the connection is considered dead.
A value of 0 uses the system default. If TCP_KEEPCNT is not
supported, this parameter must be 0. This parameter is ignored
for connections made via a Unix-domain socket.
Security and Authenticationauthentication_timeout (integer)timeout>client authentication>client authentication>timeout 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 breaks the connection. This prevents
hung clients from occupying a connection indefinitely.
The default is 60.
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.
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 at server start.
krb_srvname (string)krb_srvname> configuration parameter
Sets the Kerberos service name. See
for details. This parameter can only be set at server start.
krb_server_hostname (string)krb_server_hostname> configuration parameter
Sets the host name part of the service principal.
This, combined with krb_srvname>, is used to generate
the complete service principal, that is
krb_srvname>/>krb_server_hostname>@>REALM.
If not set, the default is the server host name. See
for details. This parameter can only be set at server start.
krb_caseins_users (boolean)krb_caseins_users configuration parameter
Sets whether Kerberos user names should be treated case-insensitively.
The default is off> (case sensitive). This parameter
can only be set at server start.
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. The @> will be stripped off
before the user name is looked up by the server.
This feature is intended as a temporary measure until a
complete solution is found. At that time, this option will
be removed.
Resource ConsumptionMemoryshared_buffers (integer)shared_buffers> configuration parameter
Sets the number of shared memory buffers used by the database
server. The default is typically 4000, but may be less if your
kernel settings will not support it (as determined during
initdb>). Each buffer is 8192 bytes, unless a
different value of BLCKSZ was chosen when building
the server. This setting must be at least 16, as well as at
least twice the value of ;
however, settings significantly higher than the minimum are
usually needed for good performance. Values of a few thousand
are recommended for production installations. This parameter can
only be set at server start.
Increasing this parameter may cause PostgreSQL>
to request more System V> shared
memory than your operating system's default configuration
allows. See for information on how to
adjust those parameters, if necessary.
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 1000. The setting can
be changed within individual sessions, but only up until the
first use of temporary tables within a 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 a lot of 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 disables the prepared-transaction
feature.
The default is 5.
This parameter can only be set at server start.
If you are not using prepared transactions, this parameter may as
well be set to zero. If you are using them, you will probably
want max_prepared_transactions to be at least
as large as , to avoid unwanted
failures at the prepare step.
Increasing this parameter may cause PostgreSQL>
to request more System V> shared
memory than your operating system's default configuration
allows. See for information on how to
adjust those parameters, if necessary.
work_mem (integer)work_mem> configuration parameter
Specifies the amount of memory to be used by internal sort operations
and hash tables before switching to temporary disk files. The value is
specified in kilobytes, and defaults to 1024 kilobytes (1 MB).
Note that for a complex query, several sort or hash operations might be
running in parallel; each one will be allowed to use as much memory
as this value specifies before it starts to put data into temporary
files. Also, several running sessions could be doing such operations
concurrently. So 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 in maintenance
operations, such as VACUUM, CREATE
INDEX>, and ALTER TABLE ADD FOREIGN KEY>.
The value is specified in kilobytes, and defaults to 16384 kilobytes
(16 MB). Since only one of these operations can be executed at
a time by a database session, and an installation normally doesn't
have very many of them happening concurrently, it's safe to set this
value significantly larger than work_mem. Larger
settings may improve performance for vacuuming and for restoring
database dumps.
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. Setting the parameter higher than
the actual kernel limit will mean that a runaway recursive function
can crash an individual backend process. The default setting is
2048 KB (two megabytes), which is conservatively small and unlikely
to risk crashes. However, it may be too small to allow execution
of complex functions.
Only superusers can change this setting.
Free Space Mapfree space map
These parameters control the size of the shared free space
map>, which tracks the locations of unused space in the database.
An undersized free space map may cause the database to consume
increasing amounts of disk space over time, because free space that
is not in the map cannot be re-used; instead PostgreSQL>
will request more disk space from the operating system when it needs
to store new data.
The last few lines displayed by a database-wide VACUUM VERBOSE>
command can help in determining if the current settings are adequate.
A NOTICE> message is also printed during such an operation
if the current settings are too low.
Increasing these parameters may cause PostgreSQL>
to request more System V> shared
memory than your operating system's default configuration
allows. See for information on how to
adjust those parameters, if necessary.
max_fsm_pages (integer)max_fsm_pages> configuration parameter
Sets the maximum number of disk pages for which free space will
be tracked in the shared free-space map. Six bytes of shared memory
are consumed for each page slot. This setting must be more than
16 * max_fsm_relations. The default is chosen
by initdb> depending on the amount of available memory,
and can range from 20000 to 200000.
This parameter can only be set at server start.
max_fsm_relations (integer)max_fsm_relations> configuration parameter
Sets the maximum number of relations (tables and indexes) for which
free space will be tracked in the shared free-space map. Roughly
seventy bytes of shared memory are consumed for each slot.
The default is 1000.
This parameter can only be set at server start.
Kernel Resource Usagemax_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 1000. 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 really support when a large number of 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.
preload_libraries (string)preload_libraries> configuration parameter
This variable specifies one or more shared libraries that are
to be preloaded at server start. A parameterless
initialization function can optionally be called for each
library. To specify that, add a colon and the name of the
initialization function after the library name. For example
'$libdir/mylib:mylib_init' would cause
mylib> to be preloaded and mylib_init>
to be executed. If more than one library is to be loaded,
separate their names with commas.
If a specified library or initialization function is not found,
the server will fail to start.
PostgreSQL procedural language
libraries may be preloaded in this way, typically by using the
syntax '$libdir/plXXX:plXXX_init' where
XXX is pgsql>, perl>,
tcl>, or python>.
By preloading a shared library (and initializing it if
applicable), the library startup time is avoided when the
library is first used. However, the time to start each new
server process may 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.
Cost-Based Vacuum Delay
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 while (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 in which it is not very
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. 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 0, 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 may have the same results as setting it
to the next higher multiple of 10.
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 1.
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.
Background Writer
Beginning in PostgreSQL> 8.0, there is a separate server
process called the background writer>, whose sole function
is to issue writes of dirty> shared buffers. The intent is
that server processes handling user queries should seldom or never have
to wait for a write to occur, because the background writer will do it.
This arrangement also reduces the performance penalty associated with
checkpoints. The background writer will continuously trickle out dirty
pages to disk, so that only a few pages will need to be forced out when
checkpoint time arrives, instead of the storm of dirty-buffer writes that
formerly occurred at each checkpoint. However there is a net overall
increase in I/O load, because where a repeatedly-dirtied page might
before have been written only once per checkpoint interval, the
background writer might write it several times in the same interval.
In most situations a continuous low load is preferable to periodic
spikes, but 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. The default value is 200. 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 may 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_percent (floating point)bgwriter_lru_percent> configuration parameter
To reduce the probability that server processes will need to issue
their own writes, the background writer tries to write buffers that
are likely to be recycled soon. In each round, it examines up to
bgwriter_lru_percent> of the buffers that are nearest to
being recycled, and writes any that are dirty.
The default value is 1.0 (this is a percentage of the total number
of shared buffers).
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
as a result of scanning soon-to-be-recycled buffers.
The default value is 5.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
bgwriter_all_percent (floating point)bgwriter_all_percent> configuration parameter
To reduce the amount of work that will be needed at checkpoint time,
the background writer also does a circular scan through the entire
buffer pool, writing buffers that are found to be dirty.
In each round, it examines up to
bgwriter_all_percent> of the buffers for this purpose.
The default value is 0.333 (this is a percentage of the total number
of shared buffers). With the default bgwriter_delay>
setting, this will allow the entire shared buffer pool to be scanned
about once per minute.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
bgwriter_all_maxpages (integer)bgwriter_all_maxpages> configuration parameter
In each round, no more than this many buffers will be written
as a result of the scan of the entire buffer pool. (If this
limit is reached, the scan stops, and resumes at the next buffer
during the next round.)
The default value is 5.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
Smaller values of bgwriter_all_percent and
bgwriter_all_maxpages reduce the extra I/O load
caused by the background writer, but leave more work to be done
at checkpoint time. To reduce load spikes at checkpoints,
increase these two values.
Similarly, smaller values of bgwriter_lru_percent and
bgwriter_lru_maxpages 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.
To disable background writing entirely,
set both maxpages values and/or both
percent values to zero.
Write Ahead Log
See also for details on WAL
tuning.
Settingsfsync> configuration parameterfsync (boolean)
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.
However, using fsync results in a
performance penalty: when a transaction is committed,
PostgreSQL must wait for the
operating system to flush the write-ahead log to disk. When
fsync is disabled, the operating system is
allowed to do its best in buffering, ordering, and delaying
writes. This can result in significantly improved performance.
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.
(Crashes of the database software itself are not>
a risk factor here. Only an operating-system-level crash
creates a risk of corruption.)
Due to the risks involved, there is no universally correct
setting for fsync. Some administrators
always disable fsync, while others only
turn it off during initial bulk data loads, where there is a clear
restart point if something goes wrong. Others
always leave fsync enabled. The default is
to enable fsync, for maximum reliability.
If you trust your operating system, your hardware, and your
utility company (or your battery backup), you can consider
disabling fsync.
This parameter 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
.
wal_sync_method (string)wal_sync_method> configuration parameter
Method used for forcing WAL updates out to disk.
If fsync is off then this setting is irrelevant,
since 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_writethrough> (call fsync()> at each commit, forcing write-through of any disk write cache)
fsync> (call fsync()> at each commit)
open_sync> (write WAL files with open()> option O_SYNC>)
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.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
full_page_writes> configuration parameterfull_page_writes (boolean)
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 a price
in 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 a corrupt database after an operating system crash
or power failure. The risks are similar to turning off
fsync>, though smaller. It may be safe to turn off
this parameter if you have hardware (such as a battery-backed disk
controller) or filesystem software (e.g., Reiser4) that reduces
the risk of partial page writes to an acceptably low level.
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_buffers (integer)wal_buffers> configuration parameter
Number of disk-page buffers allocated in shared memory for WAL data.
The default is 8. The setting need only be large enough to hold
the amount of WAL data generated by one typical transaction, since
the data is written out to disk at every transaction commit.
This parameter can only be set at server start.
Increasing this parameter may cause PostgreSQL>
to request more System V> shared
memory than your operating system's default configuration
allows. See for information on how to
adjust those parameters, if necessary.
commit_delay (integer)commit_delay> configuration parameter
Time delay between writing a commit record to the WAL buffer
and flushing the buffer out to disk, in microseconds. A
nonzero delay can allow multiple transactions to be committed
with only one fsync() system call, if
system load is high enough that additional transactions become
ready to commit within the given interval. But the delay is
just wasted 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 server process has written its
commit record. The default is zero (no delay).
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.
Checkpointscheckpoint_segments (integer)checkpoint_segments> configuration parameter
Maximum distance between automatic WAL checkpoints, in log
file segments (each segment is normally 16 megabytes). The
default is three.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
checkpoint_timeout (integer)checkpoint_timeout> configuration parameter
Maximum time between automatic WAL checkpoints, in
seconds. The default is 300 seconds.
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
checkpoint_segments> ought to be raised). The default is
30 seconds. Zero disables the warning.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
Archivingarchive_command (string)archive_command> configuration parameter
The shell command to execute to archive a completed segment of
the WAL file series. If this is an empty string (the default),
WAL archiving is disabled. Any %p> in the string is
replaced by the absolute path of the file to archive, and any
%f> is replaced by the file name only. Use
%%> to embed an actual %> character in the
command. For more information see .
This parameter can only be set in the postgresql.conf>
file or on the server command line.
It is important for the command to return a zero exit status if
and only if it succeeds. Examples:
archive_command = 'cp "%p" /mnt/server/archivedir/"%f"'
archive_command = 'copy "%p" /mnt/server/archivedir/"%f"' # Windows
Query PlanningPlanner Method Configuration
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 may be found by using one
of these configuration parameters to force the optimizer to
choose a different plan. Turning one of these settings off
permanently is seldom a good idea, however.
Better ways to improve the quality of the
plans chosen by the optimizer include adjusting the , running more
frequently, 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 scanenable_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 scanenable_indexscan> configuration parameter
Enables or disables the query planner's use of index-scan plan
types. 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's not possible 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 scanenable_seqscan> configuration parameter
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 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's not possible 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>.
Planner Cost Constants
Unfortunately, there is no well-defined method for determining
ideal values for the family of cost variables that
appear below. You are encouraged to experiment and share
your findings.
effective_cache_size (floating point)effective_cache_size> configuration parameter
Sets the planner's assumption about the effective size of the
disk cache that is available to a single index scan. 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 using
different indexes, 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 value is measured in disk pages,
which are normally 8192 bytes each. The default is 1000.
random_page_cost (floating point)random_page_cost> configuration parameter
Sets the planner'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. A higher
value makes it more likely a sequential scan will be used, a
lower value makes it more likely an index scan will be
used. The default is four.
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. This is measured as a fraction of
the cost of a sequential page fetch. 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 row during an index scan. This is measured as a
fraction of the cost of a sequential page fetch. The default
is 0.001.
cpu_operator_cost (floating point)cpu_operator_cost> configuration parameter
Sets the planner'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. The default is 0.0025.
Genetic Query Optimizergenetic query optimizationGEQOgenetic query optimizationgeqo> configuration parametergeqo (boolean)
Enables or disables genetic query optimization, which is an
algorithm that attempts to do query planning without
exhaustive searching. This is on by default. The
geqo_threshold variable provides a more
granular way to disable GEQO for certain classes of queries.
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 deterministic, exhaustive planner, but for queries with
many tables the deterministic planner takes too long.
geqo_effort
(integer)geqo_effort> configuration parameter
Controls the trade off between planning time and query plan
efficiency in GEQO. This variable must be an integer in the
range from 1 to 10. The default value is 5. 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. The pool size 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
default 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. Generations
specifies 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 default 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.
Other Planner Optionsdefault_statistics_target (integer)default_statistics_target> configuration parameter
Sets the default statistics target for table columns that have
not had a column-specific target set via ALTER TABLE
SET STATISTICS>. Larger values increase the time needed to
do ANALYZE>, but may improve the quality of the
planner's estimates. The default is 10. For more information
on the use of statistics by the PostgreSQL>
query planner, refer to .
constraint_exclusion (boolean)constraint exclusionconstraint_exclusion> configuration parameter
Enables or disables the query planner's use of table constraints to
optimize queries. The default is off>.
When this parameter is on>, the planner compares
query conditions with table 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. This can
improve performance when inheritance is used to build
partitioned tables.
Currently, constraint_exclusion> is disabled by
default because it risks incorrect results if query plans are
cached — if a table constraint is changed or dropped,
the previously generated plan might now be wrong, and there is
no built-in mechanism to force re-planning. (This deficiency
will probably be addressed in a future
PostgreSQL> release.) Another reason for
keeping it off is that the constraint checks are relatively
expensive, and in many circumstances will yield no savings.
It is recommended to turn this on only if you are actually
using partitioned tables designed to take advantage of the
feature.
Refer to for more information
on using constraint exclusion and partitioning.
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 may
yield inferior query plans. The default is 8. It is usually
wise to keep this less than .
For more information see .
join_collapse_limit (integer)join_collapse_limit> configuration parameter
The planner will rewrite explicit JOIN>
constructs (except FULL JOIN>s) into lists of
FROM> items whenever a list of no more than this many items
would result. Smaller values reduce planning time but may
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 JOIN>s. Thus, the explicit join order
specified in the query will be the actual order in which the
relations are joined. The query planner does not always choose
the optimal join order; advanced users may elect to
temporarily set this variable to 1, and then specify the join
order they desire explicitly.
For more information see .
Error Reporting and Loggingserver logWhere To Logwhere to loglog_destination (string)log_destination> configuration parameterPostgreSQL supports several methods
for logging server messages, including
stderr 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.
redirect_stderr (boolean)redirect_stderr> configuration parameter
This parameter allows messages sent to stderr> to be
captured and redirected into log files.
This method, in combination with logging to stderr>,
is often more useful than
logging to syslog>, since some types of messages
may not appear in syslog> output (a common example
is dynamic-linker failure messages).
This parameter can only be set at server start.
log_directory (string)log_directory> configuration parameter
When redirect_stderr> is enabled, this parameter
determines the directory in which log files will be created.
It may 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.
log_filename (string)log_filename> configuration parameter
When redirect_stderr 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.
If no %-escapes are present,
PostgreSQL will
append the epoch of the new log file's open time. For example,
if log_filename were server_log, then the
chosen file name would be server_log.1093827753
for a log starting at Sun Aug 29 19:02:33 2004 MST.
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 redirect_stderr 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 redirect_stderr 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 redirect_stderr 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 may occur to select a file name
different from the hour's initial file name.
syslog_facility (string)syslog_facility> configuration parameter
When logging to syslog> is enabled, this parameter
determines the syslogfacility to be used. 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 daemon.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
syslog_ident (string)syslog_identity> 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.
When To Logclient_min_messages (string)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 (string)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 NOTICE>. Note that
LOG> has a different rank here than in
client_min_messages>.
Only superusers can change this setting.
log_error_verbosity (string)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.
Only superusers can change this setting.
log_min_error_statement (string)log_min_error_statement> configuration parameter
Controls whether or not the SQL statement that causes an error
condition will also be recorded in the server log. All SQL
statements that cause an error of the specified level or
higher are logged. The default is
PANIC (effectively turning this feature
off for normal use). Valid values are DEBUG5,
DEBUG4, DEBUG3,
DEBUG2, DEBUG1,
INFO, NOTICE,
WARNING, ERROR,
FATAL, and PANIC. For
example, if you set this to ERROR then all
SQL statements causing errors, fatal errors, or panics will be
logged. Enabling this parameter can be helpful in tracking down
the source of any errors that appear in the server log.
Only superusers can change this setting.
log_min_duration_statement (integer)log_min_duration_statement> configuration parameter
Logs the statement and its duration on a single log line if its
duration is greater than or equal to the specified number of
milliseconds. Setting this to zero will print all statements
and their durations. Minus-one (the default) disables the
feature. For example, if you set it to 250
then all SQL statements that run 250ms or longer will be
logged. Enabling this parameter can be useful in tracking down
unoptimized queries in your applications. This setting is
independent of log_statement and
log_duration. Only superusers can change
this setting.
silent_mode (boolean)silent_mode> configuration parameter
Runs the server silently. If this parameter is set, the server
will automatically run in background and any controlling
terminals are disassociated.
The server's standard output and standard error are redirected
to /dev/null>, so any messages sent to them will be lost.
Unless syslog> logging is selected or
redirect_stderr> is enabled, using this parameter
is discouraged because it makes it impossible to see error messages.
This parameter can only be set at server start.
Here is a list of the various message severity levels used in
these settings:
DEBUG[1-5]
Provides information for use by developers.
INFO
Provides information implicitly requested by the user,
e.g., during VACUUM VERBOSE>.
NOTICE
Provides information that may be helpful to users, e.g.,
truncation of long identifiers and the creation of indexes as part
of primary keys.
WARNING
Provides warnings to the user, e.g., COMMIT>
outside a transaction block.
ERROR
Reports an error that caused the current command to abort.
LOG
Reports information of interest to administrators, e.g.,
checkpoint activity.
FATAL
Reports an error that caused the current session to abort.
PANIC
Reports an error that caused all sessions to abort.
What To Logdebug_print_parse (boolean)debug_print_rewritten (boolean)debug_print_plan (boolean)debug_pretty_print (boolean)debug_print_parse> configuration parameterdebug_print_rewritten> configuration parameterdebug_print_plan> configuration parameterdebug_pretty_print> configuration parameter
These parameters enable various debugging output to be emitted.
For each executed query, they print
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. client_min_messages or
log_min_messages must be
DEBUG1 or lower to actually send this output
to the client or the server log, respectively.
These parameters are off by default.
log_connections (boolean)log_connections> configuration parameter
This outputs a line to the server log detailing each successful
connection. This is off by default, although it is probably very
useful. 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.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
log_disconnections (boolean)log_disconnections> configuration parameter
This outputs a line in the server log similar to
log_connections but at session termination,
and includes the duration of the session. This is off by
default.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
log_duration (boolean)log_duration> configuration parameter
Causes the duration of every completed statement which satisfies
log_statement> to be logged. When using this option,
if you are not using syslog>, it is recommended
that you log the PID or session ID using log_line_prefix>
so that you can link the statement message to the later
duration message using the process ID or session ID. The default is
off>. Only superusers can change this setting.
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. The default is an empty string.
Each recognized escape is replaced as outlined
below - anything else that looks like an escape is ignored. Other
characters are copied straight to the log line. Some escapes are
only recognized by session processes, and do not apply to
background processes such as the postmaster. Syslog>
produces its own
time stamp and process ID information, so you probably do not want to
use those escapes if you are using syslog>.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
EscapeEffectSession only%uUser nameyes%dDatabase nameyes%rRemote host name or IP address, and remote portyes%hRemote host name or IP addressyes%pProcess IDno%tTime stamp (no milliseconds)no%mTime stamp with millisecondsno%iCommand tag: This is the command that generated the log line.yes%cSession ID: A unique identifier for each session.
It is 2 4-byte hexadecimal numbers (without leading zeros)
separated by a dot. The numbers
are the session start time and the process ID, so this can also
be used as a space saving way of printing these items.yes%lNumber of the log line for each process, starting at 1no%sSession start time stampyes%xTransaction IDyes%qDoes not produce any output, but tells non-session
processes to stop at this point in the string. Ignored by
session processes.no%%Literal %>nolog_statement (string)log_statement> configuration parameter
Controls which SQL statements are logged. Valid values are
none>, ddl>, mod>, and
all>. ddl> logs all data definition
commands like CREATE>, ALTER>, and
DROP> commands. mod> logs all
ddl> statements, plus INSERT>,
UPDATE>, DELETE>, TRUNCATE>,
and COPY FROM>. PREPARE> and
EXPLAIN ANALYZE> statements are also logged if their
contained command is of an appropriate type.
The default is none>. Only superusers can change this
setting.
The EXECUTE statement is not considered a
ddl> or mod> statement. When it is logged,
only the name of the prepared statement is reported, not the
actual prepared statement.
When a function is defined in the
PL/pgSQLserver-side language, any queries
executed by the function will only be logged the first time that the
function is invoked in a particular session. This is because
PL/pgSQL keeps a cache of the
query plans produced for the SQL statements in the function.
log_hostname (boolean)log_hostname> configuration parameter
By default, connection log messages only show the IP address of the
connecting host. Turning on this parameter 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.
Run-Time StatisticsStatistics Monitoringlog_statement_stats (boolean)log_parser_stats (boolean)log_planner_stats (boolean)log_executor_stats (boolean)log_statement_stats> configuration parameterlog_parser_stats> configuration parameterlog_planner_stats> configuration parameterlog_executor_stats> configuration parameter
For each query, write performance statistics of the respective
module to the server log. This is a crude profiling
instrument. 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.
Query and Index Statistics Collector
These parameters control a server-wide statistics collection feature.
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.
stats_start_collector (boolean)stats_start_collector> configuration parameter
Controls whether the server should start the
statistics-collection subprocess. This is on by default, but
may be turned off if you know you have no interest in
collecting statistics or running autovacuum.
This parameter can only be set at server start, because the collection
subprocess cannot be started or stopped on-the-fly. (However, the
extent to which statistics are actually gathered can be changed while
the server is running, so long as the subprocess exists.)
stats_command_string (boolean)stats_command_string> configuration parameter
Enables the collection of statistics on the currently
executing command of each session, along with the time at
which that command began execution. This parameter is off 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.
stats_block_level (boolean)stats_block_level> configuration parameter
Enables the collection of block-level statistics on database
activity. This parameter is off by default.
Only superusers can change this setting.
stats_row_level (boolean)stats_row_level> configuration parameter
Enables the collection of row-level statistics on database
activity. This parameter is off by default.
Only superusers can change this setting.
stats_reset_on_server_start (boolean)stats_reset_on_server_start> configuration parameter
If on, collected statistics are zeroed out whenever the server
is restarted. If off, statistics are accumulated across server
restarts. The default is off>. This parameter can only
be set at server start.
Automatic Vacuumingautovacuumconfiguration parameters
These settings control the behavior of the autovacuum>
feature. Refer to for
more information.
autovacuum (boolean)autovacuum> configuration parameter
Controls whether the server should run the
autovacuum daemon. This is off by default.
stats_start_collector> and stats_row_level>
must also be turned on for autovacuum to work.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
autovacuum_naptime (integer)autovacuum_naptime> configuration parameter
Specifies the delay between activity rounds for the autovacuum
daemon. In each round the daemon examines one database
and issues VACUUM> and ANALYZE> commands
as needed for tables in that database. The delay is measured
in seconds, and the default is 60.
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 1000.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
This setting can be overridden for individual tables by entries in
pg_autovacuum>.
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 500.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
This setting can be overridden for individual tables by entries in
pg_autovacuum>.
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 0.4.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
This setting can be overridden for individual tables by entries in
pg_autovacuum>.
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 0.2.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
This setting can be overridden for individual tables by entries in
pg_autovacuum>.
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 (which is the
default), the regular
value will be used.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
This setting can be overridden for individual tables by entries in
pg_autovacuum>.
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.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
This setting can be overridden for individual tables by entries in
pg_autovacuum>.
Client Connection DefaultsStatement Behaviorsearch_path (string)search_path> configuration parameterpath>for 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 component. 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 has to be a comma-separated
list of schema names. If one of the list items is
the special value $user, then the schema
having the name returned by SESSION_USER> is substituted, if there
is such a schema. (If not, $user is ignored.)
The system catalog schema, pg_catalog>, is always
searched, whether it is mentioned in the path or not. If it is
mentioned in the path then it will be searched in the specified
order. If pg_catalog> is not in the path then it will
be searched before> searching any of the path items.
It should also be noted that the temporary-table schema,
pg_temp_nnn>>, is implicitly searched before any of
these.
When objects are created without specifying a particular target
schema, they will be placed in the first schema listed
in the search path. An error is reported if the search path is
empty.
The default value for this parameter is
'"$user", public' (where the second part will be
ignored if there is no schema named public>).
This 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()>. This is not quite the same as
examining the value of search_path, since
current_schemas()> shows how the requests
appearing in search_path were resolved.
For more information on schema handling, see .
default_tablespace (string)default_tablespace> configuration parametertablespace>default>>
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.
For more information on tablespaces,
see .
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 is
occasionally useful to avoid problems such as forward references
when restoring function definitions from a dump.
transaction isolation leveldefault_transaction_isolation> configuration parameterdefault_transaction_isolation (string)
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.
read-only transactiondefault_transaction_read_only> configuration parameterdefault_transaction_read_only (boolean)
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.
statement_timeout (integer)statement_timeout> configuration parameter
Abort any statement that takes over the specified number of
milliseconds. 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 off the
limitation.
Locale and FormattingDateStyle (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.
timezone (string)timezone> configuration parametertime zone>>
Sets the time zone for displaying and interpreting time
stamps. The default is 'unknown', which means to use whatever
the system environment specifies as the time zone. See for more information.
australian_timezones (boolean)australian_timezones> configuration parametertime zone>Australian>>
If set to on, ACST,
CST, EST, and
SAT are interpreted as Australian time
zones rather than as North/South American time zones and
Saturday. The default is off>.
significant digitsfloating-pointdisplayextra_float_digits> configuration parameterextra_float_digits (integer)
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 2, 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.
client_encoding (string)client_encoding> configuration parametercharacter set>>
Sets the client-side encoding (character set).
The default is to use the database encoding.
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 postmaster log as well as to the client.
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 date and time values.
(Currently, this setting does nothing, but it may in the
future.) 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.
Other Defaultsexplain_pretty_print (boolean)explain_pretty_print> configuration parameter
Determines whether EXPLAIN VERBOSE> uses the
indented or non-indented format for displaying detailed
query-tree dumps. The default is on>.
dynamic_library_path (string)dynamic_library_path> configuration parameterdynamic 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 has to 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.
Lock Managementdeadlocktimeout duringtimeoutdeadlockdeadlock_timeout> configuration parameterdeadlock_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. The
check for deadlock is relatively slow, 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 starting the check 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 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 a lock will be released before
the waiter decides to check for deadlock.
max_locks_per_transaction (integer)max_locks_per_transaction> configuration parameter
The shared lock table is created with room to describe locks on
max_locks_per_transaction *
( +
) objects;
hence, no more than this many distinct objects can
be locked at any one time. (Thus, this parameter's name may be
confusing: it is not a hard limit on the number of locks taken
by any one transaction, but rather a maximum average value.)
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 parameter can only be set at server start.
Increasing this parameter may cause PostgreSQL>
to request more System V> shared
memory than your operating system's default configuration
allows. See for information on how to
adjust those parameters, if necessary.
Version and Platform CompatibilityPrevious PostgreSQL Versionsadd_missing_from (boolean)FROM>missing>>
add_missing_from> configuration parameter
When on, tables that are referenced by a query will be
automatically added to the FROM> clause if not
already present. This behavior does not comply with the SQL
standard and many people dislike it because it can mask mistakes
(such as referencing a table where you should have referenced
its alias). The default is off>. This variable can be
enabled for compatibility with releases of
PostgreSQL> prior to 8.1, where this behavior was
allowed by default.
Note that even when this variable is enabled, a warning
message will be emitted for each implicit FROM>
entry referenced by a query. Users are encouraged to update
their applications to not rely on this behavior, by adding all
tables referenced by a query to the query's FROM>
clause (or its USING> clause in the case of
DELETE>).
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
NULLs to be entered. However, PostgreSQL> versions
before 8.2 did not support NULLs in arrays, and therefore would
treat NULL> as specifying a normal array element with
the string value NULL>. For backwards compatibility with
applications that require the old behavior, this variable can be
turned off>.
Note that it is possible to create array values containing NULLs
even when this variable is off>.
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. In PostgreSQL>
8.1 default_with_oids> is disabled by default; in
prior versions of PostgreSQL, 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)strings>escape>>
escape_string_warning> configuration parameter
When on, a warning is issued if a backslash (\>)
appears in an ordinary string literal ('...'>
syntax). The default is off>.
Escape string syntax (E'...'>) should be used for
escapes, because in future versions of
PostgreSQL ordinary strings will have
the standard-conforming behavior of treating backslashes
literally.
regex_flavor (string)regular expressions>>
regex_flavor> configuration parameter
The regular expression flavor> can be set to
advanced>, extended>, or basic>.
The default is advanced>. The extended>
setting may be useful for exact backwards compatibility with
pre-7.4 releases of PostgreSQL>. See
for details.
sql_inheritance (boolean)sql_inheritance> configuration parameterinheritance>>
This controls the inheritance semantics, in particular whether
subtables are included by various commands by default. They were
not included in versions prior to 7.1. If you need the old
behavior you can set this variable to off>, but in
the long run you are encouraged to change your applications to
use the ONLY key word to exclude subtables.
See for more information about
inheritance.
Platform and Client Compatibilitytransform_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 correct 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 not on 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.
Preset Options
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 may 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.
integer_datetimes (boolean)integer_datetimes> configuration parameter
Reports whether PostgreSQL was built
with support for 64-bit-integer dates and times. It is set by
configuring with --enable-integer-datetimes
when building PostgreSQL. The
default value is off.
lc_collate (string)lc_collate> configuration parameter
Reports the locale in which sorting of textual data is done.
See for more information.
The value is determined when the database cluster is initialized.
lc_ctype (string)lc_ctype> configuration parameter
Reports the locale that determines character classifications.
See for more information.
The value is determined when the database cluster is initialized.
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.
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.
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.
server_encoding (string)server_encoding> configuration parametercharacter 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.
standard_conforming_strings (boolean)strings>escape>>
standard_conforming_strings> configuration parameter
Reports whether ordinary string literals
('...'>) treat backslashes literally, as specified in
the SQL standard. The value is currently always off>,
indicating that backslashes are treated as escapes. It is planned
that this will change to on> in a future
PostgreSQL release when string literal
syntax changes to meet the standard. Applications may 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.
Customized Options
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 add-on modules to be
configured in the standard ways.
custom_variable_classes (string)custom_variable_classes> configuration parameter
This variable specifies one or several class names to be used for
custom variables, in the form of a comma-separated list. A custom
variable is a variable not normally known
to PostgreSQL proper but used by some
add-on module. Such variables must have names consisting of a class
name, a dot, and a variable name. custom_variable_classes>
specifies all the class names in use in a particular installation.
This parameter can only be set in the postgresql.conf>
file or on the server command line.
The difficulty with setting custom variables in
postgresql.conf> is that the file must be read before add-on
modules have been loaded, and so custom variables would ordinarily be
rejected as unknown. When custom_variable_classes> is set,
the server will accept definitions of arbitrary variables within each
specified class. These variables will be treated as placeholders and
will have no function until the module that defines them is loaded. When a
module for a specific class is loaded, it will add the proper variable
definitions for its class name, convert any placeholder
values according to those definitions, and issue warnings for any
placeholders of its class that remain (which presumably would be
misspelled configuration variables).
Here is an example of what postgresql.conf> might contain
when using custom variables:
custom_variable_classes = 'plr,plperl'
plr.path = '/usr/lib/R'
plperl.use_strict = true
plruby.use_strict = true # generates error: unknown class name
Developer Options
The following parameters are intended for work on the
PostgreSQL source, and in some cases
to assist with recovery of severely damaged databases. There
should be no reason to use them in a production database setup.
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.
debug_assertions (boolean)debug_assertions> configuration parameter
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 parameter, the macro USE_ASSERT_CHECKING
must be defined when PostgreSQL is
built (accomplished by the configure option
). Note that
debug_assertions defaults to on>
if PostgreSQL has been built with
assertions enabled.
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 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 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 log, respectively.
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_lwlocks (boolean)trace_userlocks (boolean)trace_lock_oidmin (boolean)trace_lock_table (boolean)debug_deadlocks (boolean)log_btree_build_stats (boolean)
Various other code tracing and debugging options.
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.
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
command. Setting zero_damaged_pages> to on causes
the system to instead report a warning, zero out the damaged page,
and continue processing. This behavior will destroy data>,
namely all the rows on the damaged page. But it allows you to get
past the error and retrieve rows from any undamaged pages that may
be present in the table. So it is useful for recovering data if
corruption has occurred due to hardware or software error. You should
generally not set this on until you have given up hope of recovering
data from the damaged page(s) of a table. The
default setting is off>, and it can only be changed
by a superuser.
Short Options
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.