There are a lot of configuration parameters that affect the behavior of the database system. In this subsection, we describe how to set configuration parameters; the following subsections discuss each parameter in detail.
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
,
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.
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 option 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 ALTER USER
and ALTER DATABASE,
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 options.
Some parameters can be changed in individual SQL sessions with the SET 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 reasonably be changed without restarting
PostgreSQL. Also, some parameters can
be modified via SET
or ALTER
by superusers,
but not by ordinary users.
The SHOW command allows inspection of the current values of all parameters.
The virtual table pg_settings
(described in Section 41.35, “pg_settings
”) 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.
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 Chapter 19, Client Authentication).
By default, all three configuration files are stored
in the database cluster's data directory. The options described
in this subsection 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
)
Specifies the directory to use for data storage. This option can only be set at server start.
config_file
(string
)
Specifies the main server configuration file
(customarily called postgresql.conf
).
This option can only be set on the postmaster command line.
hba_file
(string
)
Specifies the configuration file for host-based authentication
(customarily called pg_hba.conf
).
This option can only be set at server start.
ident_file
(string
)
Specifies the configuration file for
ident authentication
(customarily called pg_ident.conf
).
This option can only be set at server start.
external_pid_file
(string
)
Specifies the name of an additional process-id (PID) file that the postmaster should create for use by server administration programs. This option can only be set at server start.
In a default installation, none of the above options are set explicitly.
Instead, the
data directory is specified by the -D
command-line
option or the PGDATA
environment variable, and the
configuration files are all found within the data directory.
If you wish to keep the configuration files elsewhere than the
data directory, the postmaster's -D
command-line option or PGDATA
environment variable
must point to the directory containing the configuration files,
and the data_directory
option must be set in
postgresql.conf
(or on the command line) to show
where the data directory is actually located. Notice that
data_directory
overrides -D
and
PGDATA
for the location
of the data directory, but not for the location of the configuration
files.
If you wish, you can specify the configuration file names and locations
individually using the options 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 options plus
data_directory
are explicitly set, then it is not necessary
to specify -D
or PGDATA
.
When setting any of these options, a relative path will be interpreted
with respect to the directory in which the postmaster
is started.
listen_addresses
(string
)
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
)
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
)
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 Section 16.5.1, “Shared Memory and Semaphores” for information on how to adjust those parameters, if necessary.
superuser_reserved_connections
(integer
)
Determines the number of connection “slots” that
are reserved for connections by PostgreSQL
superusers. At most max_connections
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
)
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
)
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 option
unix_socket_permissions
this can be used as
an additional access control mechanism for Unix-domain connections.
By default this is the empty string, which uses the default
group for the current user. This option can only be set at
server start.
unix_socket_permissions
(integer
)
Sets the access permissions of the Unix-domain socket. Unix-domain
sockets use the usual Unix file system permission set.
The option value is expected to be 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 Chapter 19, Client Authentication.
This option can only be set at server start.
bonjour_name
(string
)
Specifies the Bonjour broadcast name. By default, the computer name is used, specified as an empty string ''. This option is ignored if the server was not compiled with Bonjour support. This option can only be set at server start.
authentication_timeout
(integer
)
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. This
option can only be set at server start or in the
postgresql.conf
file. The default is 60.
ssl
(boolean
)
Enables SSL connections. Please read Section 16.8, “Secure TCP/IP Connections with SSL” before using this. The default is off. This parameter can only be set at server start.
password_encryption
(boolean
)
When a password is specified in CREATE USER or
ALTER USER
without writing either ENCRYPTED
or
UNENCRYPTED
, this option determines whether the
password is to be encrypted. The default is on (encrypt the
password).
krb_server_keyfile
(string
)
Sets the location of the Kerberos server key file. See Section 19.2.3, “Kerberos authentication” for details.
db_user_namespace
(boolean
)
This allows per-database user names. It is off by default.
If this is on, you should create users as username@dbname
.
When username
is passed by a connecting client,
@
and the database name is 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 option 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.
shared_buffers
(integer
)
Sets the number of shared memory buffers used by the database
server. The default is typically 1000, 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 max_connections;
however, settings significantly higher than the minimum are
usually needed for good performance. Values of a few thousand
are recommended for production installations. This option 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 Section 16.5.1, “Shared Memory and Semaphores” for information on how to adjust those parameters, if necessary.
temp_buffers
(integer
)
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).
work_mem
(integer
)
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
)
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
)
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.
max_fsm_pages
(integer
)
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 20000.
The last few lines of a database-wide VACUUM VERBOSE
can help in determining if the the default setting is suitable.
A NOTICE
message is also printed during such an operation
if the current setting is too low.
This option can only be set at server start.
max_fsm_relations
(integer
)
Sets the maximum number of relations (tables and indexes) for which
free space will be tracked in the shared free-space map. Roughly
fifty bytes of shared memory are consumed for each slot.
The default is 1000.
The last few lines of a database-wide VACUUM VERBOSE
can help in determining if the the default setting is suitable.
A NOTICE
message is also printed during such an operation
if the current setting is too low.
This option can only be set at server start.
max_files_per_process
(integer
)
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 option can only be set at server start.
preload_libraries
(string
)
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 option is recommended only for libraries that will be used in most sessions.
During the execution of VACUUM and ANALYZE 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
)
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
)
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
)
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
)
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
)
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.
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 section can be used to tune the behavior for local needs.
bgwriter_delay
(integer
)
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 option can only be set at server start or in the
postgresql.conf
file.
bgwriter_lru_percent
(floating point
)
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 option can only be set at server start or in the
postgresql.conf
file.
bgwriter_lru_maxpages
(integer
)
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 option can only be set at server start or in the
postgresql.conf
file.
bgwriter_all_percent
(floating point
)
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 option can only be set at server start or in the
postgresql.conf
file.
bgwriter_all_maxpages
(integer
)
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 option can only be set at server start or in the
postgresql.conf
file.
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.
See also Section 25.2, “WAL Configuration” for details on WAL tuning.
fsync
(boolean
)
If this option is on, the PostgreSQL server
will use the fsync()
system call in several places
to make sure that updates are physically written to disk. This
insures that a database cluster will 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 server 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 for bulk loads, where there is a clear restart
point if something goes wrong, whereas some administrators
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 option can only be set at server start or in the
postgresql.conf
file.
wal_sync_method
(string
)
Method used for forcing WAL updates out to disk. Possible
values are
fsync
(call fsync()
at each commit),
fdatasync
(call fdatasync()
at each commit),
fsync_writethrough
(force write-through of any disk write cache),
open_sync
(write WAL files with open()
option O_SYNC
), and
open_datasync
(write WAL files with open()
option O_DSYNC
).
Not all of these choices are available on all platforms.
If fsync
is off then this setting is irrelevant.
This option can only be set at server start or in the
postgresql.conf
file.
wal_buffers
(integer
)
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. This option can only be set at server start.
commit_delay
(integer
)
Time delay between writing a commit record to the WAL buffer
and flushing the buffer out to disk, in microseconds. A
nonzero delay 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
)
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.
checkpoint_segments
(integer
)
Maximum distance between automatic WAL checkpoints, in log
file segments (each segment is normally 16 megabytes). The
default is three. This option can only be set at server start
or in the postgresql.conf
file.
checkpoint_timeout
(integer
)
Maximum time between automatic WAL checkpoints, in
seconds. The default is 300 seconds. This option can only be
set at server start or in the postgresql.conf
file.
checkpoint_warning
(integer
)
Write a message to the server log if checkpoints caused by the filling of checkpoint segment files happen closer together than this many seconds. The default is 30 seconds. Zero turns off the warning.
archive_command
(string
)
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 Section 22.3.1, “Setting up WAL archiving”. This option can only be set at
server start or in the postgresql.conf
file.
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
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 Planner Cost Constants
, running ANALYZE more
frequently, increasing the value of the default_statistics_target configuration parameter,
and increasing the amount of statistics collected for
specific columns using ALTER TABLE SET
STATISTICS
.
enable_bitmapscan
(boolean
)
Enables or disables the query planner's use of bitmap-scan plan types. The default is on.
enable_hashagg
(boolean
)
Enables or disables the query planner's use of hashed aggregation plan types. The default is on.
enable_hashjoin
(boolean
)
Enables or disables the query planner's use of hash-join plan types. The default is on.
enable_indexscan
(boolean
)
Enables or disables the query planner's use of index-scan plan types. The default is on.
enable_mergejoin
(boolean
)
Enables or disables the query planner's use of merge-join plan types. The default is on.
enable_nestloop
(boolean
)
Enables or disables the query planner's use of nested-loop join plans. It's not possible to suppress nested-loop joins entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.
enable_seqscan
(boolean
)
Enables or disables the query planner's use of sequential scan plan types. It's not possible to suppress sequential scans entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.
enable_sort
(boolean
)
Enables or disables the query planner's use of explicit sort steps. It's not possible to suppress explicit sorts entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on.
enable_tidscan
(boolean
)
Enables or disables the query planner's use of TID scan plan types. The default is on.
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
)
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
)
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
)
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
)
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
)
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.
geqo
(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
)
Use genetic query optimization to plan queries with at least
this many FROM
items involved. (Note that an 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
)
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
)
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
)
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
)
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.
default_statistics_target
(integer
)
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 Section 13.2, “Statistics Used by the Planner”.
from_collapse_limit
(integer
)
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 geqo_threshold.
join_collapse_limit
(integer
)
The planner will rewrite explicit inner JOIN
constructs into lists of FROM
items whenever a
list of no more than this many items in total would
result. Prior to PostgreSQL 7.4, joins
specified via the JOIN
construct would
never be reordered by the query planner. The query planner has
subsequently been improved so that inner joins written in this
form can be reordered; this configuration parameter controls
the extent to which this reordering is performed.
At present, the order of outer joins specified via the
JOIN
construct is never adjusted by the query
planner; therefore, join_collapse_limit
has no
effect on this behavior. The planner may be improved to
reorder some classes of outer joins in a future release of
PostgreSQL.
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
inner 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. Another consequence of setting
this variable to 1 is that the query planner will behave more
like the PostgreSQL 7.3 query
planner, which some users might find useful for backward
compatibility reasons.
Setting this variable to a value between 1 and
from_collapse_limit
might be useful to
trade off planning time against the quality of the chosen plan
(higher values produce better plans).
log_destination
(string
)
PostgreSQL supports several methods
for logging server messages, including
stderr and
syslog. On Windows,
eventlog is also supported. Set this
option to a list of desired log destinations separated by
commas. The default is to log to stderr
only.
This option can only be set at server start or in the
postgresql.conf
configuration file.
redirect_stderr
(boolean
)
This option allows messages sent to stderr to be captured and redirected into log files. This option, 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 option can only be set at server start.
log_directory
(string
)
When redirect_stderr
is enabled, this option
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 option can only be set at server start or in the
postgresql.conf
configuration file.
log_filename
(string
)
When redirect_stderr
is enabled, this option
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 option can only be set at server start or in the
postgresql.conf
configuration file.
log_rotation_age
(integer
)
When redirect_stderr
is enabled, this option
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 option can only be set at server start or in the
postgresql.conf
configuration file.
log_rotation_size
(integer
)
When redirect_stderr
is enabled, this option
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 option can only be set at server start or in the
postgresql.conf
configuration file.
log_truncate_on_rotation
(boolean
)
When redirect_stderr
is enabled, this option 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 false, pre-existing files will be appended to in
all cases. For example, using this option 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 option can only be set at server start or in the
postgresql.conf
configuration file.
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 true
, 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 true
,
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 filename
different from the hour's initial filename.
syslog_facility
(string
)
When logging to syslog is enabled, this option
determines the syslog
“facility” 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 option can only be set at server start.
syslog_ident
(string
)
When logging to syslog is enabled, this option
determines the program name used to identify
PostgreSQL messages in
syslog logs. The default is
postgres
.
This option can only be set at server start.
client_min_messages
(string
)
Controls which message levels are sent to the client.
Valid values are DEBUG5
,
DEBUG4
, DEBUG3
, DEBUG2
,
DEBUG1
, LOG
, NOTICE
,
WARNING
, and ERROR
. 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
)
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
)
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
)
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 option 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
)
Sets a minimum statement execution time (in milliseconds)
that causes a statement to be logged. All SQL statements
that run for the time specified or longer will be logged with
their duration. Setting this to zero will print
all queries 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 option can be
useful in tracking down unoptimized queries in your applications.
Only superusers can change this setting.
silent_mode
(boolean
)
Runs the server silently. If this option is set, the server
will automatically run in background and any controlling
terminals are disassociated (same effect as
postmaster
's -S
option).
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 option
is discouraged because it makes it impossible to see error messages.
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.
debug_print_parse
(boolean
)debug_print_rewritten
(boolean
)debug_print_plan
(boolean
)debug_pretty_print
(boolean
) These options 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 options are off by default.
log_connections
(boolean
)
This outputs a line to the server log detailing each successful
connection. This is off by default, although it is probably very
useful. This option can only be set at server start or in the
postgresql.conf
configuration file.
log_disconnections
(boolean
)
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 option can only be set at server start or in the
postgresql.conf
configuration file.
log_duration
(boolean
)
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 to the
duration using the process ID or session ID. The default is off.
Only superusers can change this setting.
log_line_prefix
(string
)
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 recognised 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 option can only be set at server start or in the
postgresql.conf
configuration file.
Escape | Effect | Session only |
---|---|---|
%u |
User name | yes |
%d |
Database name | yes |
%r |
Remote host name or IP address, and remote port | yes |
%p |
Process ID | no |
%t |
Time stamp | no |
%i |
Command tag: This is the command that generated the log line. | yes |
%c |
Session 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 |
%l |
Number of the log line for each process, starting at 1 | no |
%s |
Session start time stamp | yes |
%x |
Transaction ID | yes |
%q |
Does not produce any output, but tells non-session processes to stop at this point in the string. Ignored by session processes. | no |
%% |
Literal %
|
no |
log_statement
(string
)
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
)
By default, connection log messages only show the IP address of the
connecting host. Turning on this option 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
option can only be set at server start or in the
postgresql.conf
file.
log_statement_stats
(boolean
)log_parser_stats
(boolean
)log_planner_stats
(boolean
)log_executor_stats
(boolean
) 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.
stats_start_collector
(boolean
)
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. This option can only be set at server start.
stats_command_string
(boolean
)
Enables the collection of statistics on the currently
executing command of each session, along with the time at
which that command began execution. This option 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. This data can be accessed via the
pg_stat_activity
system view; refer
to Chapter 23, Monitoring Database Activity for more information.
stats_block_level
(boolean
)
Enables the collection of block-level statistics on database
activity. This option is disabled by default. If this option
is enabled, the data that is produced can be accessed via the
pg_stat
and
pg_statio
family of system views;
refer to Chapter 23, Monitoring Database Activity for more information.
stats_row_level
(boolean
)
Enables the collection of row-level statistics on database
activity. This option is disabled by default. If this option
is enabled, the data that is produced can be accessed via the
pg_stat
and
pg_statio
family of system views;
refer to Chapter 23, Monitoring Database Activity for more information.
stats_reset_on_server_start
(boolean
)
If on, collected statistics are zeroed out whenever the server is restarted. If off, statistics are accumulated across server restarts. The default is on. This option can only be set at server start.
search_path
(string
)
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, $user
is ignored.)
The system catalog schema, pg_catalog
, is always
searched, whether it is mentioned in the path or not. If it is
mentioned in the path then it will be searched in the specified
order. If pg_catalog
is not in the path then it will
be searched before searching any of the path items.
It should also be noted that the temporary-table schema,
pg_temp_
, is implicitly searched before any of
these.
nnn
When objects are created without specifying a particular target schema, they will be placed in the first schema listed in the search path. An error is reported if the search path is empty.
The default value for this parameter is
'$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 Section 5.8, “Schemas”.
default_tablespace
(string
)
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 Section 18.6, “Tablespaces”.
check_function_bodies
(boolean
)
This parameter is normally true. When set to false, it disables validation of the function body string during CREATE FUNCTION. Disabling validation is occasionally useful to avoid problems such as forward references when restoring function definitions from a dump.
default_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 Chapter 12, Concurrency Control and SET TRANSACTION for more information.
default_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 false (read/write).
Consult SET TRANSACTION for more information.
statement_timeout
(integer
)
Abort any statement that takes over the specified number of milliseconds. A value of zero (the default) turns off the limitation.
DateStyle
(string
)
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
Section 8.5, “Date/Time Types” for more information. The
default is ISO, MDY
.
timezone
(string
)
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 Section 8.5, “Date/Time Types” for more information.
australian_timezones
(boolean
)
If set to true, ACST
,
CST
, EST
, and
SAT
are interpreted as Australian time
zones rather than as North/South American time zones and
Saturday. The default is false.
extra_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
)
Sets the client-side encoding (character set). The default is to use the database encoding.
lc_messages
(string
)
Sets the language in which messages are displayed. Acceptable values are system-dependent; see Section 20.1, “Locale Support” 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.
lc_monetary
(string
)
Sets the locale to use for formatting monetary amounts, for
example with the to_char
family of
functions. Acceptable values are system-dependent; see Section 20.1, “Locale Support” 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
)
Sets the locale to use for formatting numbers, for example
with the to_char
family of
functions. Acceptable values are system-dependent; see Section 20.1, “Locale Support” 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
)
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 Section 20.1, “Locale Support” 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.
explain_pretty_print
(boolean
)
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
)
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.
deadlock_timeout
(integer
)
This is the amount of time, in milliseconds, to wait on a lock before checking to see if there is a deadlock condition. 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
)
The shared lock table is sized on the assumption that at most
max_locks_per_transaction
*
max_connections
distinct objects will need to
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 option can only be set at server start.
add_missing_from
(boolean
)
When true
, 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 false
. 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
).
regex_flavor
(string
)
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
Section 9.7.3.1, “Regular Expression Details” for details.
sql_inheritance
(boolean
)
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
Section 5.5, “Inheritance” for more information about inheritance.
default_with_oids
(boolean
)
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 true 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.
transform_null_equals
(boolean
)
When turned on, expressions of the form
(or expr
= NULLNULL
=
) are treated as
expr
, that is, they
return true if expr
IS NULLexpr
evaluates to the null value,
and false otherwise. The correct SQL-spec-compliant behavior of
is to always
return null (unknown). Therefore this option defaults to off.
expr
= NULL
However, filtered forms in Microsoft
Access generate queries that appear to use
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.
expr
= NULL
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 Section 9.2, “Comparison Operators” for related information.
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
)
Shows 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 shared_buffers) is
influenced by block_size
. See Section 16.4.3, “Resource Consumption” for information.
integer_datetimes
(boolean
)
Shows 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
)
Shows the locale in which sorting of textual data is done. See Section 20.1, “Locale Support” for more information. The value is determined when the database cluster is initialized.
lc_ctype
(string
)
Shows the locale that determines character classifications.
See Section 20.1, “Locale Support” 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
)
Shows 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
)
Shows 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
)
Shows 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
)
Shows the database encoding (character set). It is determined when the database is created. Ordinarily, clients need only be concerned with the value of client_encoding.
server_version
(string
)
Shows the version number of the server. It is determined by the
value of PG_VERSION
when building the server.
This feature was designed to allow options 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
)
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 option can only be set at server start or in the
postgresql.conf
configuration file.
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,pljava' plr.path = '/usr/lib/R' pljava.foo = 1 plruby.bar = true # generates error, unknown class name
The following options 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
options require special source compilation flags to work at all.
debug_assertions
(boolean
)
Turns on various assertion checks. This is a debugging aid. If
you are experiencing strange problems or crashes you might want
to turn this on, as it might expose programming mistakes. To use
this option, the macro USE_ASSERT_CHECKING
must be defined when PostgreSQL is
built (accomplished by the configure
option
--enable-cassert
). Note that
debug_assertions
defaults to on if
PostgreSQL has been built with
assertions enabled.
pre_auth_delay
(integer
)
If nonzero, a delay of this many seconds occurs just after a new server process is forked, before it conducts the authentication process. This is intended to give an opportunity to attach to the server process with a debugger to trace down misbehavior in authentication.
trace_notify
(boolean
)
Generates a great amount of debugging output for the
LISTEN
and NOTIFY
commands. client_min_messages or
log_min_messages must be
DEBUG1
or lower to send this output to the
client or server log, respectively.
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
)
If true, emit WAL-related debugging output. This option is
only available if the WAL_DEBUG
macro was
defined when PostgreSQL was
compiled.
zero_damaged_pages
(boolean
)
Detection of a damaged page header normally causes
PostgreSQL to report an error, aborting the current
command. Setting zero_damaged_pages
to true 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 true 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.
For convenience there are also single letter command-line option switches available for some parameters. They are described in Table 16.1, “Short option key”.
Table 16.1. Short option key
Short option | Equivalent |
---|---|
-B |
shared_buffers = |
-d |
log_min_messages = DEBUG |
-F |
fsync = off |
-h |
listen_addresses = |
-i |
listen_addresses = '*' |
-k |
unix_socket_directory = |
-l |
ssl = on |
-N |
max_connections = |
-p |
port = |
-fb , -fh , -fi ,
-fm , -fn ,
-fs , -ft [a]
|
enable_bitmapscan = off ,
enable_hashjoin = off ,
enable_indexscan = off ,
enable_mergejoin = off ,
enable_nestloop = off ,
enable_seqscan = off ,
enable_tidscan = off
|
-s [a]
|
log_statement_stats = on |
-S [a]
|
work_mem = |
-tpa , -tpl , -te [a]
|
log_parser_stats = on ,
log_planner_stats = on ,
log_executor_stats = on
|
[a] For historical reasons, these options must be passed to
the individual server process via the $
or via PGOPTIONS from the client side, as
explained above.
|