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3.4. Run-time Configuration

There are a lot of configuration parameters that affect the behavior of the database system. Here we describe how to set them and the following subsections will discuss each in detail.

All parameter names are case-insensitive. Every parameter takes a value of one of the four types: Boolean, integer, floating point, and string. Boolean values are ON, OFF, TRUE, FALSE, YES, NO, 1, 0 (case-insensitive) or any non-ambiguous prefix of these.

One way to set these options is to edit the file postgresql.conf in the data directory. (A default file is installed there.) An example of what this file might look like is:

# This is a comment
log_connections = yes
syslog = 2
search_path = '$user, public'

As you see, options are one 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 should be single-quoted.

The configuration file is reread whenever the postmaster 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 backend processes so that existing sessions also get the new value. Alternatively, you can send the signal to a single backend process directly.

A second way to set these configuration parameters is to give them as a command line option to the postmaster, such as:

postmaster -c log_connections=yes -c syslog=2

which would have the same effect as the previous example. Command-line options override any conflicting settings in postgresql.conf.

Occasionally it is also useful to give a command line option to one particular backend session only. The environment variable PGOPTIONS can be used for this purpose on the client side:

env PGOPTIONS='-c geqo=off' psql

(This works for any libpq-based client application, not just psql.) Note that this won't work for options that are fixed when the server is started, such as the port number.

Some options can be changed in individual SQL sessions with the SET command, for example:

=> SET ENABLE_SEQSCAN TO OFF;

See the SQL command language reference for details on the syntax.

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 DATABASE and ALTER USER, respectively, are used to configure these settings. Such per-database settings override anything received from the postmaster or the configuration file, and in turn are overridden by per-user settings.

3.4.1. pg_settings

The pg_settings virtual table allows display and update of current session run-time parameters. There is one entry for each of the available parameters provided by SHOW ALL. But it is in a form that allows it to be joined with other relations and have a selection criteria applied.

An UPDATE performed on pg_settings is equivalent to executing the SET command on that named parameter. The change only affects the value used by the current session. If an UPDATE is issued within a transaction that is later aborted, the effects of the UPDATE command disappear when the transaction is rolled back. Once the surrounding transaction is committed, the effects will persist until the end of the session, unless overridden by another UPDATE or SET.

Table 3-1. pg_settings Columns

Name Type Description
name text The name of a current session run-time parameter
setting text The value of a current session run-time parameter

3.4.2. Planner and Optimizer Tuning

CPU_INDEX_TUPLE_COST (floating point)

Sets the query optimizer's estimate of the cost of processing each index tuple during an index scan. This is measured as a fraction of the cost of a sequential page fetch.

CPU_OPERATOR_COST (floating point)

Sets the optimizer's estimate of the cost of processing each operator in a WHERE clause. This is measured as a fraction of the cost of a sequential page fetch.

CPU_TUPLE_COST (floating point)

Sets the query optimizer's estimate of the cost of processing each tuple during a query. This is measured as a fraction of the cost of a sequential page fetch.

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.

EFFECTIVE_CACHE_SIZE (floating point)

Sets the optimizer's assumption about the effective size of the disk cache (that is, the portion of the kernel's disk cache that will be used for PostgreSQL data files). This is measured in disk pages, which are normally 8 kB each.

ENABLE_HASHJOIN (boolean)

Enables or disables the query planner's use of hash-join plan types. The default is on. This is used for debugging the query planner.

ENABLE_INDEXSCAN (boolean)

Enables or disables the query planner's use of index-scan plan types. The default is on. This is used to debugging the query planner.

ENABLE_MERGEJOIN (boolean)

Enables or disables the query planner's use of merge-join plan types. The default is on. This is used for debugging the query planner.

ENABLE_NESTLOOP (boolean)

Enables or disables the query planner's use of nested-loop join plans. It's not possible to suppress nested-loop joins entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on. This is used for debugging the query planner.

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. This is used for debugging the query planner.

ENABLE_SORT (boolean)

Enables or disables the query planner's use of explicit sort steps. It's not possible to suppress explicit sorts entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on. This is used for debugging the query planner.

ENABLE_TIDSCAN (boolean)

Enables or disables the query planner's use of TID scan plan types. The default is on. This is used for debugging the query planner.

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. See also the various other GEQO_ settings.

GEQO_EFFORT (integer)
GEQO_GENERATIONS (integer)
GEQO_POOL_SIZE (integer)
GEQO_RANDOM_SEED (integer)
GEQO_SELECTION_BIAS (floating point)

Various tuning parameters for the genetic query optimization algorithm: The pool size is the number of individuals in one population. Valid values are between 128 and 1024. If it is set to 0 (the default) a pool size of 2^(QS+1), where QS is the number of FROM items in the query, is taken. The effort is used to calculate a default for generations. Valid values are between 1 and 80, 40 being the default. Generations specifies the number of iterations in the algorithm. The number must be a positive integer. If 0 is specified then Effort * Log2(PoolSize) is used. The run time of the algorithm is roughly proportional to the sum of pool size and generations. The selection bias is the selective pressure within the population. Values can be from 1.50 to 2.00; the latter is the default. The random seed can be set to get reproducible results from the algorithm. If it is set to -1 then the algorithm behaves non-deterministically.

GEQO_THRESHOLD (integer)

Use genetic query optimization to plan queries with at least this many FROM items involved. (Note that a JOIN construct counts as only one FROM item.) The default is 11. For simpler queries it is usually best to use the deterministic, exhaustive planner. This parameter also controls how hard the optimizer will try to merge subquery FROM clauses into the upper query.

RANDOM_PAGE_COST (floating point)

Sets the query optimizer's estimate of the cost of a nonsequentially fetched disk page. This is measured as a multiple of the cost of a sequential page fetch.

Note: Unfortunately, there is no well-defined method for determining ideal values for the family of "COST" variables that were just described. You are encouraged to experiment and share your findings.

3.4.3. Logging and Debugging

SERVER_MIN_MESSAGES (string)

This controls how much message detail is written to the server logs. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, INFO, NOTICE, WARNING, ERROR, LOG, FATAL, and PANIC. Later values send less detail to the logs. The default is NOTICE. Note that LOG has a different precedence here than in CLIENT_MIN_MESSAGES.

Here is a summary of the various message types:

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 index creation as part of primary keys.

WARNING

Provides warnings to the user, e.g., COMMIT outside a transaction.

ERROR

Reports the error that caused a transaction to abort.

LOG

Reports information of interest to administrators, e.g., checkpoint activity.

FATAL

Reports why a backend session terminated.

PANIC

Reports why all backend sessions restarted.

CLIENT_MIN_MESSAGES (string)

This controls how much message detail is written to the client. Valid values are DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, LOG, NOTICE, WARNING, and ERROR. Later values send less information to the client. The default is NOTICE. Note that LOG has a different precedence here than in SERVER_MIN_MESSAGES. Also see that section for an explanation of the various values.

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.

DEBUG_PRINT_PARSE (boolean)
DEBUG_PRINT_REWRITTEN (boolean)
DEBUG_PRINT_PLAN (boolean)
DEBUG_PRETTY_PRINT (boolean)

These flags enable various debugging output to be sent to the server log. For each executed query, print either the query text, the resulting parse tree, the query rewriter output, or the execution plan. DEBUG_PRETTY_PRINT indents these displays to produce a more readable but much longer output format.

EXPLAIN_PRETTY_PRINT (boolean)

Determines whether EXPLAIN VERBOSE uses the indented or non-indented format for displaying detailed query-tree dumps.

HOSTNAME_LOOKUP (boolean)

By default, connection logs only show the IP address of the connecting host. If you want it to show the host name you can turn this on, but depending on your host name resolution setup it might impose a non-negligible performance penalty. This option can only be set at server start.

LOG_CONNECTIONS (boolean)

This outputs a line to the server logs 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_DURATION (boolean)

Causes the duration of every completed statement to be logged. To use this option, enable LOG_STATEMENT and LOG_PID so you can link the statement to the duration using the process ID.

LOG_MIN_ERROR_STATEMENT (string)

This controls for which message levels the SQL statement causing that message is to be recorded in the server log. All statements causing a message of the level of the setting or higher are logged. The default is PANIC (effectively turning this feature off). 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.

It is recommended you enable LOG_PID as well so you can more easily match the error statement with the error message.

LOG_PID (boolean)

Prefixes each server message in the log file with the process ID of the backend process. This is useful to sort out which messages pertain to which connection. The default is off. This parameter does not affect messages logged via syslog, which always contain the process ID.

LOG_STATEMENT (boolean)

Causes each SQL statement to be logged.

LOG_TIMESTAMP (boolean)

Prefixes each server log message with a time stamp. The default is off.

SHOW_STATEMENT_STATS (boolean)
SHOW_PARSER_STATS (boolean)
SHOW_PLANNER_STATS (boolean)
SHOW_EXECUTOR_STATS (boolean)

For each query, write performance statistics of the respective module to the server log. This is a crude profiling instrument.

SHOW_SOURCE_PORT (boolean)

Shows the outgoing port number of the connecting host in the connection log messages. You could trace back the port number to find out what user initiated the connection. Other than that, it's pretty useless and therefore off by default. This option can only be set at server start.

STATS_COMMAND_STRING (boolean)
STATS_BLOCK_LEVEL (boolean)
STATS_ROW_LEVEL (boolean)

These flags determine what information backends send to the statistics collector process: current commands, block-level activity statistics, or row-level activity statistics. All default to off. Enabling statistics collection costs a small amount of time per query, but is invaluable for debugging and performance tuning.

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.

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.

SYSLOG (integer)

PostgreSQL allows the use of syslog for logging. If this option is set to 1, messages go both to syslog and the standard output. A setting of 2 sends output only to syslog. (Some messages will still go to the standard output/error.) The default is 0, which means syslog is off. This option must be set at server start.

SYSLOG_FACILITY (string)

This option determines the syslog "facility" to be used when syslog is enabled. You may choose from LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7; the default is LOCAL0. See also the documentation of your system's syslog.

SYSLOG_IDENT (string)

If logging to syslog is enabled, this option determines the program name used to identify PostgreSQL messages in syslog log messages. The default is postgres.

TRACE_NOTIFY (boolean)

Generates a great amount of debugging output for the LISTEN and NOTIFY commands.

3.4.4. General Operation

AUTOCOMMIT (boolean)

If set to true, PostgreSQL will automatically do a COMMIT after each successful command that is not inside an explicit transaction block (that is, unless a BEGIN with no matching COMMIT has been given). If set to false, PostgreSQL will commit only upon receiving an explicit COMMIT command. This mode can also be thought of as implicitly issuing BEGIN whenever a command is received that is not already inside a transaction block. The default is true, for compatibility with historical PostgreSQL behavior. However, for maximum compatibility with the SQL specification, set it to false.

Note: Even with autocommit set to false, SET, SHOW, and RESET do not start new transaction blocks. They are run in their own transactions. Once another command is issued, a transaction block begins and any SET, SHOW, or RESET commands are considered to be part of the transaction, i.e., they are committed or rolled back depending on the completion status of the transaction. To execute a SET, SHOW, or RESET command at the start of a transaction block, use BEGIN first.

Note: As of PostgreSQL 7.3, setting autocommit to false is not well-supported. This is a new feature and is not yet handled by all client libraries and applications. Before making it the default setting in your installation, test carefully.

AUSTRALIAN_TIMEZONES (boolean)

If set to true, CST, EST, and SAT are interpreted as Australian time zones rather than as North American Central/Eastern time zones and Saturday. The default is false.

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.

BACKSLASH_QUOTE (string)

This controls whether a quote mark can be represented by \' in a string literal. The preferred, SQL-standard way to represent a quote mark is by doubling it ('') but PostgreSQL has historically also accepted \'. However, use of \' creates security risks because in some client character set encodings, there are multibyte characters in which the last byte is numerically equivalent to ASCII \. If client-side code does escaping incorrectly then a SQL-injection attack is possible. This risk can be prevented by making the server reject queries in which a quote mark appears to be escaped by a backslash. The allowed values of backslash_quote are on (allow \' always), off (reject always), and safe_encoding (allow only if client encoding does not allow ASCII \ within a multibyte character). safe_encoding is the default setting.

CLIENT_ENCODING (string)

Sets the client-side encoding for multibyte character sets. The default is to use the database encoding.

DATESTYLE (string)

Sets the display format for dates, as well as the rules for interpreting ambiguous input dates. The default is ISO, US.

DB_USER_NAMESPACE (boolean)

This allows per-database user names. It is off by default.

If this is on, 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.

Note: This feature is intended as a temporary measure until a complete solution is found. At that time, this option will be removed.

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 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 the lock will be released before the waiter decides to check for deadlock. This option can only be set at server start.

DEFAULT_TRANSACTION_ISOLATION (string)

Each SQL transaction has an isolation level, which can be either "read committed" or "serializable". This parameter controls the default isolation level of each new transaction. The default is "read committed".

Consult the PostgreSQL 7.3.21 User's Guide and the command SET TRANSACTION for more information.

DYNAMIC_LIBRARY_PATH (string)

If a dynamically loadable module needs to be opened and the specified name does not have a directory component (i.e. the name does not contain a slash), the system will search this path for the specified file. (The name that is used is the name specified in the CREATE FUNCTION or LOAD command.)

The value for dynamic_library_path has to be a colon-separated list of absolute directory names. If a directory name starts with the special value $libdir, the compiled-in PostgreSQL package library directory is substituted. This where the modules provided by the PostgreSQL distribution are installed. (Use pg_config --pkglibdir to print the name of this directory.) For example:

dynamic_library_path = '/usr/local/lib/postgresql:/home/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.

KRB_SERVER_KEYFILE (string)

Sets the location of the Kerberos server key file. See Section 6.2.3 for details.

FSYNC (boolean)

If this option is on, the PostgreSQL backend will use the fsync() system call in several places to make sure that updates are physically written to disk. This insures that a database installation will recover to a consistent state after an operating system or hardware crash. (Crashes of the database server itself are not related to this.)

However, this operation does slow down PostgreSQL because at transaction commit it has wait for the operating system to flush the write-ahead log. Without fsync, the operating system is allowed to do its best in buffering, sorting, and delaying writes, which can considerably increase 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.

For the above reasons, some administrators always leave it off, some turn it off only for bulk loads, where there is a clear restart point if something goes wrong, and some leave it on just to be on the safe side. Because it is always safe, the default is on. If you trust your operating system, your hardware, and your utility company (or better your UPS), you might want to disable fsync.

It should be noted that the performance penalty of having fsync on is considerably less in PostgreSQL version 7.1 and later. If you previously suppressed fsync for performance reasons, you may wish to reconsider your choice.

This option can only be set at server start or in the postgresql.conf file.

LC_MESSAGES (string)

Sets the language in which messages are displayed. Acceptable values are system-dependent; see Section 7.1 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 7.1 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 7.1 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 7.1 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.

MAX_CONNECTIONS (integer)

Determines the maximum number of concurrent connections to the database server. The default is 32 (unless altered while building the server). This parameter can only be set at server start.

MAX_EXPR_DEPTH (integer)

Sets the maximum expression nesting depth of the parser. The default value is high enough for any normal query, but you can raise it if needed. (But if you raise it too high, you run the risk of backend crashes due to stack overflow.)

MAX_FILES_PER_PROCESS (integer)

Sets the maximum number of simultaneously open files in each server subprocess. The default is 1000. The limit actually used by the code is the smaller of this setting and the result of sysconf(_SC_OPEN_MAX). Therefore, on systems where sysconf returns a reasonable limit, you don't need to worry about this setting. But on some platforms (notably, most BSD systems), sysconf returns a value that is much larger 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 or in the postgresql.conf configuration file; if changed in the configuration file, it only affects subsequently-started server subprocesses.

MAX_FSM_RELATIONS (integer)

Sets the maximum number of relations (tables) for which free space will be tracked in the shared free-space map. The default is 1000. This option can only be set at server start.

MAX_FSM_PAGES (integer)

Sets the maximum number of disk pages for which free space will be tracked in the shared free-space map. The default is 10000. This option can only be set at server start.

MAX_LOCKS_PER_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. The default, 64, which 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.

PASSWORD_ENCRYPTION (boolean)

When a password is specified in CREATE USER or ALTER USER without writing either ENCRYPTED or UNENCRYPTED, this flag determines whether the password is to be encrypted. The default is on (encrypt the password).

PORT (integer)

The TCP port the server listens on; 5432 by default. 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 same name as the 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.

Likewise, the current session's temporary-table schema, pg_temp_nnn, is always searched if it exists. It can be explicitly listed in the path by using the alias pg_temp. If it is not listed in the path then it is searched first (before even pg_catalog). However, the temporary schema is only searched for relation (table, view, sequence, etc) and data type names. It will never be searched for function or operator names.

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 the PostgreSQL 7.3.21 User's Guide.

STATEMENT_TIMEOUT (integer)

Aborts any statement that takes over the specified number of milliseconds. A value of zero turns off the timer.

SHARED_BUFFERS (integer)

Sets the number of shared memory buffers used by the database server. The default is 64. Each buffer is typically 8192 bytes. This must be greater than 16, as well as at least twice the value of MAX_CONNECTIONS; however, a higher value can often improve performance on modern machines. Values of at least 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 3.5.1 for information on how to adjust these parameters, if necessary.

SILENT_MODE (bool)

Runs the server silently. If this option is set, the server will automatically run in background and any controlling ttys are disassociated, thus no messages are written to standard output or standard error (same effect as postmaster's -S option). Unless some logging system such as syslog is enabled, using this option is discouraged since it makes it impossible to see error messages.

SORT_MEM (integer)

Specifies the amount of memory to be used by internal sorts and hashes before 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 sorts might be running in parallel, and each one will be allowed to use as much memory as this value specifies before it starts to put data into temporary files. Also, each running backend could be doing one or more sorts simultaneously, so the total memory used could be many times the value of SORT_MEM. Sorts are used by ORDER BY, merge joins, and CREATE INDEX.

SQL_INHERITANCE (bool)

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 keyword to exclude subtables. See the SQL language reference and the PostgreSQL 7.3.21 User's Guide for more information about inheritance.

SSL (boolean)

Enables SSL connections. Please read Section 3.7 before using this. The default is off.

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 from superuser accounts.

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.

TCPIP_SOCKET (boolean)

If this is true, then the server will accept TCP/IP connections. Otherwise only local Unix domain socket connections are accepted. It is off by default. This option can only be set at server start.

TIMEZONE (string)

Sets the time zone for displaying and interpreting timestamps. The default is to use whatever the system environment specifies as the time zone.

TRANSFORM_NULL_EQUALS (boolean)

When turned 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 behavior of expr = NULL is to always return null (unknown). Therefore this option 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 literal = operator, 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 the PostgreSQL 7.3.21 User's Guide for related information.

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.

UNIX_SOCKET_GROUP (string)

Sets the group owner of the Unix domain socket. (The owning user of the socket is always the user that starts the server.) In combination with the option UNIX_SOCKET_PERMISSIONS this can be used as an additional access control mechanism for this socket type. By default this is the empty string, which uses the default group for the current user. This option can only be set at server start.

UNIX_SOCKET_PERMISSIONS (integer)

Sets the access permissions of the Unix domain socket. Unix domain sockets use the usual Unix file system permission set. The option value is expected to be an numeric mode specification in the form accepted by the chmod and umask system calls. (To use the customary octal format the number must start with a 0 (zero).)

The default permissions are 0777, meaning anyone can connect. Reasonable alternatives are 0770 (only user and group, see also under UNIX_SOCKET_GROUP) and 0700 (only user). (Note that actually for a Unix domain socket, only write permission matters and there is no point in setting or revoking read or execute permissions.)

This access control mechanism is independent of the one described in Chapter 6.

This option can only be set at server start.

VACUUM_MEM (integer)

Specifies the maximum amount of memory to be used by VACUUM to keep track of to-be-reclaimed tuples. The value is specified in kilobytes, and defaults to 8192 kilobytes. Larger settings may improve the speed of vacuuming large tables that have many deleted tuples.

VIRTUAL_HOST (string)

Specifies the TCP/IP host name or address on which the postmaster is to listen for connections from client applications. Defaults to listening on all configured addresses (including localhost).

ZERO_DAMAGED_PAGES (boolean)

Detection of a damaged page header normally causes PostgreSQL to report an error, aborting the current transaction. 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.

3.4.5. WAL

See also Section 12.3 for details on WAL tuning.

CHECKPOINT_SEGMENTS (integer)

Maximum distance between automatic WAL checkpoints, in log file segments (each segment is normally 16 megabytes). This option can only be set at server start or in the postgresql.conf file.

CHECKPOINT_TIMEOUT (integer)

Maximum time between automatic WAL checkpoints, in seconds. This option can only be set at server start or in the postgresql.conf file.

COMMIT_DELAY (integer)

Time delay between writing a commit record to the WAL buffer and flushing the buffer out to disk, in microseconds. A nonzero delay allows multiple transactions to be committed with only one fsync system call, if system load is high enough additional transactions may 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 backend process has written its commit record.

COMMIT_SIBLINGS (integer)

Minimum number of concurrent open transactions to require before performing the COMMIT_DELAY delay. A larger value makes it more probable that at least one other transaction will become ready to commit during the delay interval.

WAL_BUFFERS (integer)

Number of disk-page buffers in shared memory for WAL logging. The default is 4. This option can only be set at server start.

WAL_DEBUG (integer)

If nonzero, turn on WAL-related debugging output on standard error.

WAL_SYNC_METHOD (string)

Method used for forcing WAL updates out to disk. Possible values are FSYNC (call fsync() at each commit), FDATASYNC (call fdatasync() at each commit), OPEN_SYNC (write WAL files with open() option O_SYNC), or OPEN_DATASYNC (write WAL files with open() option O_DSYNC). Not all of these choices are available on all platforms. This option can only be set at server start or in the postgresql.conf file.

3.4.6. Short Options

For convenience there are also single letter option switches available for many parameters. They are described in Table 3-2.

Table 3-2. Short option key

Short option Equivalent
-B x shared_buffers = x
-d x server_min_messages = DEBUGx
-F fsync = off
-h x virtual_host = x
-i tcpip_socket = on
-k x unix_socket_directory = x
-l ssl = on
-N x max_connections = x
-p x port = x
-fi, -fh, -fm, -fn, -fs, -ft[a] enable_indexscan=off, enable_hashjoin=off, enable_mergejoin=off, enable_nestloop=off, enable_seqscan=off, enable_tidscan=off
-s[a] show_statement_stats = on
-S x[a] sort_mem = x
-tpa, -tpl, -te[a] show_parser_stats=on, show_planner_stats=on, show_executor_stats=on
Notes:
a. For historical reasons, these options must be passed to the individual backend process via the -o postmaster option, for example,
$ postmaster -o '-S 1024 -s'

or via PGOPTIONS from the client side, as explained above.

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