16.4. Run-time Configuration

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.

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.

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 for information on how to adjust those parameters, if necessary.

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.

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.

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.

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.

This option can only be set at server start.

Specifies the Rendezvous 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 Rendezvous support. This option can only be set at server start.

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.

Enables SSL connections. Please read Section 16.7 before using this. The default is off. This parameter can only be set at server start.

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).

Sets the location of the Kerberos server key file. See Section 19.2.3 for details.

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.

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 for information on how to adjust those parameters, if necessary.

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.

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.

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.

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. This option can only be set at server start.

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. This option can only be set at server start.

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.

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.

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.

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.

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.

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.

The accumulated cost that will cause the vacuuming process to sleep. The default value is 200.

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). The selected buffers will always be the least recently used ones among the currently dirty buffers. 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.

In each round, no more than this percentage of the currently dirty buffers will be written (rounding up any fraction to the next whole number of buffers). The default value is 1. This option can only be set at server start or in the postgresql.conf file.

In each round, no more than this many dirty buffers will be written. The default value is 100. This option can only be set at server start or in the postgresql.conf file.

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.

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), 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.

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.

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).

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.

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.

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.

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.

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. 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:

Enables or disables the query planner's use of hashed aggregation plan types. The default is on.

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

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

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

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 JOINs. 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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Provides information for use by developers.

Provides information implicitly requested by the user, e.g., during VACUUM VERBOSE.

Provides information that may be helpful to users, e.g., truncation of long identifiers and the creation of indexes as part of primary keys.

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

Reports an error that caused the current command to abort.

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

Reports an error that caused the current session to abort.

Reports an error that caused all sessions to abort.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 for more information.

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 for more information.

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 for more information.

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.

This variable specifies the order in which schemas are searched when an object (table, data type, function, etc.) is referenced by a simple name with no schema component. When there are objects of identical names in different schemas, the one found first in the search path is used. An object that is not in any of the schemas in the search path can only be referenced by specifying its containing schema with a qualified (dotted) name.

The value for search_path has to be a comma-separated list of schema names. If one of the list items is the special value $user, then the schema having the name returned by SESSION_USER is substituted, if there is such a schema. (If not, $user is ignored.)

The system catalog schema, pg_catalog, is always searched, whether it is mentioned in the path or not. If it is mentioned in the path then it will be searched in the specified order. If pg_catalog is not in the path then it will be searched before searching any of the path items. It should also be noted that the temporary-table schema, pg_temp_nnn, is implicitly searched before any of these.

When objects are created without specifying a particular target schema, they will be placed in the first schema listed in the search path. An error is reported if the search path is empty.

The default value for this parameter is '$user, public' (where the second part will be ignored if there is no schema named public). This supports shared use of a database (where no users have private schemas, and all share use of public), private per-user schemas, and combinations of these. Other effects can be obtained by altering the default search path setting, either globally or per-user.

The current effective value of the search path can be examined via the SQL function current_schemas(). This is not quite the same as examining the value of search_path, since current_schemas() shows how the requests appearing in search_path were resolved.

For more information on schema handling, see Section 5.8.

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.

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.

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 and SET TRANSACTION for more information.

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.

Abort any statement that takes over the specified number of milliseconds. A value of zero (the default) turns off the limitation.

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 for more information. The default is ISO, MDY.

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 for more information.

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.

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.

Sets the client-side encoding (character set). The default is to use the database encoding.

Sets the language in which messages are displayed. Acceptable values are system-dependent; see Section 20.1 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.

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 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.

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 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.

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 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.

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

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:

When true, tables that are referenced by a query will be automatically added to the FROM clause if not already present. The default is true for compatibility with previous releases of PostgreSQL. However, this behavior is not SQL-standard, and many people dislike it because it can mask mistakes (such as referencing a table where you should have referenced its alias). Set to false for the SQL-standard behavior of rejecting references to tables that are not listed in FROM.

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 for details.

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 for more information about inheritance.

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 8.0.1 default_with_oids defaults to true. This is also the behavior of previous versions of PostgreSQL. However, assuming that tables will contain OIDs by default is not encouraged. This option will probably default to false in a future release of PostgreSQL.

To ease compatibility with applications that make use of OIDs, this option should left enabled. To ease compatibility with future versions of PostgreSQL, this option should be disabled, and applications that require OIDs on certain tables should explicitly specify WITH OIDS when those tables are created.

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 SQL-spec-compliant 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 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 for related information.