# Server Run-time Environment

This chapter discusses how to set up and run the database server and the interactions with the operating system.

## The PostgreSQL User Account

As with any other server daemon that is connected to outside world, it is advisable to run PostgreSQL under a separate user account. This user account should only own the data that is managed by the server, and should not be shared with other daemons. (For example, using the user nobody is a bad idea.) It is not advisable to install executables owned by this user because compromised systems could then modify their own binaries.

To add a Unix user account to your system, look for a command `useradd` or `adduser`. The user name `postgres` is often used but is by no means required.

## Creating a Database Cluster

Before you can do anything, you must initialize a database storage area on disk. We call this a database cluster. (SQL uses the term catalog cluster instead.) A database cluster is a collection of databases is accessible by a single instance of a running database server. After initialization, a database cluster will contain a database named `template1`. As the name suggests, this will be used as a template for subsequently created databases; it should not be used for actual work. (See managing-databases for information about creating databases.)

In file system terms, a database cluster will be a single directory under which all data will be stored. We call this the data directory or data area. It is completely up to you where you choose to store your data. There is no default, although locations such as `/usr/local/pgsql/data` or `/var/lib/pgsql/data` are popular. To initialize a database cluster, use the command `initdb`, which is installed with PostgreSQL. The desired file system location of your database system is indicated by the `-D` option, for example

    $ initdb -D /usr/local/pgsql/data

Note that you must execute this command while logged into the PostgreSQL user account, which is described in the previous section.

As an alternative to the `-D` option, you can set the environment variable `PGDATA`.

`initdb` will attempt to create the directory you specify if it does not already exist. It is likely that it will not have the permission to do so (if you followed our advice and created an unprivileged account). In that case you should create the directory yourself (as root) and change the owner to be the PostgreSQL user. Here is how this might be done:

    root# mkdir /usr/local/pgsql/data
    root# chown postgres /usr/local/pgsql/data
    root# su postgres
    postgres$ initdb -D /usr/local/pgsql/data

`initdb` will refuse to run if the data directory looks like it it has already been initialized.

Because the data directory contains all the data stored in the database, it is essential that it be secured from unauthorized access. `initdb` therefore revokes access permissions from everyone but the PostgreSQL user.

However, while the directory contents are secure, the default client authentication setup allows any local user to connect to the database and even become the database superuser. If you don't trust other local users, we recommend you use `initdb`'s `-W` or `--pwprompt` option to assign a password to the database superuser. After `initdb`, modify the `pg_hba.conf` file to use `md5` or `password` instead of `trust` authentication before you start the server for the first time. (Other, approaches include using `ident` authentication or file system permissions to restrict connections. See client-authentication for more information.)

`initdb` also initializes the default locale for the database cluster. Normally, it will just take the locale settings in the environment and apply them to the initialized database. It is possible to specify a different locale for the database; more information about that can be found in locale. One surprise you might encounter while running `initdb` is a notice similar to this:

    The database cluster will be initialized with locale de_DE.
    This locale setting will prevent the use of indexes for pattern matching
    operations.  If that is a concern, rerun initdb with the collation order
    set to "C".  For more information see the Administrator's Guide.

This is intended to warn you that the currently selected locale will cause indexes to be sorted in an order that prevents them from being used for `LIKE` and regular-expression searches. If you need good performance in such searches, you should set your current locale to `C` and re-run `initdb`, e.g., by running `initdb --lc-collate=C`. The sort order used within a particular database cluster is set by `initdb` and cannot be changed later, short of dumping all data, rerunning `initdb`, and reloading the data. So it's important to make this choice correctly the first time.

## Starting the Database Server

Before anyone can access the database, you must start the database server. The database server is called postmaster. The postmaster must know where to find the data it is supposed to use. This is done with the `-D` option. Thus, the simplest way to start the server is:

    $ postmaster -D /usr/local/pgsql/data

which will leave the server running in the foreground. This must be done while logged into the PostgreSQL user account. Without `-D`, the server will try to use the data directory in the environment variable `PGDATA`. If neither of these succeed, it will fail.

To start the postmaster in the background, use the usual shell syntax:

    $ postmaster -D /usr/local/pgsql/data > logfile 2>&1 &

It is an important to store the server's `stdout` and `stderr` output somewhere, as shown above. It will help for auditing purposes and to diagnose problems. (See logfile-maintenance for a more thorough discussion of log file handling.)

The postmaster also takes a number of other command line options. For more information, see the reference page and runtime-config below. In particular, in order for the server to accept TCP/IP connections (rather than just Unix domain socket ones), you must specify the `-i` option.

This shell syntax can get tedious quickly. Therefore the shell script wrapper pg_ctl is provided to simplify some tasks. For example:

    pg_ctl start -l logfile

will start the server in the background and put the output into the named log file. The `-D` option has the same meaning here as in the postmaster. pg_ctl is also capable of stopping the server.

Normally, you will want to start the database server when the computer boots. Autostart scripts are operating system-specific. There are a few distributed with PostgreSQL in the `/contrib/start-scripts` directory. This may require root privileges.

Different systems have different conventions for starting up daemons at boot time. Many systems have a file `/etc/rc.local` or `/etc/rc.d/rc.local`. Others use `rc.d` directories. Whatever you do, the server must be run by the PostgreSQL user account and not by root or any other user. Therefore you probably should form your commands using `su -c '...' postgres`. For example:

    su -c 'pg_ctl start -D /usr/local/pgsql/data -l serverlog' postgres

Here are a few more operating system specific suggestions. (Always replace these with the proper installation directory and the user name.)

- For FreeBSD, look at the file `contrib/start-scripts/freebsd` in the PostgreSQL source distribution.

- On OpenBSD, add the following lines to the file `/etc/rc.local`:

      if [ -x /usr/local/pgsql/bin/pg_ctl -a -x /usr/local/pgsql/bin/postmaster ]; then
          su - -c '/usr/local/pgsql/bin/pg_ctl start -l /var/postgresql/log -s' postgres
          echo -n ' postgresql'
      fi

- On Linux systems either add

      /usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data

  to `/etc/rc.d/rc.local` or look at the file `contrib/start-scripts/linux` in the PostgreSQL source distribution.

- On NetBSD, either use the FreeBSD or Linux start scripts, depending on preference.

- On Solaris, create a file called `/etc/init.d/postgresql` which should contain the following line:

      su - postgres -c "/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data"

  Then, create a symbolic link to it in `/etc/rc3.d` as `S99postgresql`.

While the postmaster is running, its PID is in the file `postmaster.pid` in the data directory. This is used to prevent multiple postmasters running in the same data directory, and can also be used for shutting down the postmaster.

### Server Start-up Failures

There are several common reasons the postmaster might fail to start. Check the postmaster's log file, or start it by hand (without redirecting standard output or standard error) and see what error messages appear. Some of the error messages are self-explanatory, but some are not, as shown below:

    FATAL: StreamServerPort: bind() failed: Address already in use
            Is another postmaster already running on that port?

This usually means just what it suggests: you tried to start another postmaster on the same port where one is already running. However, if the kernel error message is not `Address already in use` or some variant of that, there may be a different problem. For example, trying to start a postmaster on a reserved port number may draw something like:

    $ postmaster -i -p 666
    FATAL: StreamServerPort: bind() failed: Permission denied
            Is another postmaster already running on that port?

A message like:

    IpcMemoryCreate: shmget(key=5440001, size=83918612, 01600) failed: Invalid argument
    FATAL 1:  ShmemCreate: cannot create region

probably means your kernel's limit on the size of shared memory is smaller than the buffer area PostgreSQL is trying to create (83918612 bytes in this example). Or it could mean that you don't have System-V-style shared memory support configured into your kernel at all. As a temporary workaround, you can try starting the postmaster with a smaller-than-normal number of buffers (`-B` switch). You will eventually want to reconfigure your kernel to increase the allowed shared memory size. You may see this message when trying to start multiple postmasters on the same machine if their total space requested exceeds the kernel limit.

An error like:

    IpcSemaphoreCreate: semget(key=5440026, num=16, 01600) failed: No space left on device

does not mean you've run out of disk space. It means your kernel's limit on the number of `System V` semaphores is smaller than the number PostgreSQL wants to create. As above, you may be able to work around the problem by starting the postmaster with a reduced number of allowed connections (`-N` switch), but you'll eventually want to increase the kernel limit.

If you get an illegal system call error, it is likely that shared memory or semaphores are not supported in your kernel at all. In that case your only option is to reconfigure the kernel to enable these features.

Details about configuring `System V` IPC facilities are given in sysvipc.

### Client Connection Problems

Although the error conditions possible on the client side are quite varied and application-dependent, a few of them might be directly related to how the server was started up. Conditions other than those shown below should be documented with the respective client application.

    psql: could not connect to server: Connection refused
            Is the server running on host server.joe.com and accepting
            TCP/IP connections on port 5432?

This is the generic I couldn't find a server to talk to failure. It looks like the above when TCP/IP communication is attempted. A common mistake is to forget the `-i` option to allow the postmaster to accept TCP/IP connections.

Alternatively, you'll get this when attempting Unix-socket communication to a local postmaster:

    psql: could not connect to server: Connection refused
            Is the server running locally and accepting
            connections on Unix domain socket "/tmp/.s.PGSQL.5432"?

The last line is useful in verifying that the client is trying to connect to the right place. If there is in fact no postmaster running there, the kernel error message will typically be either `Connection refused` or `No such file or directory`, as illustrated. (It is important to realize that `Connection refused` in this context does not mean that the postmaster got your connection request and rejected it -- that case will produce a different message, as shown in client-authentication-problems.) Other error messages such as `Connection timed out` may indicate more fundamental problems, like lack of network connectivity.

## 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.

### 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`.

### 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

### 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.

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.

### 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.

### 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.

  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.

  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.

`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.

  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 &cite-user; 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 kerberos-auth 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 doing `fsync`s is considerably less in PostgreSQL version 7.1 and later. If you previously suppressed `fsync`s 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 locale 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 locale 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 locale 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 locale 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. 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 the &cite-user;.

`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 sysvipc 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 &cite-user; for more information about inheritance.

`SSL` (`boolean`)

- Enables SSL connections. Please read ssl-tcp 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 &cite-user; 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 client-authentication.

  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`).

### WAL

See also wal-configuration 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.

### Short Options

For convenience there are also single letter option switches available for many parameters. They are described in runtime-config-short-table.

### Short option key

Short option | Equivalent
`-B `x`` | `shared_buffers = `x``
`-d `x`` | `server_min_messages = DEBUG`x``
`-F` | `fsync = off`
`-h `x`` | `virtual_host = `x``
`-i` | `tcpip_socket = on`
`-k `x`` | `unix_socket_directory = `x``
`-l` | `ssl = on`
`-N `x`` | `max_connections = `x``
`-p `x`` | `port = `x``
`-fi`, `-fh`, `-fm`, `-fn`, `-fs`, `-ft` 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. | `enable_indexscan=off`, `enable_hashjoin=off`, `enable_mergejoin=off`, `enable_nestloop=off`, `enable_seqscan=off`, `enable_tidscan=off`
`-s` | `show_statement_stats = on`
`-S `x`` | `sort_mem = `x``
`-tpa`, `-tpl`, `-te` | `show_parser_stats=on`, `show_planner_stats=on`, `show_executor_stats=on`

## Managing Kernel Resources

A large PostgreSQL installation can quickly exhaust various operating system resource limits. (On some systems, the factory defaults are so low that you don't even need a really large installation.) If you have encountered this kind of problem, keep reading.

### Shared Memory and Semaphores

Shared memory and semaphores are collectively referred to as `System V` IPC (together with message queues, which are not relevant for PostgreSQL). Almost all modern operating systems provide these features, but not all of them have them turned on or sufficiently sized by default, especially systems with BSD heritage. (For the `QNX` and `BeOS` ports, PostgreSQL provides its own replacement implementation of these facilities.)

The complete lack of these facilities is usually manifested by an Illegal system call error upon postmaster start. In that case there's nothing left to do but to reconfigure your kernel -- PostgreSQL won't work without them.

When PostgreSQL exceeds one of the various hard IPC limits, the postmaster will refuse to start and should leave an instructive error message describing the problem encountered and what to do about it. (See also postmaster-start-failures.) The relevant kernel parameters are named consistently across different systems; sysvipc-parameters gives an overview. The methods to set them, however, vary. Suggestions for some platforms are given below. Be warned that it is often necessary to reboot your machine, and possibly even recompile the kernel, to change these settings.

### `System V` IPC parameters

Name | Description | Reasonable values
`SHMMAX` | Maximum size of shared memory segment (bytes) | 250kB + 8.2 kB * `shared_buffers` + 14.2 kB * `max_connections` or infinity
`SHMMIN` | Minimum size of shared memory segment (bytes) | 1
`SHMALL` | Total amount of shared memory available (bytes or pages) | if bytes, same as `SHMMAX`; if pages, `ceil(SHMMAX/PAGE_SIZE)`
`SHMSEG` | Maximum number of shared memory segments per process | only 1 segment is needed, but the default is much higher
`SHMMNI` | Maximum number of shared memory segments system-wide | like `SHMSEG` plus room for other applications
`SEMMNI` | Maximum number of semaphore identifiers (i.e., sets) | `>= ceil(max_connections / 16)`
`SEMMNS` | Maximum number of semaphores system-wide | `ceil(max_connections / 16) * 17` + room for other applications
`SEMMSL` | Maximum number of semaphores per set | >= 17
`SEMMAP` | Number of entries in semaphore map | see text
`SEMVMX` | Maximum value of semaphore | >= 255 (The default is often 32767, don't change unless asked to.)

The most important shared memory parameter is `SHMMAX`, the maximum size, in bytes, of a shared memory segment. If you get an error message from `shmget` like Invalid argument, it is possible that this limit has been exceeded. The size of the required shared memory segment varies both with the number of requested buffers (`-B` option) and the number of allowed connections (`-N` option), although the former is the most significant. (You can, as a temporary solution, lower these settings to eliminate the failure.) As a rough approximation, you can estimate the required segment size by multiplying the number of buffers and the block size (8 kB by default) plus ample overhead (at least half a megabyte). Any error message you might get will contain the size of the failed allocation request.

Less likely to cause problems is the minimum size for shared memory segments (`SHMMIN`), which should be at most approximately 256 kB for PostgreSQL (it is usually just 1). The maximum number of segments system-wide (`SHMMNI`) or per-process (`SHMSEG`) should not cause a problem unless your system has them set to zero. Some systems also have a limit on the total amount of shared memory in the system; see the platform-specific instructions below.

PostgreSQL uses one semaphore per allowed connection (`-N` option), in sets of 16. Each such set will also contain a 17th semaphore which contains a magic number, to detect collision with semaphore sets used by other applications. The maximum number of semaphores in the system is set by `SEMMNS`, which consequently must be at least as high as the connection setting plus one extra for each 16 allowed connections (see the formula in sysvipc-parameters). The parameter `SEMMNI` determines the limit on the number of semaphore sets that can exist on the system at one time. Hence this parameter must be at least `ceil(max_connections / 16)`. Lowering the number of allowed connections is a temporary workaround for failures, which are usually confusingly worded No space left on device, from the function `semget()`.

In some cases it might also be necessary to increase `SEMMAP` to be at least on the order of `SEMMNS`. This parameter defines the size of the semaphore resource map, in which each contiguous block of available semaphores needs an entry. When a semaphore set is freed it is either added to an existing entry that is adjacent to the freed block or it is registered under a new map entry. If the map is full, the freed semaphores get lost (until reboot). Fragmentation of the semaphore space could over time lead to fewer available semaphores than there should be.

The `SEMMSL` parameter, which determines how many semaphores can be in a set, must be at least 17 for PostgreSQL.

Various other settings related to semaphore undo, such as `SEMMNU` and `SEMUME`, are not of concern for PostgreSQL.

`BSD/OS`

- ### Shared Memory

  By default, only 4 MB of shared memory is supported. Keep in mind that shared memory is not pageable; it is locked in RAM. To increase the number of shared buffers supported by the postmaster, add the following to your kernel configuration file. A `SHMALL` value of 1024 represents 4 MB of shared memory. The following increases the maximum shared memory area to 32 MB:

      options "SHMALL=8192"
      options "SHMMAX=\(SHMALL*PAGE_SIZE\)"

  For those running 4.1 or later, just make the above changes, recompile the kernel, and reboot. For those running earlier releases, use `bpatch` to find the `sysptsize` value in the current kernel. This is computed dynamically at boot time.

      $ bpatch -r sysptsize
      0x9 = 9

  Next, add `SYSPTSIZE` as a hard-coded value in the kernel configuration file. Increase the value you found using bpatch. Add 1 for every additional 4 MB of shared memory you desire.

      options "SYSPTSIZE=16"

  `sysptsize` cannot be changed by `sysctl`.

  ### Semaphores

  You may need to increase the number of semaphores. By default, PostgreSQL allocates 34 semaphores, which is over half the default system total of 60.

  Set the values you want in your kernel configuration file, e.g.:

      options "SEMMNI=40"
      options "SEMMNS=240"
      options "SEMUME=40"
      options "SEMMNU=120"

`FreeBSD`

`NetBSD`

`OpenBSD`

- The options `SYSVSHM` and `SYSVSEM` need to be enabled when the kernel is compiled. (They are by default.) The maximum size of shared memory is determined by the option `SHMMAXPGS` (in pages). The following shows an example of how to set the various parameters:

      options         SYSVSHM
      options         SHMMAXPGS=4096
      options         SHMSEG=256

      options         SYSVSEM
      options         SEMMNI=256
      options         SEMMNS=512
      options         SEMMNU=256
      options         SEMMAP=256

  (On `NetBSD` and `OpenBSD` the key word is actually `option` singular.)

  You may also want to use the sysctl setting to lock shared memory into RAM and prevent it from being paged out to swap.

`HP-UX`

- The default settings tend to suffice for normal installations. On HP-UX 10, the factory default for `SEMMNS` is 128, which might be too low for larger database sites.

  IPC parameters can be set in the System Administration Manager (SAM) under Kernel Configuration Configurable Parameters. Hit Create A New Kernel when you're done.

`Linux`

- The default shared memory limit (both `SHMMAX` and `SHMALL`) is 32 MB in 2.2 kernels, but it can be changed in the `proc` file system (without reboot). For example, to allow 128 MB:

      $ echo 134217728 >/proc/sys/kernel/shmall
      $ echo 134217728 >/proc/sys/kernel/shmmax

  You could put these commands into a script run at boot-time.

  Alternatively, you can use

  sysctl

  8, if available, to control these parameters. Look for a file called `/etc/sysctl.conf` and add lines like the following to it:

      kernel.shmall = 134217728
      kernel.shmmax = 134217728

  This file is usually processed at boot time, but sysctl can also be called explicitly later.

  Other parameters are sufficiently sized for any application. If you want to see for yourself look in `/usr/src/linux/include/asm-`xxx`/shmpara m.h` and `/usr/src/linux/include/linux/sem.h`.

`MacOS X`

- Edit the file `/System/Library/StartupItems/SystemTuning/SystemTuning` and change the following values:

      sysctl -w kern.sysv.shmmax
      sysctl -w kern.sysv.shmmin
      sysctl -w kern.sysv.shmmni
      sysctl -w kern.sysv.shmseg
      sysctl -w kern.sysv.shmall

  These values have the same meanings on MacOS X as those listed for previous operating systems.

`SCO OpenServer`

- In the default configuration, only 512 kB of shared memory per segment is allowed, which is about enough for `-B 24 -N 12`. To increase the setting, first change directory to `/etc/conf/cf.d`. To display the current value of `SHMMAX`, in bytes, run

      ./configure -y SHMMAX

  To set a new value for `SHMMAX`, run:

      ./configure SHMMAX=value

  where `value` is the new value you want to use (in bytes). After setting `SHMMAX`, rebuild the kernel

      ./link_unix

  and reboot.

`Solaris`

- At least in version 2.6, the default maximum size of a shared memory segments is too low for PostgreSQL. The relevant settings can be changed in `/etc/system`, for example:

      set shmsys:shminfo_shmmax=0x2000000
      set shmsys:shminfo_shmmin=1
      set shmsys:shminfo_shmmni=256
      set shmsys:shminfo_shmseg=256

      set semsys:seminfo_semmap=256
      set semsys:seminfo_semmni=512
      set semsys:seminfo_semmns=512
      set semsys:seminfo_semmsl=32

  You need to reboot for the changes to take effect.

  See also for information on shared memory under Solaris.

`UnixWare`

- On UnixWare 7, the maximum size for shared memory segments is 512 kB in the default configuration. This is enough for about `-B 24 -N 12`. To display the current value of `SHMMAX`, run

      /etc/conf/bin/idtune -g SHMMAX

  which displays the current, default, minimum, and maximum values, in bytes. To set a new value for `SHMMAX`, run:

      /etc/conf/bin/idtune SHMMAX value

  where `value` is the new value you want to use (in bytes). After setting `SHMMAX`, rebuild the kernel

      /etc/conf/bin/idbuild -B

  and reboot.

### Resource Limits

Unix-like operating systems enforce various kinds of resource limits that might interfere with the operation of your PostgreSQL server. Of particular importance are limits on the number of processes per user, the number of open files per process, and the amount of memory available to each process. Each of these have a hard and a soft limit. The soft limit is what actually counts but it can be changed by the user up to the hard limit. The hard limit can only be changed by the root user. The system call `setrlimit` is responsible for setting these parameters. The shell's built-in command `ulimit` (Bourne shells) or `limit` (csh) is used to control the resource limits from the command line. On BSD-derived systems the file `/etc/login.conf` controls the various resource limits set during login. See

login.conf

5 for details. The relevant parameters are `maxproc`, `openfiles`, and `datasize`. For example:

    default:\
    ...
            :datasize-cur=256M:\
            :maxproc-cur=256:\
            :openfiles-cur=256:\
    ...

(`-cur` is the soft limit. Append `-max` to set the hard limit.)

Kernels can also have system-wide limits on some resources.

- On Linux `/proc/sys/fs/file-max` determines the maximum number of open files that the kernel will support. It can be changed by writing a different number into the file or by adding an assignment in `/etc/sysctl.conf`. The maximum limit of files per process is fixed at the time the kernel is compiled; see `/usr/src/linux/Documentation/proc.txt` for more information.

The PostgreSQL server uses one process per connection so you should provide for at least as many processes as allowed connections, in addition to what you need for the rest of your system. This is usually not a problem but if you run several servers on one machine things might get tight.

The factory default limit on open files is often set to socially friendly values that allow many users to coexist on a machine without using an inappropriate fraction of the system resources. If you run many servers on a machine this is perhaps what you want, but on dedicated servers you may want to raise this limit.

On the other side of the coin, some systems allow individual processes to open large numbers of files; if more than a few processes do so then the system-wide limit can easily be exceeded. If you find this happening, and don't want to alter the system-wide limit, you can set PostgreSQL's `max_files_per_process` configuration parameter to limit the consumption of open files.

## Shutting Down the Server

There are several ways to shut down the database server. You control the type of shutdown by sending different signals to the server process.

`SIGTERM`

- After receiving `SIGTERM`, the postmaster disallows new connections, but lets existing backends end their work normally. It shuts down only after all of the backends terminate normally. This is Smart Shutdown.

`SIGINT`

- The postmaster disallows new connections and sends all existing backends `SIGTERM`, which will cause them to abort their current transactions and exit promptly. It then waits for the backends to exit and finally shuts down. This is Fast Shutdown.

`SIGQUIT`

- This is Immediate Shutdown, which will cause the postmaster to send a `SIGQUIT` to all backends and exit immediately (without properly shutting itself down). The backends likewise exit immediately upon receiving `SIGQUIT`. This will lead to recovery (by replaying the WAL log) upon next start-up. This is recommended only in emergencies.

It is best not to use `SIGKILL` to shut down the postmaster. This will prevent the postmaster from releasing shared memory and semaphores, which may then have to be done by manually.

The PID of the postmaster process can be found using the ps program, or from the file `postmaster.pid` in the data directory. So for example, to do a fast shutdown:

    $ kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`

The program pg_ctl is a shell script that provides a more convenient interface for shutting down the postmaster.

## Secure TCP/IP Connections with SSL

PostgreSQL has native support for using SSL connections to encrypt client/server communications for increased security. This requires OpenSSL be installed on both client and server systems and support enabled at build time (see installation).

With SSL support compiled in, the PostgreSQL server can be started with SSL support by setting the parameter `ssl` to on in `postgresql.conf`. When starting in SSL mode, the server will look for the files `server.key` and `server.crt` in the data directory. These files should contain the server private key and certificate respectively. These files must be set up correctly before an SSL-enabled server can start. If the private key is protected with a passphrase, the server will prompt for the passphrase and will not start until it has been entered.

The server will listen for both standard and SSL connections on the same TCP/IP port, and will negotiate with any connecting client on whether to use SSL. See client-authentication about how to force the server to require use of SSL for certain connections.

For details on how to create your server private key and certificate, refer to the OpenSSL documentation. A simple self-signed certificate can be used to get started for testing, but a certificate signed by a certificate authority (CA) (either one of the global CAs or a local one) should be used in production so the client can verify the server's identity. To create a quick self-signed certificate, use the following OpenSSL command:

    openssl req -new -text -out server.req

Fill out the information that `openssl` asks for. Make sure that you enter the local host name as Common Name; the challenge password can be left blank. The script will generate a key that is passphrase protected; it will not accept a passphrase that is less than four characters long. To remove the passphrase (as you must if you want automatic start-up of the server), run the commands

    openssl rsa -in privkey.pem -out server.key
    rm privkey.pem

Enter the old passphrase to unlock the existing key. Now do

    openssl req -x509 -in server.req -text -key server.key -out server.crt
    chmod og-rwx server.key

to turn the certificate into a self-signed certificate and to copy the key and certificate to where the server will look for them.

## Secure TCP/IP Connections with SSH Tunnels

### Acknowledgement

Idea taken from an email by Gene Selkov, Jr. (selkovjr@mcs.anl.gov) written on 1999-09-08 in response to a question from Eric Marsden.

One can use SSH to encrypt the network connection between clients and a PostgreSQL server. Done properly, this provides an adequately secure network connection.

First make sure that an SSH server is running properly on the same machine as PostgreSQL and that you can log in using `ssh` as some user. Then you can establish a secure tunnel with a command like this from the client machine:

    ssh -L 3333:foo.com:5432 joe@foo.com

The first number in the `-L` argument, 3333, is the port number of your end of the tunnel; it can be chosen freely. The second number, 5432, is the remote end of the tunnel -- the port number your server is using. The name or the address in between the port numbers is the host with the database server you are going to connect to. In order to connect to the database server using this tunnel, you connect to port 3333 on the local machine:

    psql -h localhost -p 3333 template1

To the database server it will then look as though you are really user `joe@foo.com` and it will use whatever authentication procedure was set up for this user. In order for the tunnel setup to succeed you must be allowed to connect via `ssh` as `joe@foo.com`, just as if you had attempted to use `ssh` to set up a terminal session.

Several other applications exist that can provide secure tunnels using a procedure similar in concept to the one just described.