## Chapter 17. Server Configuration

There are many configuration parameters that affect the behavior of the database system. In the first section of this chapter, we describe how to set configuration parameters. The subsequent sections discuss each parameter in detail.

## 17.1. Setting Parameters

All parameter names are case-insensitive. Every parameter takes a value of one of four types: Boolean, integer, floating point, or string. Boolean values may be written as `ON`, `OFF`, `TRUE`, `FALSE`, `YES`, `NO`, `1`, `0` (all case-insensitive) or any unambiguous prefix of these.

One way to set these parameters is to edit the file `postgresql.conf`, which is normally kept in the data directory. (`initdb` installs a default copy there.) An example of what this file might look like is:

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

One parameter is specified per line. The equal sign between name and value is optional. Whitespace is insignificant and blank lines are ignored. Hash marks (`#`) introduce comments anywhere. Parameter values that are not simple identifiers or numbers must be single-quoted. To embed a single quote in a parameter value, write either two quotes (preferred) or backslash-quote.

The configuration file is reread whenever the `postmaster` process receives a SIGHUP signal (which is most easily sent by means of `pg_ctl reload`). The `postmaster` also propagates this signal to all currently running server processes so that existing sessions also get the new value. Alternatively, you can send the signal to a single server process directly. Some parameters can only be set at server start; any changes to their entries in the configuration file will be ignored until the server is restarted.

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

    postmaster -c log_connections=yes -c log_destination='syslog'

Command-line options override any conflicting settings in `postgresql.conf`. Note that this means you won't be able to change the value on-the-fly by editing `postgresql.conf`, so while the command-line method may be convenient, it can cost you flexibility later.

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

    env PGOPTIONS='-c geqo=off' psql

(This works for any libpq-based client application, not just psql.) Note that this won't work for parameters that are fixed when the server is started or that must be specified in `postgresql.conf`.

Furthermore, it is possible to assign a set of option settings to a user or a database. Whenever a session is started, the default settings for the user and database involved are loaded. The commands ALTER USER and ALTER DATABASE, respectively, are used to configure these settings. Per-database settings override anything received from the `postmaster` command-line or the configuration file, and in turn are overridden by per-user settings; both are overridden by per-session options.

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

    SET ENABLE_SEQSCAN TO OFF;

If `SET` is allowed, it overrides all other sources of values for the parameter. Some parameters cannot be changed via `SET`: for example, if they control behavior that cannot reasonably be changed without restarting PostgreSQL. Also, some parameters can be modified via `SET` or `ALTER` by superusers, but not by ordinary users.

The SHOW command allows inspection of the current values of all parameters.

The virtual table `pg_settings` (described in Section 42.41, “`pg_settings`”) also allows displaying and updating session run-time parameters. It is equivalent to `SHOW` and `SET`, but can be more convenient to use because it can be joined with other tables, or selected from using any desired selection condition.