Postgres-XC 1.1 Documentation
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# 17.1. Setting Parameters

## 17.1.1. Parameter Names and Values

All parameter names are case-insensitive. Every parameter takes a value of one of five types: Boolean, integer, floating point, string or enum. Boolean values can be written as `on`, `off`, `true`, `false`, `yes`, `no`, `1`, `0` (all case-insensitive) or any unambiguous prefix of these.

Some settings specify a memory or time value. Each of these has an implicit unit, which is either kilobytes, blocks (typically eight kilobytes), milliseconds, seconds, or minutes. Default units can be found by referencing `pg_settings`.`unit`. For convenience, a different unit can also be specified explicitly. Valid memory units are `kB` (kilobytes), `MB` (megabytes), and `GB` (gigabytes); valid time units are `ms` (milliseconds), `s` (seconds), `min` (minutes), `h` (hours), and `d` (days). Note that the multiplier for memory units is 1024, not 1000.

Parameters of type "enum" are specified in the same way as string parameters, but are restricted to a limited set of values. The allowed values can be found from `pg_settings`.`enumvals`. Enum parameter values are case-insensitive.

## 17.1.2. Setting Parameters via the Configuration File

One way to set these parameters is to edit the file `postgresql.conf`, which is normally kept in the data directory. (A default copy is installed there when the database cluster directory is initialized.) An example of what this file might look like is:

    # This is a comment
    log_connections = yes
    log_destination = 'syslog'
    search_path = '"$user", public'
    shared_buffers = 128MB

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 (`#`) designate the remainder of the line as a comment. 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.

In addition to parameter settings, the `postgresql.conf` file can contain include directives, which specify another file to read and process as if it were inserted into the configuration file at this point. This feature allows a configuration file to be divided into physically separate parts. Include directives simply look like:

    include 'filename'

If the file name is not an absolute path, it is taken as relative to the directory containing the referencing configuration file. Inclusions can be nested.

There is also an `include_if_exists` directive, which acts the same as the `include` directive, except for the behavior when the referenced file does not exist or cannot be read. A regular `include` will consider this an error condition, but `include_if_exists` merely logs a message and continues processing the referencing configuration file.

The configuration file is reread whenever the main coordinator/datanode process receives a SIGHUP signal (which is most easily sent by means of `pg_ctl reload`). The main server process also propagates this signal to all currently running coordinator/datanode 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 start; any changes to their entries in the configuration file will be ignored until it is restarted. Invalid parameter settings in the configuration file are likewise ignored (but logged) during SIGHUP processing.

## 17.1.3. Other Ways to Set Parameters

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

    postgres --coordinator -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 might 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 parameter 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 ROLE and ALTER DATABASE, respectively, are used to configure these settings. Per-database settings override anything received from the `postgres` command-line or the configuration file, and in turn are overridden by per-user settings; both are overridden by per-session settings.

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 be changed without restarting the entire coordinator/datanode. Also, some parameters require superuser permission to change via `SET` or `ALTER`.

## 17.1.4. Examining Parameter Settings

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

The virtual table `pg_settings` also allows displaying and updating session run-time parameters; see Section 47.67 for details and a description of the different variable types and when they can be changed. `pg_settings` 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. It also contains more information about each parameter than is available from `SHOW`.

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