PostgreSQL 9.0.7 Documentation
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# 18.14. Customized Options

This feature was designed to allow parameters not normally known to PostgreSQL to be added by add-on modules (such as procedural languages). This allows add-on modules to be configured in the standard ways.

`custom_variable_classes` (`string`)

  This variable specifies one or several class names to be used for custom variables, in the form of a comma-separated list. A custom variable is a variable not normally known to PostgreSQL proper but used by some add-on module. Such variables must have names consisting of a class name, a dot, and a variable name. `custom_variable_classes` specifies all the class names in use in a particular installation. This parameter can only be set in the `postgresql.conf` file or on the server command line.

The difficulty with setting custom variables in `postgresql.conf` is that the file must be read before add-on modules have been loaded, and so custom variables would ordinarily be rejected as unknown. When `custom_variable_classes` is set, the server will accept definitions of arbitrary variables within each specified class. These variables will be treated as placeholders and will have no function until the module that defines them is loaded. When a module for a specific class is loaded, it will add the proper variable definitions for its class name, convert any placeholder values according to those definitions, and issue warnings for any unrecognized placeholders of its class that remain.

Here is an example of what `postgresql.conf` might contain when using custom variables:

    custom_variable_classes = 'plpgsql,plperl'
    plpgsql.variable_conflict = use_variable
    plperl.use_strict = true
    plruby.use_strict = true        # generates error: unknown class name

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