17.14. 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.

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 placeholders of its class that remain (which presumably would be misspelled configuration variables).

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

custom_variable_classes = 'plr,plperl'
plr.path = '/usr/lib/R'
plperl.use_strict = true
plruby.use_strict = true        # generates error: unknown class name