How do I set and get custom database variables? Using PGAdmin III I can right-click on a database, navigate to the Variables tab, and put a variable name-value property on the database itself.  Is there a way to customize these?  I saw an application_name variable, but I'd like to have an application_version variable.
You have to add the variable to the end of your postgresql.conf like

#------------------------------------------------------------------------------
# CUSTOMIZED OPTIONS
#------------------------------------------------------------------------------

custom_variable_classes = 'general'     # list of custom variable class names

general.application_version = 'v1.0'


When restarted, you can add general.application_version manually in PgAdmin (it will not show up in the drop-down - at least not for me).  Otherwise, you can use it just like any other PostgreSQL variable.

Update for version 9.2 and newer

From PostgreSQL 9.2 onwards, we don't have to set custom_variable_classes anymore.  One can set whatever variable they want - the only limitation seems to be it still has to have two parts:

SET something TO 'bla';
ERROR:  unrecognized configuration parameter "something"

SET thing.something TO 'bla';
SET


I guess this is so to avoid collision with the builtin parameters.