Recall from Section 16.4 that the PostgreSQL server provides a large number of run-time configuration variables. You can set database-specific default values for many of these settings.
For example, if for some reason you want to disable the GEQO optimizer for a given database, you'd ordinarily have to either disable it for all databases or make sure that every connecting client is careful to issue SET geqo TO off;. To make this setting the default you can execute the command
ALTER DATABASE mydb SET geqo TO off;
This will save the setting (but not set it immediately) and in subsequent connections it will appear as though SET geqo TO off; had been called right before the session started. Note that users can still alter this setting during the session; it will only be the default. To undo any such setting, use ALTER DATABASE dbname RESET varname;.