Recall from Chapter 19 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 within a particular database, you can execute the command:
ALTER DATABASE mydb SET geqo TO off;
This will save the setting (but not set it immediately). In subsequent connections to this database it will appear as though SET geqo TO off; had been executed just before the session started. Note that users can still alter this setting during their sessions; it will only be the default. To undo any such setting, use ALTER DATABASE .dbname RESET varname