If I have a query that reads from system tables like pg_class,
pg_namespace, pg_attribute, pg_type, etc. and I'd like to cache the
results in my application is there any fast way to detect when any
changes have been made to these system catalogs? I don't need to know
exactly what has changed. Some kind of a global "database version" would
do, just so I know that I need to invalidate my cache (the database
definition is rarely modified in practice).