Way to quickly detect if database tables/columns/etc. were modified?

[Evan Martin <postgresql2@xxxxxxxxxxxxxxxxx>]

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).



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