Are Cataloge and relation cache in PostgreSQL per connection or global to the server?

Question

I am trying to figure out if Postgres' relation and catalog cache are per connection or per server? I did find that plan caches are per connection. It would make sense for the cache to exist for as long as the server is running, but I can't find any documentation on it.

Answer 1

Is catalog and relation cache kept in shared memory or in session memory?

Both.

All relations in PostgreSQL are cached in shared memory up to the limit configured by shared_buffers parameter. This includes also system catalog (pg_class, pg_attribute, etc).

When a session is running, it builds its own internal cache in private memory for many things - including query plans, name-oid mappings etc.

On query level you can review caching effect with EXPLAIN command - see this answer.

On global level you can inspect contents of shared buffers using a standard extension named pg_buffercache.

Also, if we speak about caching you can't ignore the O/S filesystem caching machinery. See this excellent presentation.

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Answer 2

Those caches are per connection. They are not held in shared memory.