This is one of the important topics for PostgreSQL Database Administrator.
Yes, it is true that PostgreSQL has a caching system.

But I didn’t find any perfect solution for clearing the cache of PostgreSQL Server.
If you want to clear cache, you should restart the PostgreSQL Server.

In this post, I am sharing a short note on PostgreSQL Discard functionality to discard temp, the query plan for running sessions.

Using DISCARD, you can release internal resources of the Database Sessions.

DISCARD TEMP: Using this, you can drop all temporary tables which are created in the current session.

DISCARD PLANS: Using this, you can release all internal cached query plans.

DISCARD ALL: Using this, you can reset a session to its original state. You cannot execute this command inside the transaction block.

Once you have executed DISCARD ALL, internally following sequence runs.

SET SESSION AUTHORIZATION DEFAULT;
RESET ALL;
DEALLOCATE ALL;
CLOSE ALL;
UNLISTEN *;
SELECT pg_advisory_unlock_all();
DISCARD PLANS;
DISCARD TEMP;