It means if I run VACUUM ANALYZE on pg_attribute or pg_class table, it is completely safe? Can you please also please confirm what is meant by "mid-level" vacuum?
To clarify my last message, VACUUM ANALYZE is a trade-off between autovacuum, that can be considered as a lazy non-blocking operation, and VACUUM FULL (eager & blocking one).
The `mid-level` in my previous mail was used to pinpoint an intermediate blacking & resource consumption situation.
VACUUM ANALYZE will:
- remove dead tuples definition
- refresh statistics (can improve execution plans for queries)
Have a look at this website, it explains that better than me ;) :
I have a question that autovacuum is running on sys tables like pg_class, pg_attribute, is it a normal thing? Further, what is dead tuples are not removed from Autovacuum, can we remove the dead tuples using FULL VACUUMM or pg_repack on sys tables?
pg_repack cannot run on system tables, it will FAIL with an explicit error message explaining the limitation.
Each time you perform DDL operations (CREATE, DROP, ALTER), rows are inserted/updated or deleted into the system tables : pg_class, pg_attribute ...
Autovacuum operations perform "low-level" operations, it can be interesting to perform "middle-level" vacuum with VACUUM ANALYZE... that is not blocking, but will be more a resource-consuming operation than autovacuum.
Performing VACUUM FULL operation will block access to these pillar tables of your database.
If your application/users can handle it, go ahead!
At work on this kind of operation, I set a statement_timeout, in order to properly stop the process if it is over a defined amount of time.