My question: What happens if not MVCC-safe command committed after snapshot but before lock? On Mon, May 20, 2024 at 12:33 PM Guillaume Lelarge <guillaume@xxxxxxxxxxxx> wrote: > > Hi, > > Le lun. 20 mai 2024 à 11:27, PetSerAl <petseral@xxxxxxxxx> a écrit : >> >> How pg_dump interact with not MVCC-safe commands? >> >> As I understand, pg_dump first take snapshot and then lock all tables >> it intended to dump. What happens if not MVCC-safe command committed >> after snapshot but before lock? From comment to pg_dump.c I understand >> that it may fail with 'cache lookup failed' error. But, can it happen, >> that pg_dump not fail, but instead capture inconsistent dump? For >> example TRUNCATE committed after snapshot and pg_dump will see result >> of TRUNCATE but not result of other commands in TRUNCATE transaction? >> >> > > You can't truncate an already existing table while pg_dump is running. TRUNCATE needs an exclusive lock, and pg_dump already has a lock on all tables of the database it's dumping. So TRUNCATE will be blocked until pg_dump finishes all its work. > > (The same will happen for VACUUM FULL, CLUSTER and some (all?) ALTER TABLE commands.) > > > -- > Guillaume.
