Re: pg_dump and not MVCC-safe commands

Hi,

Le lun. 20 mai 2024 à 11:27, PetSerAl <petseral(at)gmail(dot)com> 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.