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help / color / mirror / Atom feedFrom: Guillaume Lelarge <[email protected]>
To: PetSerAl <[email protected]>
Cc: [email protected]
Subject: Re: pg_dump and not MVCC-safe commands
Date: Mon, 20 May 2024 11:33:45 +0200
Message-ID: <CAECtzeUQ3hJYPM6k6_XqZLq=DygHbQ3+5YQLrv-ngcht2rQjQg@mail.gmail.com> (raw)
In-Reply-To: <CAKygsHQHhd4rzYcNCd8OJMH46saT1E+9+5MT2CgOCGgf_yaQAw@mail.gmail.com>
References: <CAKygsHQHhd4rzYcNCd8OJMH46saT1E+9+5MT2CgOCGgf_yaQAw@mail.gmail.com>
Hi,
Le lun. 20 mai 2024 à 11:27, PetSerAl <[email protected]> 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.
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Subject: Re: pg_dump and not MVCC-safe commands
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