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Re: pg_dump locks a table on dumping ... ?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Marc G(dot) Fournier" <scrappy(at)postgresql(dot)org>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: pg_dump locks a table on dumping ... ?
Date: 2004-03-23 17:02:40
Message-ID: 21464.1080061360@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
"Marc G. Fournier" <scrappy(at)postgresql(dot)org> writes:
> 'k, just had someone tell me that they believed that a pg_dump will lock a
> table while the dump is happening ... my first thought/response was that
> this would defeat the point of MVCC, no?  And would make for a terrible
> backup procedure on a very large database, since you'd just have made your
> database useless for the duration ...

> Can someone confirm/deny that the lock happens?

pg_dump takes a reader's lock (AccessShareLock).  This does not prevent
reads or writes of the table.  It does prevent dropping the table,
making schema alterations to it (such as column add/drop), or anything
else that wants an exclusive lock.  I think that VACUUM FULL is the only
thing particularly likely to cause a conflict in a running database.

			regards, tom lane

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