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Date: Wed, 6 Dec 2006 14:45:27 +0100
From: Martijn van Oosterhout <kleptog@svana.org>
To: Dave Cramer <pg@fastcrypt.com>
Cc: Jens Schipkowski <jens.schipkowski@apus.co.at>,
	Joost Kraaijeveld <J.Kraaijeveld@askesis.nl>,
	pgsql-general@postgresql.org, pgsql-performance@postgresql.org
Subject: Re: [PERFORM] Locking in PostgreSQL?
Message-ID: <20061206134527.GA11211@svana.org>
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On Wed, Dec 06, 2006 at 08:26:45AM -0500, Dave Cramer wrote:
> >>Unless you specifically ask for it postgresql doesn't lock any =20
> >>rows when you update data.
> >>
> >Thats not right. UPDATE will force a RowExclusiveLock to rows =20
> >matching the WHERE clause, or all if no one is specified.
>=20
> Apparently I've completely misunderstood MVCC then.... My =20
> understanding is that unless you do a select ... for update then =20
> update the rows will not be locked .

I think it comes down to what you mean by RowExclusiveLock. In MVCC,
writers don't block readers, so even if someone executes an update on a
row, readers (SELECT statements) will not be blocked.

So it's not a lock as such, more a "I've updated this row, go find the
new version if that's appropriate for your snapshot".

Have a nice day,
--=20
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to l=
itigate.

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