Re: FOR SHARE vs FOR UPDATE locks

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Jim Nasby <decibel(at)decibel(dot)org>, Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: FOR SHARE vs FOR UPDATE locks
Date: 2007-02-01 04:36:15
Message-ID: 200702010436.l114aFm15778@momjian.us
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Added to TODO:

* Fix problem when multiple subtransactions of the same outer transaction
hold different types of locks, and one subtransaction aborts

http://archives.postgresql.org/pgsql-hackers/2006-11/msg01011.php
http://archives.postgresql.org/pgsql-hackers/2006-12/msg00001.php

---------------------------------------------------------------------------

Tom Lane wrote:
> Jim Nasby <decibel(at)decibel(dot)org> writes:
> > As for possibly using the in-memory store of multiple CIDs affecting
> > a tuple, could that not work if that store contained enough
> > information to 'rollback' the lock to it's original state when
> > restoring to a savepoint? AFAIK other backends would only need to
> > know what the current lock being held was, they wouldn't need to know
> > the history of it themselves...
>
> One of the interesting problems is that if you upgrade shared lock to
> exclusive and then want to roll that back, you might need to un-block
> other processes that tried to acquire share lock after you acquired
> exclusive. We have no way to do that in the current implementation.
> (Any such processes will be blocked on your transaction ID lock, which
> you can't release without possibly unblocking the wrong processes.)
>
> regards, tom lane
>
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--
Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +

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