To: Robert Haas <robertmhaas@gmail.com>
cc: Noah Misch <noah@leadboat.com>, Nikolas Everett <nik9000@gmail.com>,
	pgsql-performance@postgresql.org
Subject: Re: Exhaustive list of what takes what locks 
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Wed, 23 Feb 2011 12:21:12 -0500"
Date: Wed, 23 Feb 2011 12:31:19 -0500
Message-ID: <2316.1298482279@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Robert Haas <robertmhaas@gmail.com> writes:
> ...but that begs the question of why DROP INDEX needs an
> AccessExclusiveLock.  It probably needs such a lock *on the index* but
> I don't see why we'd need it on the table.

Some other session might be in process of planning a query on the table.
It would be sad if the index it had chosen turned out to have vanished
meanwhile.  You could perhaps confine DROP INDEX's ex-lock to the index,
but only at the price of making the planner take out a lock on every
index it considers even transiently.  Which isn't going to be a net
improvement.

(While we're on the subject, I have strong suspicions that most of what
Simon did this cycle on ALTER TABLE lock strength reduction is
hopelessly broken and will have to be reverted.  It's on my to-do list
to try to break that patch during beta, and I expect to succeed.)

			regards, tom lane