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Re: Async commands (like drop index)

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Neil Conway <neilc(at)samurai(dot)com>
Cc: "Jim C(dot) Nasby" <decibel(at)decibel(dot)org>, "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Async commands (like drop index)
Date: 2007-05-18 17:29:35
Message-ID: 20070518172934.GA15691@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-hackers
Neil Conway wrote:
> On Fri, 2007-18-05 at 11:47 -0500, Jim C. Nasby wrote:
> > Assuming the concurrent psql stuff gets in, do you still see a use for
> > this?
> 
> I think concurrent psql (and/or async libpq) is the right way to handle
> this sort of requirement. "DROP INDEX NOWAIT" is hacky, and would be
> difficult (impossible?) to implement in a reasonable manner: the backend
> is fundamentally single-threaded. Also, how does the client learn when
> the DROP INDEX actually finishes? The client would either need to poll
> the database, or we'd need to implement something like select() --
> neither is a very appealing alternative.

I think what Joshua really wants is an equivalent of this:

start:
	BEGIN;
	LOCK TABLE foo IN ACCESS EXCLUSIVE MODE NOWAIT;
	-- if fail, rollback and go to start
	DROP INDEX foo_idx;
	COMMIT;

The idea is that the lock is only acquired if immediately available,
thus not blocking other queries which would otherwise be blocked behind
the DROP INDEX.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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