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Re: Support for REINDEX CONCURRENTLY

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Cc: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <stark(at)mit(dot)edu>
Subject: Re: Support for REINDEX CONCURRENTLY
Date: 2012-10-03 20:41:36
Message-ID: 201210032241.36655.andres@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wednesday, October 03, 2012 10:12:58 PM Michael Paquier wrote:
> On 2012/10/03, at 23:52, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > On Wednesday, October 03, 2012 04:28:59 PM Tom Lane wrote:
> >> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> >>> Maybe I am missing something here, but reindex concurrently should do
> >>> 1) BEGIN
> >>> 2) Lock table in share update exlusive
> >>> 3) lock old index
> >>> 3) create new index
> >>> 4) obtain session locks on table, old index, new index
> >>> 5) commit
> >>> 6) process till newindex->insisready (no new locks)
> >>> 7) process till newindex->indisvalid (no new locks)
> >>> 8) process till !oldindex->indisvalid (no new locks)
> >>> 9) process till !oldindex->indisready (no new locks)
> >>> 10) drop all session locks
> >>> 11) lock old index exlusively which should be "invisible" now
> >>> 12) drop old index
> >> 
> >> You can't drop the session locks until you're done.  Consider somebody
> >> else trying to do a DROP TABLE between steps 10 and 11, for instance.
> > 
> > Yea, the session lock on the table itself probably shouldn't be dropped.
> > If were holding only that one there shouldn't be any additional deadlock
> > dangers when dropping the index due to lock upgrades as were doing the
> > normal dance any DROP INDEX does. They seem pretty unlikely in a !valid
> > !ready table
> 
> Just à note...
> My patch drops the locks on parent table and indexes at the end of process,
> after dropping the old indexes ;)
I think that might result in deadlocks with concurrent sessions in some 
circumstances if those other sessions already have a lower level lock on the 
index. Thats why I think dropping the lock on the index and then reacquiring 
an access exlusive might be necessary.
Its not a too likely scenario, but why not do it right if its just 3 lines...

Andres
-- 
Andres Freund		http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services


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