Message-ID: <4DB37385.3090209@emolecules.com>
Date: Sat, 23 Apr 2011 17:49:09 -0700
From: Craig James <craig_james@emolecules.com>
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: "pgsql-admin@postgresql.org" <pgsql-admin@postgresql.org>
Subject: Re: select for update
References: <4DB1BFC2.7020806@emolecules.com> <10313.1303505924@sss.pgh.pa.us>
	<4DB20AEC.90400@emolecules.com> <1161.1303528668@sss.pgh.pa.us>
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On 4/22/11 8:17 PM, Tom Lane wrote:
> Craig James<craig_james@emolecules.com>  writes:
>> On 4/22/11 1:58 PM, Tom Lane wrote:
>>> Craig James<craig_james@emolecules.com>   writes:
>>>> select objectid from archive where db_id is null limit 1 for update
>>> The interaction between LIMIT and FOR UPDATE changed in 9.0 ... what
>>> PG version are you using?
>> 8.4.4
> Well, note what it says in the 8.4 SELECT reference page:
>
> 				Caution
>
> 	It is possible for a SELECT command using both LIMIT and FOR
> 	UPDATE/SHARE clauses to return fewer rows than specified by
> 	LIMIT. This is because LIMIT is applied first. The command
> 	selects the specified number of rows, but might then block
> 	trying to obtain a lock on one or more of them. Once the SELECT
> 	unblocks, the row might have been deleted or updated so that it
> 	does not meet the query WHERE condition anymore, in which case
> 	it will not be returned.
>
> I think what's probably happening to you is you're getting a NULL not
> because there isn't a matching row, but because somebody is updating the
> first matching row underneath you and then the LIMIT prevents finding
> any other matches.  However, that pseudo-code is too pseudo to tell
> whether this theory is correct.
Thanks, it sounds like this is exactly what's happening.  It happens very rarely (a few times per month), so this makes sense.

I think I just need a two-step approach:

   $object_id = $dbh->selectrow_array("select min(objectid) from archive where db_id is null");
   if ($object_id) {
     $db_id = $dbh->selectrow_array("select db_id from archive where objectid = $object_id for update");
     ... double check that db_id is still NULL, repeat if someone else grabbed it...
   }
> (9.0 handles these situations in a less unintuitive fashion, btw.)
We'll be migrating soon, thanks.

Craig