Re: select for update

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