Re: Problems with an update-from statement and pg-8.1.4 - Mailing list pgsql-performance

Rafael Martinez wrote:
> On Wed, 2006-12-06 at 14:55 -0500, Ted Allen wrote:
>
>> Stephan Szabo wrote:
>>
>>> On Wed, 6 Dec 2006, Rafael Martinez wrote:
>>>
>>>> mailstats=# EXPLAIN update mail SET spamscore = '-5.026'  FROM mail m,
>>>> mail_received mr where mr.mail_id = m.mail_id AND mr.queue_id =
>>>> '1GrxLs-0004N9-I1' and mr.mailhost = '129.240.10.47';
>>>>
>>>>
>>> I don't think this statement does what you expect. You're ending up with
>>> two copies of mail in the above one as "mail" and one as "m". You probably
>>> want to remove the mail m in FROM and use mail rather than m in the
>>> where clause.
>>>
>>>
>>>
>> Worse yet I think your setting "spamcore" for EVERY row in mail to
>> '-5.026'.  The above solution should fix it though.
>>
>> -- Ted
>>
>>
>
> Thanks for the answers. I think the 'problem' is explain in the
> documentation:
>
> "fromlist
>
> A list of table expressions, allowing columns from other tables to
> appear in the WHERE condition and the update expressions. This is
> similar to the list of tables that can be specified in the FROMClause of
> a SELECT statement. Note that the target table must not appear in the
> fromlist, unless you intend a self-join (in which case it must appear
> with an alias in the fromlist)".
>
> And as you said, we can not have 'mail m' in the FROM clause. I have
> contacted the developers and they will change the statement. I gave then
> these 2 examples:
>
> -------------------------------------------------------------------------------
> mailstats=# EXPLAIN update mail SET spamscore = '-5.026'  FROM
> mail_received mr where mr.mail_id = mail.mail_id AND mr.queue_id =
> '1GrxLs-0004N9-I1' and mr.mailhost = '129.240.10.47';
>                                               QUERY PLAN
> ------------------------------------------------------------------------------------------------------
>  Nested Loop  (cost=0.00..6.54 rows=1 width=57)
>    ->  Index Scan using received_queue_id_index on mail_received mr
> (cost=0.00..3.20 rows=1 width=4)
>          Index Cond: ((queue_id)::text = '1GrxLs-0004N9-I1'::text)
>          Filter: (mailhost = '129.240.10.47'::inet)
>    ->  Index Scan using mail_pkey on mail  (cost=0.00..3.32 rows=1
> width=57)
>          Index Cond: ("outer".mail_id = mail.mail_id)
> (6 rows)
>
> mailstats=# explain  update mail SET spamscore = '-5.026' where mail_id
> = (select mail_id from mail_received where queue_id = '1GrxLs-0004N9-I1'
> and mailhost = '129.240.10.47');
>                                              QUERY PLAN
> -----------------------------------------------------------------------------------------------------
>  Index Scan using mail_pkey on mail  (cost=3.20..6.52 rows=1 width=57)
>    Index Cond: (mail_id = $0)
>    InitPlan
>      ->  Index Scan using received_queue_id_index on mail_received
> (cost=0.00..3.20 rows=1 width=4)
>            Index Cond: ((queue_id)::text = '1GrxLs-0004N9-I1'::text)
>            Filter: (mailhost = '129.240.10.47'::inet)
> (6 rows)
> -------------------------------------------------------------------------------
>
Look again at the estimated costs of those two query plans.  You haven't
gained anything there.  Try this out:

EXPLAIN UPDATE mail
SET spamscore = '-5.026'
FROM mail_received mr
WHERE mail.mail_id = mr.mail_id AND mr.queue_id = '1GrxLs-0004N9-I1' ;

--
erik jones <erik@myemma.com>
software development
emma(r)