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

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)
-------------------------------------------------------------------------------

regards,
--
Rafael Martinez, <r.m.guerrero@usit.uio.no>
Center for Information Technology Services
University of Oslo, Norway

PGP Public Key: http://folk.uio.no/rafael/