Message-ID: <43792355.1020501@noao.edu>
Date: Mon, 14 Nov 2005 16:52:53 -0700
From: Steve Wampler <swampler@noao.edu>
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To: Scott Lamb <slamb@slamb.org>
CC: Postgres-performance <pgsql-performance@postgresql.org>
Subject: Re: Help speeding up delete
References: <43790A99.9050603@noao.edu>
	<5E90D296-FA4A-4E48-92B4-396AADE35EF2@slamb.org>
In-Reply-To: <5E90D296-FA4A-4E48-92B4-396AADE35EF2@slamb.org>
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Scott Lamb wrote:
> On Nov 14, 2005, at 2:07 PM, Steve Wampler wrote:
> 
>> # SELECT at.id FROM "tmp_table2" at, "tmp_tabl2e" a
>> #   WHERE at.id=a.id and a.name='obsid' and a.value='oid080505';
> 
> 
> Isn't this equivalent?
> 
> select id from tmp_table2 where name = 'obsid' and value = 'oid080505';

Probably, the user based the above on a query designed to find
all rows with the same id as those rows that have a.name='obsid' and
a.value='oid080505'.  However, I think the above would work to locate
all the ids, which is all we need for the delete (see below)

>> # DELETE FROM "tmp_table2" WHERE id IN
>> # (SELECT at.id FROM "tmp_table2" at, "tmp_table2" a
>> #   WHERE at.id=a.id and a.name='obsid' and a.value='oid080505');
> 
> 
> and this?
> 
> delete from tmp_table2 where name = 'obsid' and value = 'oid080505';
> 
> Why are you doing a self-join using id, which I assume is a primary key?

Because I think we need to.  The above would only delete rows that have
name = 'obsid' and value = 'oid080505'.  We need to delete all rows that
have the same ids as those rows.  However, from what you note, I bet
we could do:

   DELETE FROM "tmp_table2" WHERE id IN
      (SELECT id FROM "temp_table2" WHERE name = 'obsid' and value= 'oid080505');

However, even that seems to have a much higher cost than I'd expect:

   lab.devel.configdb=# explain delete from "tmp_table2" where id in
        (select id from tmp_table2 where name='obsid' and value = 'oid080505');
   NOTICE:  QUERY PLAN:

   Seq Scan on tmp_table2  (cost=0.00..65705177237.26 rows=769844 width=6)
     SubPlan
       ->  Materialize  (cost=42674.32..42674.32 rows=38 width=50)
             ->  Seq Scan on tmp_table2  (cost=0.00..42674.32 rows=38 width=50)

   EXPLAIN

And, sure enough, is taking an extrordinarily long time to run (more than
10 minutes so far, compared to < 10seconds for the select).  Is this
really typical of deletes?  It appears (to me) to be the Seq Scan on tmp_table2
that is the killer here.  If we put an index on, would it help?  (The user
claims she tried that and it's EXPLAIN cost went even higher, but I haven't
checked that...)

Thanks!
-- 
Steve Wampler -- swampler@noao.edu
The gods that smiled on your birth are now laughing out loud.