Message-ID: <43792BA3.3060702@noao.edu>
Date: Mon, 14 Nov 2005 17:28:19 -0700
From: Steve Wampler <swampler@noao.edu>
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To: Postgres-performance <pgsql-performance@postgresql.org>
Subject: Re: Help speeding up delete
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	<43792355.1020501@noao.edu>
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Joshua Marsh wrote:
> 
> 
> On 11/14/05, *Steve Wampler* <swampler@noao.edu
> <mailto:swampler@noao.edu>> wrote:
> 
>     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)
> 
> 
> For one reason or the other, the planner things a sequential scan is the
> best solution. Try turning off seq_scan before the query and see if it
> changes the plan (set enable_seqscan off;). 
> 
> I've seen this problem with sub queries and that usually solves it.
> 

Hmmm, not only does it still use sequential scans, it thinks it'll take
even longer:

   set enable_seqscan to off;
   SET VARIABLE
   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=100000000.00..160237039405992.50 rows=800836 width=6)
     SubPlan
       ->  Materialize  (cost=100043604.06..100043604.06 rows=45 width=26)
             ->  Seq Scan on tmp_table2  (cost=100000000.00..100043604.06 rows=45 width=26)

   EXPLAIN

But the advice sounds like it *should* have helped...

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