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Re: Wrong plan sequential scan instead of an index one

From: Gaetano Mendola <mendola(at)bigfoot(dot)com>
To: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Cc: Michael Fuhr <mike(at)fuhr(dot)org>
Subject: Re: Wrong plan sequential scan instead of an index one
Date: 2007-03-30 11:32:34
Message-ID: 460CF552.5000903@bigfoot.com (view raw or flat)
Thread:
Lists: pgsql-performance
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Michael Fuhr wrote:
> On Fri, Mar 30, 2007 at 12:08:26PM +0200, Gaetano Mendola wrote:
>> Claus Guttesen wrote:
>>> Try changing random_page_cost from the default 4 to 2 in postgresql.conf:
>>>
>>> random_page_cost = 2
>> I have tuned that number already at 2.5, lowering it to 2 doesn't change
>> the plan.
> 
> The following 19-fold overestimate is influencing the rest of the
> plan:
> 
>   ->  Seq Scan on l_pvcp  (cost=0.00..2.17 rows=19 width=4) (actual time=0.066..0.081 rows=1 loops=1)
>         Filter: (value ~~* '%pi%'::text)
> 
> Have you tried increasing the statistics target on l_pvcp.value?
> I ran your queries against canned data in 8.2.3 and better statistics
> resulted in more accurate row count estimates for this and other
> parts of the plan.  I don't recall if estimates for non-leading-character
> matches in earlier versions can benefit from better statistics.
> 


test=# alter table l_pvcp alter column value set statistics 1000;
ALTER TABLE
test=# analyze l_pvcp;
ANALYZE
test=# explain analyze SELECT COUNT(id) FROM t_oa_2_00_card WHERE pvcp in (select id from l_pvcp where value ilike '%pi%');
                                                              QUERY PLAN
- ---------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=154321.83..154321.84 rows=1 width=8) (actual time=4948.627..4948.628 rows=1 loops=1)
   ->  Hash IN Join  (cost=2.22..153877.08 rows=177898 width=8) (actual time=2.262..4940.395 rows=7801 loops=1)
         Hash Cond: ("outer".pvcp = "inner".id)
         ->  Seq Scan on t_oa_2_00_card  (cost=0.00..147695.25 rows=880125 width=12) (actual time=0.040..3850.074 rows=877682 loops=1)
         ->  Hash  (cost=2.17..2.17 rows=19 width=4) (actual time=0.073..0.073 rows=1 loops=1)
               ->  Seq Scan on l_pvcp  (cost=0.00..2.17 rows=19 width=4) (actual time=0.052..0.067 rows=1 loops=1)
                     Filter: (value ~~* '%pi%'::text)
 Total runtime: 4948.717 ms
(8 rows)


and nothing changed.


Regards
Gaetano Mendola

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