Re: force the use of a particular index

От: Scott Cain
Тема: Re: force the use of a particular index
Дата: ,
Msg-id: 1057929459.5766.7.camel@localhost.localdomain
(см: обсуждение, исходный текст)
Ответ на: Re: force the use of a particular index  (Rod Taylor)
Список: pgsql-performance

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force the use of a particular index  (Scott Cain, )
 Re: force the use of a particular index  (Rod Taylor, )
  Re: force the use of a particular index  (Scott Cain, )
   Re: force the use of a particular index  (Rod Taylor, )
 Re: force the use of a particular index  (Rod Taylor, )
  Re: force the use of a particular index  (Scott Cain, )
 Re: force the use of a particular index  (Scott Cain, )
  Re: force the use of a particular index  (Rod Taylor, )
   Re: force the use of a particular index  (Scott Cain, )
 Re: force the use of a particular index  (Tom Lane, )
  Re: force the use of a particular index  (Scott Cain, )
   Re: force the use of a particular index  (Tom Lane, )

The problem (at least as it appears to me) is not that it is performing
a table scan instead of an index scan, it is that it is using the wrong
index.  Here is the output from EXPLAIN ANALYZE:

                                                                         QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------------------
 Unique  (cost=494008.47..494037.59 rows=166 width=54) (actual time=114660.37..114660.38 rows=1 loops=1)
   ->  Sort  (cost=494008.47..494012.63 rows=1664 width=54) (actual time=114660.37..114660.37 rows=1 loops=1)
         Sort Key: f.name, fl.fmin, fl.fmax, fl.strand, f.type_id, f.feature_id
         ->  Nested Loop  (cost=0.00..493919.44 rows=1664 width=54) (actual time=2596.13..114632.90 rows=1 loops=1)
               ->  Index Scan using feature_pkey on feature f  (cost=0.00..134601.43 rows=52231 width=40) (actual
time=105.74..56048.87rows=13825 loops=1) 
                     Filter: (type_id = 219)
               ->  Index Scan using featureloc_idx1 on featureloc fl  (cost=0.00..6.87 rows=1 width=14) (actual
time=4.23..4.23rows=0 loops=13825) 
                     Index Cond: ("outer".feature_id = fl.feature_id)
                     Filter: ((srcfeature_id = 6) AND (fmin <= 2585581) AND (fmax >= 2565581))
 Total runtime: 114660.91 msec

This is the same regardless of enable_seqscan's setting.  The index that
it is using on featureloc (featureloc_idx1) is on the foreign key
feature_id.  It should instead be using another index, featureloc_idx3,
which is built on (srcfeature_id, fmin, fmax).

I should also mention that I've done a VACUUM FULL ANALYZE on this
database, and I've been using it for a while, and this is the primary
type of query I perform on the database.

Thanks,
Scott



On Fri, 2003-07-11 at 06:51, Rod Taylor wrote:
> On Thu, 2003-07-10 at 15:18, Scott Cain wrote:
> > Hello,
> >
> > I am wondering if there is a way to force the use of a particular index
> > when doing a query.  I have two tables that are pretty big (each >3
> > million rows), and when I do a join between them the performance is
> > generally quite poor as it does not use the indexes that I think it
> > should use.  Here is an example query:
>
> Please send the EXPLAIN ANALYZE results for that query with and without
> sequential scans enabled.
>
> set enable_seqscan = true;
> EXPLAIN ANALYZE <query>;
>
> set enable_seqscan = false;
> EXPLAIN ANALYZE <query>;
--
------------------------------------------------------------------------
Scott Cain, Ph. D.                                         
GMOD Coordinator (http://www.gmod.org/)                     216-392-3087
Cold Spring Harbor Laboratory



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