Re: postgres 8.2 seems to prefer Seq Scan

From: Michael Fuhr
Subject: Re: postgres 8.2 seems to prefer Seq Scan
Date: ,
Msg-id: 20070406213147.GA12788@winnie.fuhr.org
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In response to: postgres 8.2 seems to prefer Seq Scan  ("Alex Deucher")
Responses: Re: postgres 8.2 seems to prefer Seq Scan  ("Alex Deucher")
Re: postgres 8.2 seems to prefer Seq Scan  ("Alex Deucher")
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postgres 8.2 seems to prefer Seq Scan  ("Alex Deucher", )
 Re: postgres 8.2 seems to prefer Seq Scan  (Michael Fuhr, )
  Re: postgres 8.2 seems to prefer Seq Scan  ("Alex Deucher", )
  Re: postgres 8.2 seems to prefer Seq Scan  ("Alex Deucher", )
   Re: postgres 8.2 seems to prefer Seq Scan  ("Alex Deucher", )
    Re: postgres 8.2 seems to prefer Seq Scan  ("Alex Deucher", )

On Fri, Apr 06, 2007 at 04:38:33PM -0400, Alex Deucher wrote:
> One more anomaly between 7.4 and 8.2.  DB dumped from 7.4 and loaded
> onto 8.2, both have locale set to C.  8.2 seems to prefer Seq Scans
> for the first query while the ordering in the second query seems to
> perform worse on 8.2.  I ran analyze.  I've tried with the encoding
> set to UTF-8 and SQL_ASCII; same numbers and plans.  Any ideas how to
> improve this?

Are you sure the data sets are identical?  The 7.4 query returned
0 rows; the 8.2 query returned 1 row.  If you're running the same
query against the same data in both versions then at least one of
them appears to be returning the wrong result.  Exactly which
versions of 7.4 and 8.2 are you running?

Have you analyzed all tables in both versions?  The row count
estimate in 7.4 is much closer to reality than in 8.2:

7.4
>   ->  Index Scan using pnum_idx on event  (cost=0.00..3.37 rows=19
> width=172) (actual time=0.063..0.063 rows=0 loops=1)
>         Index Cond: ((pnum)::text = 'AB5819188'::text)

8.2
>   ->  Index Scan using pnum_idx on event  (cost=0.00..3147.63
> rows=1779 width=171) (actual time=0.030..0.033 rows=1 loops=1)
>         Index Cond: ((pnum)::text = 'AB5819188'::text)

If analyzing the event table doesn't improve the row count estimate
then try increasing the statistics target for event.pnum and analyzing
again.  Example:

ALTER TABLE event ALTER pnum SET STATISTICS 100;
ANALYZE event;

You can set the statistics target as high as 1000 to get more
accurate results at the cost of longer ANALYZE times.

--
Michael Fuhr


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