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Re: problem with pg_statistics

From: Andre Schubert <andre(dot)schubert(at)km3(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: mkoi-pg(at)aon(dot)at, pgsql-performance(at)postgresql(dot)org
Subject: Re: problem with pg_statistics
Date: 2003-06-27 06:13:06
Message-ID: 20030627081306.71ffbb2e.andre.schubert@km3.de (view raw or flat)
Thread:
Lists: pgsql-performance
On Thu, 26 Jun 2003 12:03:52 -0400
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Manfred Koizar <mkoi-pg(at)aon(dot)at> writes:
> > On Thu, 26 Jun 2003 10:08:05 -0400, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
> > wrote:
> >> Try reducing random_page_cost
> 
> > With index scan cost being more than 25 * seq scan cost, I guess that
> > - all other things held equal - even random_page_cost = 1 wouldn't
> > help.
> 
> Oh, you're right, I was comparing the wrong estimated costs.  Yeah,
> changing random_page_cost won't fix it.
> 
> > Or there's something wrong with correlation?
> 
> That seems like a good bet.  Andre, is this table likely to be
> physically ordered by time_stamp, or nearly so?  If so, do you
> expect that condition to persist, or is it just an artifact of
> a test setup?
> 

Sorry forgot the pg_stat query...


SELECT * FROM pg_stats where tablename = 'tbl_traffic' and attname = 'time_stamp';
  tablename  |  attname   | null_frac | avg_width | n_distinct |                                                                                                       
                                                  most_common_vals                                                                                                     
                                                    |                         most_common_freqs                          |                                             
                                                                                                                        histogram_bounds                               
                                                                                                                                       | correlation 
-------------+------------+-----------+-----------+------------+-------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------+--------------------------------------------------------------------+---------------------------------------------
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
---------------------------------------------------------------------------------------------------------------------------------------+-------------
 tbl_traffic | time_stamp |         0 |         8 |     104009 | {"2003-06-03 19:12:01.059625+02","2003-02-03 19:52:06.666296+01","2003-02-13 09:59:45.415763+01","2003
-02-28 18:10:28.536399+01","2003-04-11 18:09:42.30363+02","2003-04-26 20:35:50.110235+02","2003-05-03 11:09:32.991507+02","2003-05-20 09:53:51.271853+02","2003-05-21 2
0:55:59.155387+02","2003-06-02 02:38:28.823182+02"} | {0.00133333,0.001,0.001,0.001,0.001,0.001,0.001,0.001,0.001,0.001} | {"2002-07-01 00:00:00+02","2003-02-21 01:59:
46.107696+01","2003-03-11 15:00:37.418521+01","2003-03-26 18:14:50.028972+01","2003-04-10 13:43:20.75909+02","2003-04-27 09:03:19.592213+02","2003-05-08 22:35:41.99761
6+02","2003-05-22 15:34:42.932958+02","2003-06-03 00:53:05.870782+02","2003-06-15 08:45:41.154875+02","2003-06-27 07:18:30.265868+02"} |   -0.479749
(1 row)

Thanks, as

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