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Re: Poor Performance on Postgres 8.0

From: Pallav Kalva <pkalva(at)deg(dot)cc>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PERFORM <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Poor Performance on Postgres 8.0
Date: 2005-01-28 20:58:19
Message-ID: 41FAA76B.7070704@deg.cc (view raw or flat)
Thread:
Lists: pgsql-performance
On 7.4 I get

select * from pg_stats
 where tablename = 'attribute' and attname = 'fknamestringid';

 schemaname | tablename |    attname     | null_frac | avg_width | 
n_distinct |                  most_common_vals                   
|                                  
most_common_freqs                                  |                     
histogram_bounds                     | correlation
------------+-----------+----------------+-----------+-----------+------------+-----------------------------------------------------+-------------------------------------------------------------------------------------+----------------------------------------------------------+-------------
 common     | attribute | fknamestringid |         0 |         4 
|        124 | {2524,2434,2523,2599,2595,2592,2596,2528,2586,2446} | 
{0.132333,0.13,0.0766667,0.0373333,0.0366667,0.0333333,0.031,0.029,0.0263333,0.019} 
| {2433,2441,2455,2462,2473,2479,2484,2492,2505,2574,2598} |    -0.22864
(1 row)

On 8

select * from pg_stats
where tablename = 'attribute' and attname = 'fknamestringid';

 schemaname | tablename |    attname     | null_frac | avg_width | 
n_distinct |                  most_common_vals                   
|                                 
most_common_freqs                                 |                     
histogram_bounds                     | correlation
------------+-----------+----------------+-----------+-----------+------------+-----------------------------------------------------+-----------------------------------------------------------------------------------+----------------------------------------------------------+-------------
 common     | attribute | fknamestringid |         0 |         4 
|         80 | {2524,2434,2530,2522,2525,2523,2527,2526,2574,2531} | 
{0.219333,0.199333,0.076,0.0643333,0.0616667,0.05,0.0453333,0.042,0.04,0.0286667} 
| {2437,2528,2529,2538,2539,2540,2554,2562,2575,2584,2637} |   0.0274016


Tom Lane wrote:

>I was able to duplicate this behavior with dummy data that had only a
>few distinct values for fknamestringid --- the planner then thinks that
>the index probe into attribute will match a lot of rows and hence take a
>long time.  Could we see your pg_stats row for fknamestringid, ie
>
>select * from pg_stats
>where tablename = 'attribute' and attname = 'fknamestringid';
>
>It would be interesting to see the same for your 7.4 installation too.
>
>			regards, tom lane
>
>  
>



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