Re: Poor Performance on Postgres 8.0

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
>
>
>