Re: VACUUM ANALYZE downgrades performance

On 11/30/2004 7:30 AM Dmitry Karasik said::

>Hi all,
>
>On v7.4.5 I noticed downgrade in the planner, namely favoring
>sequential scan over index scan. The proof:
>
>   create table a ( a integer);
>   create index aidx on a(a);
>   explain analyze select * from a where a = 0;
>   -- Index Scan using aidx on a  (cost=0.00..17.07 rows=5 width=4) (actual
>   --   time=0.029..0.029 rows=0 loops=1)
>   -- Index Cond: (a = 0)
>   vacuum analyze;
>   explain analyze select * from a where a = 0;
>   -- Seq Scan on a (cost=0.00..0.00 rows=1 width=4) (actual time=0.009..0.009
>   --   rows=0 loops=1)
>   -- Filter: (a = 0)
>
>I do realize that there might be reasons why this happens over an empty
>table, but what is way worse that when the table starts actually to fill,
>the seq scan is still there, and the index is simply not used. How
>that could be so ...mmm... shortsighted, and what is more important,
>how to avoid this? I hope the answer is not 'run vacuum analyze each 5 seconds'.
>
>
>
Look at the ACTUAL TIME.   It dropped from 0.029ms (using the index
scan) to 0.009ms (using a sequential scan.)

Index scans are not always faster, and the planner/optimizer knows
this.  VACUUM ANALYZE is best run when a large proportion of data has
been updated/loaded or in the off hours to refresh the statistics on
large datasets.