On 30 Nov 2004 14:30:37 +0100, Dmitry Karasik <[EMAIL PROTECTED]> wrote:
> 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)

Looks to me like the seq scan is a better plan.  The "actual time" went down.

> 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'.

See this thread
(http://archives.postgresql.org/pgsql-hackers/2004-11/msg00985.php and
http://archives.postgresql.org/pgsql-hackers/2004-11/msg01080.php) for
an ongoing discussion of the issue.

Mike Rylander
GPLS -- PINES Development
Database Developer

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