=?iso-8859-15?Q?Pierre-Fr=E9d=E9ric_Caillaud?= <lists(at)boutiquenumerique(dot)com> writes:
> Now, if I LIMIT the query to 10 rows, the index should be used all the
> time, because it will always return few rows... well, it doesn't !
Not at all. From the planner's point of view, the LIMIT is going to
reduce the cost by about a factor of 10/1403, since the underlying plan
step will only be run partway through. That's not going to change the
decision about which underlying plan step is cheapest: 10/1403 of a
cheaper plan is still always less than 10/1403 of a more expensive plan.
Later, you note that LIMIT with ORDER BY does affect the plan choice
--- that's because in that situation one plan alternative has a much
higher startup cost than the other (namely the cost of a sort step).
A small LIMIT can allow the fast-startup plan to be chosen even though
it would be estimated to be the loser if run to completion.
regards, tom lane
In response to
pgsql-performance by date
|Next:||From: Pierre-Frédéric Caillaud||Date: 2004-09-07 06:51:54|
|Subject: Re: The usual sequential scan, but with LIMIT ! |
|Previous:||From: Markus Schaber||Date: 2004-09-06 15:56:18|
|Subject: Re: Multiple Uniques|
pgsql-general by date
|Next:||From: Joshua D. Drake||Date: 2004-09-06 17:22:12|
|Subject: Re: RAD with postgreSQL|
|Previous:||From: Tom Lane||Date: 2004-09-06 16:25:11|
|Subject: Re: tg_relation doesn't seem to have the attribute names! |