=?iso-8859-15?Q?Pierre-Fr=E9d=E9ric_Caillaud?= <lists@boutiquenumerique.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