Re: postgres chooses objectively wrong index

On 18/3/26 19:38, Merlin Moncure wrote:
> On Tue, Mar 17, 2026 at 11:27 PM Alexey Ermakov <alexius(dot)work(at)gmail(dot)com
> I think the planner is not giving enough bonus for an exact match versus
> an inexact match on partial index mathcing,  (A=A should be better than
> A IN(A,B,C)), and it's unclear why the planner things bitmap heap + sort
> is outperforming a raw read off the index base on marginal estimated row
> counts.  Lowering random_page_cost definitely biases the plan I like,
> but it skews both estimates.

One ongoing shortcoming is that cardinality estimation takes place early
in the optimisation process and uses all filter conditions. This can be
frustrating because a partial index covers just part of the table and
could give the optimiser better statistics. If we ignored the index
condition, we might get a more accurate estimate.

I haven’t tried to redesign this part myself. If it were simple, someone
likely would have fixed it by now. Maybe Tom has some ideas about it.

--
regards, Andrei Lepikhov,
pgEdge