On Thu, Mar 19, 2026 at 1:09 AM Andrei Lepikhov <lepihov@gmail.com> wrote:
On 18/3/26 19:38, Merlin Moncure wrote: > On Tue, Mar 17, 2026 at 11:27 PM Alexey Ermakov <alexius.work@gmail.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.
Thanks. I understand the challenge with estimation around partial indexes. Something deeper seems to be at play here.
Poking around more, I see that the bad plans are related to bloat. A simple REINDEX of one of the indexes made the problem disappear; however, what's odd is that the estimates didn't really change although the net plan cost certainly did. It's also worth noting ANALYZE doesn't help, only REINDEX does.
I keep coming back to this: the bitmap scan noted above makes no sense. I'm trying to figure out what is steering the planner in that direction and eliminate it.
This problem reliably reproduces about once a month (taking down production). I'll wait for it to recur and look at it with fresh eyes.
