Re: row estimate for partial index

Harmen <harmen@lijzij.de> writes:
> Works well enough. However, we now have an org_id which has > 10% of the rows,
> but only a handful rows where "deleted is null" matches (so the org has a lot
> of "deleted" contacts). The planner doesn't like this and it falls back to a
> full table scan for the above query.

> I've added a dedicated index just for that org_id, to see if that helps:
>     "org123" btree (id) WHERE deleted IS NULL AND org_id = 123
> The planner seems to use it now, however the row estimate is way off:

Yeah, so that indicates that it isn't producing a good selectivity
estimate for the combination of those two conditions: it will assume
the org_id and deleted columns are independent, which per your statements
they are not.

If you are running a reasonably recent PG version you should be able to
fix that by setting up "extended statistics" on that pair of columns:

https://www.postgresql.org/docs/current/planner-stats.html#PLANNER-STATS-EXTENDED

(I might be wrong, but I think that will help even when one of
the troublesome conditions is a null-check.  If it doesn't, then
we have something to improve there ...)

            regards, tom lane