On Sat, Apr 22, 2023 at 11:21 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Steinar Kaldager <steinar.kaldager@oda.com> writes: > First-time potential contributor here. We recently had an incident due > to a sudden 1000x slowdown of a Postgres query (from ~10ms to ~10s) > due to a join with a foreign key that was often null. We found that it > was caused by a merge join with an index scan on one join path -- > whenever the non-null data happened to be such that the merge join > couldn't be terminated early, the index would proceed to scan all of > the null rows and filter each one out individually. Since this was an > inner join, this was pointless; the nulls would never have matched the > join clause anyway.
Hmm. I don't entirely understand why the existing stop-at-nulls logic in nodeMergejoin.c didn't fix this for you. Maybe somebody has broken that? See the commentary for MJEvalOuterValues/MJEvalInnerValues.
I think it's just because the MergeJoin didn't see a NULL foo_id value from test_bar tuples because all such tuples are removed by the filter 'test_bar.active', thus it does not have a chance to stop at nulls.
# select count(*) from test_bar where foo_id is null and active; count ------- 0 (1 row)
Instead, the index scan on test_bar will have to scan all the tuples with NULL foo_id because none of them satisfies the qual clause.