Robert Haas <robertmhaas@gmail.com> writes:
> The relevant code is in neqsel(). It estimates the fraction of rows
> that will be equal, and then does 1 - that number. Evidently, the
> query planner thinks that l1.l_suppkey = l2.l_suppkey would almost
> always be true, and therefore l1.l_suppkey <> l2.l_suppkey will almost
> always be false. I think the presumed selectivity of l1.l_suppkey =
> l2.l_suppkey is being computed by var_eq_non_const(), but I'm a little
> puzzled by that function is managing to produce a selectivity estimate
> of, essentially, 1.
No, I believe it's going through neqjoinsel and thence to eqjoinsel_semi.
This query will have been flattened into a semijoin.
I can reproduce a similarly bad estimate in the regression database:
regression=# explain select * from tenk1 a where exists(select * from tenk1 b where a.thousand = b.thousand and
a.twothousand<> b.twothousand); QUERY PLAN
-------------------------------------------------------------------------Hash Semi Join (cost=583.00..1067.25 rows=1
width=244) Hash Cond: (a.thousand = b.thousand) Join Filter: (a.twothousand <> b.twothousand) -> Seq Scan on tenk1 a
(cost=0.00..458.00 rows=10000 width=244) -> Hash (cost=458.00..458.00 rows=10000 width=8) -> Seq Scan on
tenk1b (cost=0.00..458.00 rows=10000 width=8)
(6 rows)
The problem here appears to be that we don't have any MCV list for
the "twothousand" column (because it has a perfectly flat distribution),
and the heuristic that eqjoinsel_semi is using for the no-MCVs case
is falling down badly.
regards, tom lane
