On Friday, June 19, 2020, Laurenz Albe <
laurenz.albe@cybertec.at> wrote:
> I am absolutely aware that those are hashed sub plans below a seqscan and that Postgres therefore has to scan all tuples of the table. But the problem is that upper nodes (which are excluded from
> this example for simplicity) think they will receive 1301316 rows from the seqscan, when in fact they will probably only see a hand full, which the planner could have (easily?) deduced by taking the
> greater of the two subplan row estimates.
>
> What am I missing, or is this perhaps a shortfall of the planner?
The subplans are executed *fpr each row* found in "transactions",
and the estimate on the subplans is *per execution".
I understood Tom’s nearby answer but not this one. This seems to be referring to correlated subplans which the examples are not.
David J.