Re: More efficient RI checks - take 2

Pavel Stehule <pavel.stehule@gmail.com> wrote:

> čt 23. 4. 2020 v 7:06 odesílatel Antonin Houska <ah@cybertec.at> napsal:
>
>  Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
>  > But it's not entirely clear to me that we know the best plan for a
>  > statement-level RI action with sufficient certainty to go that way.
>  > Is it really the case that the plan would not vary based on how
>  > many tuples there are to check, for example?
>
>  I'm concerned about that too. With my patch the checks become a bit slower if
>  only a single row is processed. The problem seems to be that the planner is
>  not entirely convinced about that the number of input rows, so it can still
>  build a plan that expects many rows. For example (as I mentioned elsewhere in
>  the thread), a hash join where the hash table only contains one tuple. Or
>  similarly a sort node for a single input tuple.
>
> without statistics the planner expect about 2000 rows table , no?

I think that at some point it estimates the number of rows from the number of
table pages, but I don't remember details.

I wanted to say that if we constructed the plan "manually", we'd need at least
two substantially different variants: one to check many rows and the other to
check a single row.

--
Antonin Houska
Web: https://www.cybertec-postgresql.com