Re: A performance issue with Memoize

On Thu, Oct 26, 2023 at 12:07 PM Andrei Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru>
wrote:

> Do you've thought about the case, fixed with the commit 1db5667? As I
> see, that bugfix still isn't covered by regression tests. Could your
> approach of a PARAM_EXEC slot reusing break that case?

Hm, I don't think so. The issue fixed by commit 1db5667 was caused by
sharing PARAM_EXEC slots between different levels of NestLoop. AFAICS
it's safe to share PARAM_EXEC slots within the same level of NestLoop.

The change here is about sharing PARAM_EXEC slots between subquery's
subplan_params and outer-relation variables, which happens within the
same level of NestLoop.

Actually, even without this change, we'd still share PARAM_EXEC slots
between subquery's subplan_params and outer-relation variables in some
cases. As an example, consider

explain (costs off)
select * from t t1 left join
(t t2 left join
lateral (select t1.a as t1a, t2.a as t2a, * from t t3) s
on t2.b = s.b)
on t1.b = s.b and t1.a = t2.a;
QUERY PLAN
-------------------------------------------------------
Nested Loop Left Join
-> Seq Scan on t t1
-> Nested Loop
Join Filter: (t1.a = t2.a)
-> Seq Scan on t t2
-> Subquery Scan on s
Filter: ((t1.b = s.b) AND (t2.b = s.b))
-> Seq Scan on t t3
(8 rows)

For outer-relation Var 't1.a' from qual 't1.a = t2.a', it shares
PARAM_EXEC slot 0 with the PlannerParamItem for 't1.a' within the
subquery (from its targetlist).

Did you notice a case that the change here breaks?

Hi Tom, could you share your insights on this issue and the proposed
fix?

Thanks
Richard