Re: Removing unneeded self joins

On Thu, May 2, 2024 at 6:08 PM Alexander Korotkov <aekorotkov(at)gmail(dot)com>
wrote:

> On Thu, May 2, 2024 at 12:45 PM Andrei Lepikhov
> <a(dot)lepikhov(at)postgrespro(dot)ru> wrote:
> > One question for me is: Do we anticipate other lateral self-references
> > except the TABLESAMPLE case? Looking into the extract_lateral_references
> > implementation, I see the only RTE_SUBQUERY case to be afraid of. But we
> > pull up subqueries before extracting lateral references. So, if we have
> > a reference to a subquery, it means we will not flatten this subquery
> > and don't execute SJE. Do we need more code, as you have written in the
> > first patch?
>
> I think my first patch was crap anyway. Your explanation seems
> reasonable to me. I'm not sure this requires any more code. Probably
> it would be enough to add more comments about this.

The tablesample case is not the only factor that can cause a relation to
have a lateral dependency on itself after self-join removal. It can
also happen with PHVs. As an example, consider

explain (costs off)
select * from t t1
left join lateral
(select t1.a as t1a, * from t t2) t2
on true
where t1.a = t2.a;
server closed the connection unexpectedly

This is because after self-join removal, a PlaceHolderInfo's ph_lateral
might contain rels mentioned in ph_eval_at, which we should get rid of.

For the tablesample case, I agree that we should not consider relations
with TABLESAMPLE clauses as candidates to be removed. Removing such a
relation could potentially change the syntax of the query, as shown by
Alexander's example. It seems to me that we can just check that in
remove_self_joins_recurse, while we're collecting the base relations
that are considered to be candidates for removal.

This leads to the attached patch. This patch also includes some code
refactoring for the surrounding code.

Thanks
Richard