Re: Removing unneeded self joins

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