Re: Making Vars outer-join aware

On Mon, Jul 11, 2022 at 3:38 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Here's v2 of this patch series. It's functionally identical to v1,
> but I've rebased it over the recent auto-node-support-generation
> changes, and also extracted a few separable bits in hopes of making
> the main planner patch smaller. (It's still pretty durn large,
> unfortunately.) Unlike the original submission, each step will
> compile on its own, though the intermediate states mostly don't
> pass all regression tests.

Noticed a different behavior from previous regarding PlaceHolderVar.
Take the query below as an example:

select a.i, ss.jj from a left join (select b.i, b.j + 1 as jj from b) ss
on a.i = ss.i;

Previously the expression 'b.j + 1' would not be wrapped in a
PlaceHolderVar, since it contains a Var of the subquery and meanwhile it
does not contain any non-strict constructs. And now in the patch, we
would insert a PlaceHolderVar for it, in order to have a place to store
varnullingrels. So the plan for the above query now becomes:

# explain (verbose, costs off) select a.i, ss.jj from a left join
(select b.i, b.j + 1 as jj from b) ss on a.i = ss.i;
QUERY PLAN
----------------------------------
Hash Right Join
Output: a.i, ((b.j + 1))
Hash Cond: (b.i = a.i)
-> Seq Scan on public.b
Output: b.i, (b.j + 1)
-> Hash
Output: a.i
-> Seq Scan on public.a
Output: a.i
(9 rows)

Note that the evaluation of expression 'b.j + 1' now occurs below the
outer join. Is this something we need to be concerned about?

Thanks
Richard