Re: Significant Execution Time Difference Between PG13.14 and PG16.4 for Query on information_schema Tables.

I wrote:
> We didn't see this particular behavior before 2489d76c49 because
> pullup_replace_vars avoided inserting a PHV:
> * If it contains a Var of the subquery being pulled up, and
> * does not contain any non-strict constructs, then it's
> * certainly nullable so we don't need to insert a
> * PlaceHolderVar.
> I dropped that case in 2489d76c49 because now we need to attach
> nullingrels to the expression. You could imagine attaching the
> nullingrels to the contained Var(s) instead of putting a PHV on top,
> but that seems like a mess and I'm not quite sure it's semantically
> the same. In any case it wouldn't fix adjacent cases where there is
> a non-strict construct in the subquery output expression.

I realized that actually we do have the mechanism for making that
work: we could apply add_nulling_relids to the expression, if it
meets those same conditions. This is a kluge really, but it would
restore the status quo ante in a fairly localized fashion that
seems like it might be safe enough to back-patch into v16.

Here's a WIP patch that does it like that. One problem with it
is that it requires rcon->relids to be calculated in cases where
we didn't need that before, which is probably not *that* expensive
but it's annoying. If we go forward with this, I'm thinking about
changing add_nulling_relids' API contract to say "if target_relid
is NULL then all level-zero Vars/PHVs are modified", so that we
don't need that relid set in non-LATERAL cases.

The other problem with this is that it breaks one test case in
memoize.sql: a query that formerly generated a memoize plan
now does not use memoize. I am not sure why not --- does that
mean anything to you?

regards, tom lane