Re: Check lateral references within PHVs for memoize cache keys

On Sun, Jul 9, 2023 at 1:28 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:

> More generally, it's not clear to me why we should need to look inside
> lateral PHVs in the first place.  Wouldn't the lateral PHV itself
> serve fine as a cache key?


Do you mean we use the lateral PHV directly as a cache key?  Hmm, it
seems to me that we'd have problem if the PHV references rels that are
inside the PHV's syntactic scope.  For instance

select * from t t1 left join
    lateral (select t1.a+t2.a as t1a, t2.a as t2a from t t2) s on true
where s.t1a = s.t2a;

The PHV references t1.a so it's lateral.  But it also references t2.a,
so if we use the PHV itself as cache key, the plan would look like

               QUERY PLAN
----------------------------------------
 Nested Loop
   ->  Seq Scan on t t1
   ->  Memoize
         Cache Key: (t1.a + t2.a)
         Cache Mode: binary
         ->  Seq Scan on t t2
               Filter: ((t1.a + a) = a)
(7 rows)

which is an invalid plan as the cache key contains t2.a.

Thanks
Richard