Re: BUG #15781: subselect on foreign table (postgres_fdw) can crash (segfault)

I wrote:
> Well, that's just coincidental for the case where the problem fdw_expr is
> a Param. I haven't tried to figure out exactly what upper-path generation
> thinks it should put into fdw_exprs, but is it really only Params?

Oh, this is interesting:

regression=# explain verbose
select exists(select c1 from ft4) as c, avg(c1) from ft4 where ft4.c1 = 42;
QUERY PLAN
-----------------------------------------------------------------------------------
Foreign Scan (cost=200.07..246.67 rows=1 width=33)
Output: ($0), (avg(ft4.c1))
Relations: Aggregate on (public.ft4)
Remote SQL: SELECT $1::boolean, avg(c1) FROM "S 1"."T 3" WHERE ((c1 = 42))
InitPlan 1 (returns $0)
-> Foreign Scan on public.ft4 ft4_1 (cost=100.00..212.39 rows=3413 width=0)
Remote SQL: SELECT NULL FROM "S 1"."T 3"
(7 rows)

That would crash if I tried to execute it, but:

regression=# explain verbose
select case when exists(select c1 from ft4) then 1 else 2 end as c, avg(c1) from ft4 where ft4.c1 = 42;
QUERY PLAN
-----------------------------------------------------------------------------------
Foreign Scan (cost=200.07..246.67 rows=1 width=36)
Output: CASE WHEN $0 THEN 1 ELSE 2 END, (avg(ft4.c1))
Relations: Aggregate on (public.ft4)
Remote SQL: SELECT avg(c1) FROM "S 1"."T 3" WHERE ((c1 = 42))
InitPlan 1 (returns $0)
-> Foreign Scan on public.ft4 ft4_1 (cost=100.00..212.39 rows=3413 width=0)
Remote SQL: SELECT NULL FROM "S 1"."T 3"
(7 rows)

That's just fine. So there is something stupid happening in creation
of the fdw_scan_tlist when a relation tlist item is a bare Param,
which doesn't happen if the same Param is buried in a larger expression.

regards, tom lane