Re: ERROR: no relation entry for relid 6

Thanks for pushing it!

I've managed to find some problems on master with the help of the new
assertions.  First the query below would trigger the new assertions.

create table t (a int unique, b int);

explain (costs off)
select 1 from t t1 left join
    (select t2.a, 1 as c from
     t t2 left join t t3 on t2.a = t3.a) s
on true left join t t4 on true where s.a < s.c;
server closed the connection unexpectedly

Note that 's.c' would be wrapped in PHV so the qual 's.a < s.c' is
actually 't2.a < PHV(1)', and it is one of t3's joinquals.  When we're
removing the t2/t3 join, we decide that this PHV is no longer needed so
we remove it entirely rather than just remove references from it.  But
actually its phrels still have references to t3 and t2/t3 join.  So when
we put back the qual 's.a < s.c', we will trigger the new assertions.

At first glance I thought we can just remove the new assertions.  But
then I figured out that the problem is more complex than that.  If the
PHV contains lateral references, removing the PHV entirely would cause
us to lose the information about the lateral references, and that may
cause wrong query results in some cases.  Consider the query below
(after removing the two new assertions, or in a non-assert build).

explain (costs off)
select 1 from t t1 left join
    lateral (select t2.a, coalesce(t1.a, 1) as c from
             t t2 left join t t3 on t2.a = t3.a) s
on true left join t t4 on true where s.a < s.c;
                    QUERY PLAN
--------------------------------------------------
 Nested Loop Left Join
   ->  Nested Loop
         ->  Seq Scan on t t1
         ->  Materialize
               ->  Seq Scan on t t2
                     Filter: (a < COALESCE(a, 1))
   ->  Materialize
         ->  Seq Scan on t t4
(8 rows)

The PHV of 'coalesce(t1.a, 1)' has lateral reference to t1 but we'd lose
this information because we've removed this PHV entirely in
remove_rel_from_query.  As a consequence, we'd fail to extract the
lateral dependency for t2 and fail to build the nestloop parameters for
the t1/t2 join.  And that causes wrong query results.  We can see that
if we insert some data into the table.

insert into t select 1,1;
insert into t select 2,2;

On v15 the query above gives

 ?column?
----------
        1
        1
(2 rows)

but on master it gives

 ?column?
----------
(0 rows)

I haven't thought through how to fix it, but I suspect that we may need
to do more checking before we decide to remove PHVs in
remove_rel_from_query.

Thanks
Richard