Re: Killing off removed rels properly

Hmm, there's something else going on here.  After getting rid of the
assertion failure, I see that the plan looks like

# explain MERGE INTO tt USING st ON tt.tid = st.sid WHEN NOT MATCHED THEN INSERT 
VALUES (st.sid);
                         QUERY PLAN                          
-------------------------------------------------------------
 Merge on tt  (cost=0.00..35.50 rows=0 width=0)
   ->  Seq Scan on st  (cost=0.00..35.50 rows=2550 width=10)
(2 rows)

which is fairly nonsensical and doesn't match v15's plan:

 Merge on tt  (cost=0.15..544.88 rows=0 width=0)
   Merge on ttp tt_1
   ->  Nested Loop Left Join  (cost=0.15..544.88 rows=32512 width=14)
         ->  Seq Scan on st  (cost=0.00..35.50 rows=2550 width=4)
         ->  Index Scan using ttp_pkey on ttp tt_1  (cost=0.15..0.19 rows=1 widt
h=14)
               Index Cond: (tid = st.sid)

It looks like we're somehow triggering the elide-a-left-join code
when we shouldn't?  That explains why the target table's RelOptInfo
has gone missing and broken make_modifytable's expectations.
That code is still unnecessarily fragile so I intend to rearrange it,
but there's more to do here.

			regards, tom lane