Rethink nullingrel marking rules in build_joinrel_tlist().

The logic for when to add the current outer join's own relid
to the nullingrels sets of output Vars and PHVs was overly
complicated and underly correct.  Not sure why I didn't think
of this before, but since what we want is marking per the
syntactic structure, we can just consult our records about
the syntactic structure, ie syn_righthand/syn_lefthand.

Also, tighten the rule about when to add the commute_above_r
bits, in hopes of eliminating some squishy reasoning.  I do not
know of a reason to think that that's broken as-is, but this way
seems better.

Per bug #17781 from Robins Tharakan.

Discussion: https://postgr.es/m/17781-c0405c8b3cd5e072@postgresql.org

1 files changed, 44 insertions, 0 deletions

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index eea8978fad..18b5e8f750 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -5008,6 +5008,50 @@ select a1.id from
  Seq Scan on a a1
 (1 row)
 
+-- another example (bug #17781)
+explain (costs off)
+select ss1.f1
+from int4_tbl as t1
+  left join (int4_tbl as t2
+             right join int4_tbl as t3 on null
+             left join (int4_tbl as t4
+                        right join int8_tbl as t5 on null)
+               on t2.f1 = t4.f1
+             left join ((select null as f1 from int4_tbl as t6) as ss1
+                        inner join int8_tbl as t7 on null)
+               on t5.q1 = t7.q2)
+    on false;
+           QUERY PLAN           
+--------------------------------
+ Nested Loop Left Join
+   Join Filter: false
+   ->  Seq Scan on int4_tbl t1
+   ->  Result
+         One-Time Filter: false
+(5 rows)
+
+-- variant with Var rather than PHV coming from t6
+explain (costs off)
+select ss1.f1
+from int4_tbl as t1
+  left join (int4_tbl as t2
+             right join int4_tbl as t3 on null
+             left join (int4_tbl as t4
+                        right join int8_tbl as t5 on null)
+               on t2.f1 = t4.f1
+             left join ((select f1 from int4_tbl as t6) as ss1
+                        inner join int8_tbl as t7 on null)
+               on t5.q1 = t7.q2)
+    on false;
+           QUERY PLAN           
+--------------------------------
+ Nested Loop Left Join
+   Join Filter: false
+   ->  Seq Scan on int4_tbl t1
+   ->  Result
+         One-Time Filter: false
+(5 rows)
+
 -- check that join removal works for a left join when joining a subquery
 -- that is guaranteed to be unique by its GROUP BY clause
 explain (costs off)