Fix planner error (or assert trap) with nested set operations.

As reported by Sean Johnston in bug #14614, since 9.6 the planner can fail
due to trying to look up the referent of a Var with varno 0.  This happens
because we generate such Vars in generate_append_tlist, for lack of any
better way to describe the output of a SetOp node.  In typical situations
nothing really cares about that, but given nested set-operation queries
we will call estimate_num_groups on the output of the subquery, and that
wants to know what a Var actually refers to.  That logic used to look at
subquery->targetList, but in commit 3fc6e2d7f I'd switched it to look at
subroot->processed_tlist, ie the actual output of the subquery plan not the
parser's idea of the result.  It seemed like a good idea at the time :-(.
As a band-aid fix, change it back.

Really we ought to have an honest way of naming the outputs of SetOp steps,
which suggests that it'd be a good idea for the parser to emit an RTE
corresponding to each one.  But that's a task for another day, and it
certainly wouldn't yield a back-patchable fix.

Report: https://postgr.es/m/20170407115808.25934.51866@wrigleys.postgresql.org

2 files changed, 31 insertions, 0 deletions

diff --git a/src/test/regress/expected/union.out b/src/test/regress/expected/union.out
index 4d697bada72..5c4edd1c166 100644
--- a/src/test/regress/expected/union.out
+++ b/src/test/regress/expected/union.out
@@ -320,6 +320,31 @@ SELECT q1 FROM int8_tbl EXCEPT ALL SELECT DISTINCT q2 FROM int8_tbl ORDER BY 1;
 
 SELECT q1 FROM int8_tbl EXCEPT ALL SELECT q1 FROM int8_tbl FOR NO KEY UPDATE;
 ERROR:  FOR NO KEY UPDATE is not allowed with UNION/INTERSECT/EXCEPT
+-- nested cases
+(SELECT 1,2,3 UNION SELECT 4,5,6) INTERSECT SELECT 4,5,6;
+ ?column? | ?column? | ?column? 
+----------+----------+----------
+        4 |        5 |        6
+(1 row)
+
+(SELECT 1,2,3 UNION SELECT 4,5,6 ORDER BY 1,2) INTERSECT SELECT 4,5,6;
+ ?column? | ?column? | ?column? 
+----------+----------+----------
+        4 |        5 |        6
+(1 row)
+
+(SELECT 1,2,3 UNION SELECT 4,5,6) EXCEPT SELECT 4,5,6;
+ ?column? | ?column? | ?column? 
+----------+----------+----------
+        1 |        2 |        3
+(1 row)
+
+(SELECT 1,2,3 UNION SELECT 4,5,6 ORDER BY 1,2) EXCEPT SELECT 4,5,6;
+ ?column? | ?column? | ?column? 
+----------+----------+----------
+        1 |        2 |        3
+(1 row)
+
 --
 -- Mixed types
 --
diff --git a/src/test/regress/sql/union.sql b/src/test/regress/sql/union.sql
index 48e6850798e..5e0eff2ca30 100644
--- a/src/test/regress/sql/union.sql
+++ b/src/test/regress/sql/union.sql
@@ -112,6 +112,12 @@ SELECT q1 FROM int8_tbl EXCEPT ALL SELECT DISTINCT q2 FROM int8_tbl ORDER BY 1;
 
 SELECT q1 FROM int8_tbl EXCEPT ALL SELECT q1 FROM int8_tbl FOR NO KEY UPDATE;
 
+-- nested cases
+(SELECT 1,2,3 UNION SELECT 4,5,6) INTERSECT SELECT 4,5,6;
+(SELECT 1,2,3 UNION SELECT 4,5,6 ORDER BY 1,2) INTERSECT SELECT 4,5,6;
+(SELECT 1,2,3 UNION SELECT 4,5,6) EXCEPT SELECT 4,5,6;
+(SELECT 1,2,3 UNION SELECT 4,5,6 ORDER BY 1,2) EXCEPT SELECT 4,5,6;
+
 --
 -- Mixed types
 --