Make setrefs.c match by ressortgroupref even for plain Vars.

Previously, we skipped using search_indexed_tlist_for_sortgroupref()
if the tlist expression being sought in the child plan node was merely
a Var.  This is purely an optimization, based on the theory that
search_indexed_tlist_for_var() is faster, and one copy of a Var should
be as good as another.  However, the GROUPING SETS patch broke the
latter assumption: grouping columns containing the "same" Var can
sometimes have different outputs, as shown in the test case added here.
So do it the hard way whenever a ressortgroupref marking exists.

(If this seems like a bottleneck, we could imagine building a tlist index
data structure for ressortgroupref values, as we do for Vars.  But I'll
let that idea go until there's some evidence it's worthwhile.)

Back-patch to 9.6.  The problem also exists in 9.5 where GROUPING SETS
came in, but this patch is insufficient to resolve the problem in 9.5:
there is some obscure dependency on the upper-planner-pathification
work that happened in 9.6.  Given that this is such a weird corner case,
and no end users have complained about it, it doesn't seem worth the work
to develop a fix for 9.5.

Patch by me, per a report from Heikki Linnakangas.  (This does not fix
Heikki's original complaint, just the follow-on one.)

Discussion: https://postgr.es/m/aefc657e-edb2-64d5-6df1-a0828f6e9104@iki.fi

1 files changed, 3 insertions, 4 deletions

diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index 1382b679748..fa9a3f0b47b 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -1744,8 +1744,8 @@ set_upper_references(PlannerInfo *root, Plan *plan, int rtoffset)
 		TargetEntry *tle = (TargetEntry *) lfirst(l);
 		Node	   *newexpr;
 
-		/* If it's a non-Var sort/group item, first try to match by sortref */
-		if (tle->ressortgroupref != 0 && !IsA(tle->expr, Var))
+		/* If it's a sort/group item, first try to match by sortref */
+		if (tle->ressortgroupref != 0)
 		{
 			newexpr = (Node *)
 				search_indexed_tlist_for_sortgroupref(tle->expr,
@@ -2113,7 +2113,6 @@ search_indexed_tlist_for_non_var(Expr *node,
 
 /*
  * search_indexed_tlist_for_sortgroupref --- find a sort/group expression
- *		(which is assumed not to be just a Var)
  *
  * If a match is found, return a Var constructed to reference the tlist item.
  * If no match, return NULL.
@@ -2644,7 +2643,7 @@ is_converted_whole_row_reference(Node *node)
 
 	if (IsA(convexpr->arg, Var))
 	{
-		Var *var = castNode(Var, convexpr->arg);
+		Var		   *var = castNode(Var, convexpr->arg);
 
 		if (var->varattno == 0)
 			return true;