Build column mapping for grouping sets in all required cases.

The previous coding frequently failed to fail because for one it's
unusual to have rollup clauses with one column, and for another
sometimes the wrong mapping didn't cause obvious problems.

Author: Jeevan Chalke
Reviewed-By: Andrew Gierth
Discussion: CAM2+6=W=9=hQOipH0HAPbkun3Z3TFWij_EiHue0_6UX=oR=1kw@mail.gmail.com
Backpatch: 9.5, where grouping sets were introduced

src/backend/optimizer/plan/planner.c

@@ -2401,13 +2401,8 @@ build_grouping_chain(PlannerInfo *root,
 	 * Prepare the grpColIdx for the real Agg node first, because we may need
 	 * it for sorting
 	 */
-	if (list_length(rollup_groupclauses) > 1)
-	{
-		Assert(rollup_lists && llast(rollup_lists));
-
-		top_grpColIdx =
-			remap_groupColIdx(root, llast(rollup_groupclauses));
-	}
+	if (parse->groupingSets)
+		top_grpColIdx = remap_groupColIdx(root, llast(rollup_groupclauses));
 
 	/*
 	 * If we need a Sort operation on the input, generate that.

src/test/regress/expected/groupingsets.out

@@ -587,4 +587,27 @@ select array(select row(v.a,s1.*) from (select two,four, count(*) from onek grou
  {"(2,0,0,250)","(2,0,2,250)","(2,0,,500)","(2,1,1,250)","(2,1,3,250)","(2,1,,500)","(2,,0,250)","(2,,1,250)","(2,,2,250)","(2,,3,250)","(2,,,1000)"}
 (2 rows)
 
+-- Grouping on text columns
+select sum(ten) from onek group by two, rollup(four::text) order by 1;
+ sum  
+------
+ 1000
+ 1000
+ 1250
+ 1250
+ 2000
+ 2500
+(6 rows)
+
+select sum(ten) from onek group by rollup(four::text), two order by 1;
+ sum  
+------
+ 1000
+ 1000
+ 1250
+ 1250
+ 2000
+ 2500
+(6 rows)
+
 -- end

src/test/regress/sql/groupingsets.sql

@@ -162,4 +162,8 @@ group by rollup(ten);
 select * from (values (1),(2)) v(a) left join lateral (select v.a, four, ten, count(*) from onek group by cube(four,ten)) s on true order by v.a,four,ten;
 select array(select row(v.a,s1.*) from (select two,four, count(*) from onek group by cube(two,four) order by two,four) s1) from (values (1),(2)) v(a);
 
+-- Grouping on text columns
+select sum(ten) from onek group by two, rollup(four::text) order by 1;
+select sum(ten) from onek group by rollup(four::text), two order by 1;
+
 -- end