Improve EXPLAIN to print the grouping columns in Agg and Group nodes.

Per request from Kevin Grittner.

doc/src/sgml/ref/explain.sgml

@@ -396,12 +396,13 @@ PREPARE query(int, int) AS SELECT sum(bar) FROM test
 EXPLAIN ANALYZE EXECUTE query(100, 200);
 
                                                        QUERY PLAN                                                       
--------------------------------------------------------------------------------------------------------------------------
- HashAggregate  (cost=39.53..39.53 rows=1 width=8) (actual time=0.661..0.672 rows=7 loops=1)
-   ->  Index Scan using test_pkey on test  (cost=0.00..32.97 rows=1311 width=8) (actual time=0.050..0.395 rows=99 loops=1)
+------------------------------------------------------------------------------------------------------------------------
+ HashAggregate  (cost=9.54..9.54 rows=1 width=8) (actual time=0.156..0.161 rows=11 loops=1)
+   Group Key: foo
+   ->  Index Scan using test_pkey on test  (cost=0.29..9.29 rows=50 width=8) (actual time=0.039..0.091 rows=99 loops=1)
          Index Cond: ((id > $1) AND (id < $2))
- Total runtime: 0.851 ms
-(4 rows)
+ Total runtime: 0.225 ms
+(5 rows)
 </programlisting>
   </para>
 

src/backend/commands/explain.c

@@ -76,7 +76,11 @@ static void show_sort_keys(SortState *sortstate, List *ancestors,
 			   ExplainState *es);
 static void show_merge_append_keys(MergeAppendState *mstate, List *ancestors,
 					   ExplainState *es);
-static void show_sort_keys_common(PlanState *planstate,
+static void show_agg_keys(AggState *astate, List *ancestors,
+			  ExplainState *es);
+static void show_group_keys(GroupState *gstate, List *ancestors,
+				ExplainState *es);
+static void show_sort_group_keys(PlanState *planstate, const char *qlabel,
 					 int nkeys, AttrNumber *keycols,
 					 List *ancestors, ExplainState *es);
 static void show_sort_info(SortState *sortstate, ExplainState *es);
@@ -1341,7 +1345,14 @@ ExplainNode(PlanState *planstate, List *ancestors,
 										   planstate, es);
 			break;
 		case T_Agg:
+			show_agg_keys((AggState *) planstate, ancestors, es);
+			show_upper_qual(plan->qual, "Filter", planstate, ancestors, es);
+			if (plan->qual)
+				show_instrumentation_count("Rows Removed by Filter", 1,
+										   planstate, es);
+			break;
 		case T_Group:
+			show_group_keys((GroupState *) planstate, ancestors, es);
 			show_upper_qual(plan->qual, "Filter", planstate, ancestors, es);
 			if (plan->qual)
 				show_instrumentation_count("Rows Removed by Filter", 1,
@@ -1693,7 +1704,7 @@ show_sort_keys(SortState *sortstate, List *ancestors, ExplainState *es)
 {
 	Sort	   *plan = (Sort *) sortstate->ss.ps.plan;
 
-	show_sort_keys_common((PlanState *) sortstate,
+	show_sort_group_keys((PlanState *) sortstate, "Sort Key",
 						 plan->numCols, plan->sortColIdx,
 						 ancestors, es);
 }
@@ -1707,13 +1718,55 @@ show_merge_append_keys(MergeAppendState *mstate, List *ancestors,
 {
 	MergeAppend *plan = (MergeAppend *) mstate->ps.plan;
 
-	show_sort_keys_common((PlanState *) mstate,
+	show_sort_group_keys((PlanState *) mstate, "Sort Key",
 						 plan->numCols, plan->sortColIdx,
 						 ancestors, es);
 }
 
+/*
+ * Show the grouping keys for an Agg node.
+ */
+static void
+show_agg_keys(AggState *astate, List *ancestors,
+			  ExplainState *es)
+{
+	Agg		   *plan = (Agg *) astate->ss.ps.plan;
+
+	if (plan->numCols > 0)
+	{
+		/* The key columns refer to the tlist of the child plan */
+		ancestors = lcons(astate, ancestors);
+		show_sort_group_keys(outerPlanState(astate), "Group Key",
+							 plan->numCols, plan->grpColIdx,
+							 ancestors, es);
+		ancestors = list_delete_first(ancestors);
+	}
+}
+
+/*
+ * Show the grouping keys for a Group node.
+ */
+static void
+show_group_keys(GroupState *gstate, List *ancestors,
+				ExplainState *es)
+{
+	Group	   *plan = (Group *) gstate->ss.ps.plan;
+
+	/* The key columns refer to the tlist of the child plan */
+	ancestors = lcons(gstate, ancestors);
+	show_sort_group_keys(outerPlanState(gstate), "Group Key",
+						 plan->numCols, plan->grpColIdx,
+						 ancestors, es);
+	ancestors = list_delete_first(ancestors);
+}
+
+/*
+ * Common code to show sort/group keys, which are represented in plan nodes
+ * as arrays of targetlist indexes
+ */
 static void
-show_sort_keys_common(PlanState *planstate, int nkeys, AttrNumber *keycols,
+show_sort_group_keys(PlanState *planstate, const char *qlabel,
+					 int nkeys, AttrNumber *keycols,
 					 List *ancestors, ExplainState *es)
 {
 	Plan	   *plan = planstate->plan;
@@ -1748,7 +1801,7 @@ show_sort_keys_common(PlanState *planstate, int nkeys, AttrNumber *keycols,
 		result = lappend(result, exprstr);
 	}
 
-	ExplainPropertyList("Sort Key", result, es);
+	ExplainPropertyList(qlabel, result, es);
 }
 
 /*

src/test/regress/expected/aggregates.out

@@ -659,12 +659,13 @@ explain (costs off)
                              QUERY PLAN                              
 ---------------------------------------------------------------------
  HashAggregate
+   Group Key: $0
    InitPlan 1 (returns $0)
      ->  Limit
            ->  Index Only Scan Backward using tenk1_unique2 on tenk1
                  Index Cond: (unique2 IS NOT NULL)
    ->  Result
-(6 rows)
+(7 rows)
 
 select distinct max(unique2) from tenk1;
  max  
@@ -806,6 +807,7 @@ explain (costs off)
                                           QUERY PLAN                                          
 ----------------------------------------------------------------------------------------------
  HashAggregate
+   Group Key: $0, $1
    InitPlan 1 (returns $0)
      ->  Limit
            ->  Merge Append
@@ -831,7 +833,7 @@ explain (costs off)
                  ->  Index Only Scan Backward using minmaxtest3i on minmaxtest3 minmaxtest3_1
                        Index Cond: (f1 IS NOT NULL)
    ->  Result
-(26 rows)
+(27 rows)
 
 select distinct min(f1), max(f1) from minmaxtest;
  min | max 

src/test/regress/expected/matview.out

@@ -22,8 +22,9 @@ EXPLAIN (costs off)
      QUERY PLAN      
 ---------------------
  HashAggregate
+   Group Key: type
    ->  Seq Scan on t
-(2 rows)
+(3 rows)
 
 CREATE MATERIALIZED VIEW tm AS SELECT type, sum(amt) AS totamt FROM t GROUP BY type WITH NO DATA;
 SELECT relispopulated FROM pg_class WHERE oid = 'tm'::regclass;
@@ -59,8 +60,9 @@ EXPLAIN (costs off)
  Sort
    Sort Key: t.type
    ->  HashAggregate
+         Group Key: t.type
          ->  Seq Scan on t
-(4 rows)
+(5 rows)
 
 CREATE MATERIALIZED VIEW tvm AS SELECT * FROM tv ORDER BY type;
 SELECT * FROM tvm;
@@ -82,8 +84,9 @@ EXPLAIN (costs off)
 ---------------------------
  Aggregate
    ->  HashAggregate
+         Group Key: t.type
          ->  Seq Scan on t
-(3 rows)
+(4 rows)
 
 CREATE MATERIALIZED VIEW tvvm AS SELECT * FROM tvv;
 CREATE VIEW tvvmv AS SELECT * FROM tvvm;

src/test/regress/expected/union.out

@@ -494,12 +494,13 @@ explain (costs off)
                     QUERY PLAN                     
 ---------------------------------------------------
  HashAggregate
+   Group Key: ((t1.a || t1.b))
    ->  Append
          ->  Index Scan using t1_ab_idx on t1
                Index Cond: ((a || b) = 'ab'::text)
          ->  Index Only Scan using t2_pkey on t2
                Index Cond: (ab = 'ab'::text)
-(6 rows)
+(7 rows)
 
 reset enable_seqscan;
 reset enable_indexscan;
@@ -553,16 +554,17 @@ SELECT * FROM
 WHERE x < 4
 ORDER BY x;
                        QUERY PLAN                       
---------------------------------
+--------------------------------------------------------
  Sort
    Sort Key: ss.x
    ->  Subquery Scan on ss
          Filter: (ss.x < 4)
          ->  HashAggregate
+               Group Key: (1), (generate_series(1, 10))
                ->  Append
                      ->  Result
                      ->  Result
-(8 rows)
+(9 rows)
 
 SELECT * FROM
   (SELECT 1 AS t, generate_series(1,10) AS x

src/test/regress/expected/window.out

@@ -620,11 +620,12 @@ select first_value(max(x)) over (), y
   from (select unique1 as x, ten+four as y from tenk1) ss
   group by y;
                  QUERY PLAN                  
--------------------------------
+---------------------------------------------
  WindowAgg
    ->  HashAggregate
+         Group Key: (tenk1.ten + tenk1.four)
          ->  Seq Scan on tenk1
-(3 rows)
+(4 rows)
 
 -- test non-default frame specifications
 SELECT four, ten,