Implement partition-wise grouping/aggregation.

If the partition keys of input relation are part of the GROUP BY
clause, all the rows belonging to a given group come from a single
partition.  This allows aggregation/grouping over a partitioned
relation to be broken down * into aggregation/grouping on each
partition.  This should be no worse, and often better, than the normal
approach.

If the GROUP BY clause does not contain all the partition keys, we can
still perform partial aggregation for each partition and then finalize
aggregation after appending the partial results.  This is less certain
to be a win, but it's still useful.

Jeevan Chalke, Ashutosh Bapat, Robert Haas.  The larger patch series
of which this patch is a part was also reviewed and tested by Antonin
Houska, Rajkumar Raghuwanshi, David Rowley, Dilip Kumar, Konstantin
Knizhnik, Pascal Legrand, and Rafia Sabih.

Discussion: http://postgr.es/m/CAM2+6=V64_xhstVHie0Rz=KPEQnLJMZt_e314P0jaT_oJ9MR8A@mail.gmail.com

doc/src/sgml/config.sgml

@@ -3821,6 +3821,26 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-enable-partitionwise-aggregate" xreflabel="enable_partitionwise_aggregate">
+      <term><varname>enable_partitionwise_aggregate</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>enable_partitionwise_aggregate</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Enables or disables the query planner's use of partitionwise grouping
+        or aggregation, which allows grouping or aggregation on a partitioned
+        tables performed separately for each partition.  If the <literal>GROUP
+        BY</literal> clause does not include the partition keys, only partial
+        aggregation can be performed on a per-partition basis, and
+        finalization must be performed later.  Because partitionwise grouping
+        or aggregation can use significantly more CPU time and memory during
+        planning, the default is <literal>off</literal>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-enable-seqscan" xreflabel="enable_seqscan">
       <term><varname>enable_seqscan</varname> (<type>boolean</type>)
       <indexterm>

src/backend/optimizer/README

@@ -1079,6 +1079,7 @@ busy for a long time to come.
 
 Partitionwise joins
 -------------------
+
 A join between two similarly partitioned tables can be broken down into joins
 between their matching partitions if there exists an equi-join condition
 between the partition keys of the joining tables. The equi-join between
@@ -1102,3 +1103,16 @@ any two partitioned relations with same partitioning scheme point to the same
 PartitionSchemeData object.  This reduces memory consumed by
 PartitionSchemeData objects and makes it easy to compare the partition schemes
 of joining relations.
+
+Partition-wise aggregates/grouping
+----------------------------------
+
+If the GROUP BY clause has contains all of the partition keys, all the rows
+that belong to a given group must come from a single partition; therefore,
+aggregation can be done completely separately for each partition. Otherwise,
+partial aggregates can be computed for each partition, and then finalized
+after appending the results from the individual partitions.  This technique of
+breaking down aggregation or grouping over a partitioned relation into
+aggregation or grouping over its partitions is called partitionwise
+aggregation.  Especially when the partition keys match the GROUP BY clause,
+this can be significantly faster than the regular method.

src/backend/optimizer/path/allpaths.c

@@ -134,8 +134,6 @@ static void subquery_push_qual(Query *subquery,
 static void recurse_push_qual(Node *setOp, Query *topquery,
 				  RangeTblEntry *rte, Index rti, Node *qual);
 static void remove_unused_subquery_outputs(Query *subquery, RelOptInfo *rel);
-static void add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
-						List *live_childrels);
 
 
 /*
@@ -1326,7 +1324,7 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
  * parameterization or ordering. Similarly it collects partial paths from
  * non-dummy children to create partial append paths.
  */
-static void
+void
 add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 						List *live_childrels)
 {
@@ -1413,8 +1411,12 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 		 * If child has an unparameterized cheapest-total path, add that to
 		 * the unparameterized Append path we are constructing for the parent.
 		 * If not, there's no workable unparameterized path.
+		 *
+		 * With partitionwise aggregates, the child rel's pathlist may be
+		 * empty, so don't assume that a path exists here.
 		 */
-		if (childrel->cheapest_total_path->param_info == NULL)
+		if (childrel->pathlist != NIL &&
+			childrel->cheapest_total_path->param_info == NULL)
 			accumulate_append_subpath(childrel->cheapest_total_path,
 									  &subpaths, NULL);
 		else
@@ -1682,6 +1684,13 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 			RelOptInfo *childrel = (RelOptInfo *) lfirst(lcr);
 			Path	   *subpath;
 
+			if (childrel->pathlist == NIL)
+			{
+				/* failed to make a suitable path for this child */
+				subpaths_valid = false;
+				break;
+			}
+
 			subpath = get_cheapest_parameterized_child_path(root,
 															childrel,
 															required_outer);

src/backend/optimizer/path/costsize.c

@@ -135,6 +135,7 @@ bool		enable_mergejoin = true;
 bool		enable_hashjoin = true;
 bool		enable_gathermerge = true;
 bool		enable_partitionwise_join = false;
+bool		enable_partitionwise_aggregate = false;
 bool		enable_parallel_append = true;
 bool		enable_parallel_hash = true;
 

src/backend/optimizer/plan/createplan.c

@@ -1670,7 +1670,15 @@ create_sort_plan(PlannerInfo *root, SortPath *best_path, int flags)
 	subplan = create_plan_recurse(root, best_path->subpath,
 								  flags | CP_SMALL_TLIST);
 
-	plan = make_sort_from_pathkeys(subplan, best_path->path.pathkeys, NULL);
+	/*
+	 * make_sort_from_pathkeys() indirectly calls find_ec_member_for_tle(),
+	 * which will ignore any child EC members that don't belong to the given
+	 * relids. Thus, if this sort path is based on a child relation, we must
+	 * pass its relids.
+	 */
+	plan = make_sort_from_pathkeys(subplan, best_path->path.pathkeys,
+								   IS_OTHER_REL(best_path->subpath->parent) ?
+								   best_path->path.parent->relids : NULL);
 
 	copy_generic_path_info(&plan->plan, (Path *) best_path);
 

src/backend/utils/misc/guc.c

@@ -923,6 +923,15 @@ static struct config_bool ConfigureNamesBool[] =
 		false,
 		NULL, NULL, NULL
 	},
+	{
+		{"enable_partitionwise_aggregate", PGC_USERSET, QUERY_TUNING_METHOD,
+			gettext_noop("Enables partitionwise aggregation and grouping."),
+			NULL
+		},
+		&enable_partitionwise_aggregate,
+		false,
+		NULL, NULL, NULL
+	},
 	{
 		{"enable_parallel_append", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of parallel append plans."),

src/backend/utils/misc/postgresql.conf.sample

@@ -306,6 +306,7 @@
 #enable_sort = on
 #enable_tidscan = on
 #enable_partitionwise_join = off
+#enable_partitionwise_aggregate = off
 #enable_parallel_hash = on
 
 # - Planner Cost Constants -

src/include/nodes/relation.h

@@ -553,6 +553,7 @@ typedef enum RelOptKind
 	RELOPT_OTHER_MEMBER_REL,
 	RELOPT_OTHER_JOINREL,
 	RELOPT_UPPER_REL,
+	RELOPT_OTHER_UPPER_REL,
 	RELOPT_DEADREL
 } RelOptKind;
 
@@ -570,12 +571,15 @@ typedef enum RelOptKind
 	 (rel)->reloptkind == RELOPT_OTHER_JOINREL)
 
 /* Is the given relation an upper relation? */
-#define IS_UPPER_REL(rel) ((rel)->reloptkind == RELOPT_UPPER_REL)
+#define IS_UPPER_REL(rel)	\
+	((rel)->reloptkind == RELOPT_UPPER_REL || \
+	 (rel)->reloptkind == RELOPT_OTHER_UPPER_REL)
 
 /* Is the given relation an "other" relation? */
 #define IS_OTHER_REL(rel) \
 	((rel)->reloptkind == RELOPT_OTHER_MEMBER_REL || \
-	 (rel)->reloptkind == RELOPT_OTHER_JOINREL)
+	 (rel)->reloptkind == RELOPT_OTHER_JOINREL || \
+	 (rel)->reloptkind == RELOPT_OTHER_UPPER_REL)
 
 typedef struct RelOptInfo
 {
@@ -2291,6 +2295,73 @@ typedef struct JoinPathExtraData
 	Relids		param_source_rels;
 } JoinPathExtraData;
 
+/*
+ * Various flags indicating what kinds of grouping are possible.
+ *
+ * GROUPING_CAN_USE_SORT should be set if it's possible to perform
+ * sort-based implementations of grouping.  When grouping sets are in use,
+ * this will be true if sorting is potentially usable for any of the grouping
+ * sets, even if it's not usable for all of them.
+ *
+ * GROUPING_CAN_USE_HASH should be set if it's possible to perform
+ * hash-based implementations of grouping.
+ *
+ * GROUPING_CAN_PARTIAL_AGG should be set if the aggregation is of a type
+ * for which we support partial aggregation (not, for example, grouping sets).
+ * It says nothing about parallel-safety or the availability of suitable paths.
+ */
+#define GROUPING_CAN_USE_SORT       0x0001
+#define GROUPING_CAN_USE_HASH       0x0002
+#define GROUPING_CAN_PARTIAL_AGG	0x0004
+
+/*
+ * What kind of partitionwise aggregation is in use?
+ *
+ * PARTITIONWISE_AGGREGATE_NONE: Not used.
+ *
+ * PARTITIONWISE_AGGREGATE_FULL: Aggregate each partition separately, and
+ * append the results.
+ *
+ * PARTITIONWISE_AGGREGATE_PARTIAL: Partially aggregate each partition
+ * separately, append the results, and then finalize aggregation.
+ */
+typedef enum
+{
+	PARTITIONWISE_AGGREGATE_NONE,
+	PARTITIONWISE_AGGREGATE_FULL,
+	PARTITIONWISE_AGGREGATE_PARTIAL
+} PartitionwiseAggregateType;
+
+/*
+ * Struct for extra information passed to subroutines of create_grouping_paths
+ *
+ * flags indicating what kinds of grouping are possible.
+ * partial_costs_set is true if the agg_partial_costs and agg_final_costs
+ * 		have been initialized.
+ * agg_partial_costs gives partial aggregation costs.
+ * agg_final_costs gives finalization costs.
+ * target is the PathTarget to be used while creating paths.
+ * target_parallel_safe is true if target is parallel safe.
+ * havingQual gives list of quals to be applied after aggregation.
+ * targetList gives list of columns to be projected.
+ * patype is the type of partitionwise aggregation that is being performed.
+ */
+typedef struct
+{
+	/* Data which remains constant once set. */
+	int			flags;
+	bool		partial_costs_set;
+	AggClauseCosts agg_partial_costs;
+	AggClauseCosts agg_final_costs;
+
+	/* Data which may differ across partitions. */
+	PathTarget *target;
+	bool		target_parallel_safe;
+	Node	   *havingQual;
+	List	   *targetList;
+	PartitionwiseAggregateType patype;
+} GroupPathExtraData;
+
 /*
  * For speed reasons, cost estimation for join paths is performed in two
  * phases: the first phase tries to quickly derive a lower bound for the

src/include/optimizer/cost.h

@@ -68,6 +68,7 @@ extern PGDLLIMPORT bool enable_mergejoin;
 extern PGDLLIMPORT bool enable_hashjoin;
 extern PGDLLIMPORT bool enable_gathermerge;
 extern PGDLLIMPORT bool enable_partitionwise_join;
+extern PGDLLIMPORT bool enable_partitionwise_aggregate;
 extern PGDLLIMPORT bool enable_parallel_append;
 extern PGDLLIMPORT bool enable_parallel_hash;
 extern PGDLLIMPORT int	constraint_exclusion;

src/include/optimizer/paths.h

@@ -236,5 +236,7 @@ extern bool has_useful_pathkeys(PlannerInfo *root, RelOptInfo *rel);
 extern PathKey *make_canonical_pathkey(PlannerInfo *root,
 					   EquivalenceClass *eclass, Oid opfamily,
 					   int strategy, bool nulls_first);
+extern void add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
+						List *live_childrels);
 
 #endif							/* PATHS_H */

src/test/regress/expected/sysviews.out

@@ -70,24 +70,25 @@ select count(*) >= 0 as ok from pg_prepared_xacts;
 -- This is to record the prevailing planner enable_foo settings during
 -- a regression test run.
 select name, setting from pg_settings where name like 'enable%';
-           name            | setting 
----------------------------+---------
- enable_bitmapscan         | on
- enable_gathermerge        | on
- enable_hashagg            | on
- enable_hashjoin           | on
- enable_indexonlyscan      | on
- enable_indexscan          | on
- enable_material           | on
- enable_mergejoin          | on
- enable_nestloop           | on
- enable_parallel_append    | on
- enable_parallel_hash      | on
- enable_partitionwise_join | off
- enable_seqscan            | on
- enable_sort               | on
- enable_tidscan            | on
-(15 rows)
+              name              | setting 
+--------------------------------+---------
+ enable_bitmapscan              | on
+ enable_gathermerge             | on
+ enable_hashagg                 | on
+ enable_hashjoin                | on
+ enable_indexonlyscan           | on
+ enable_indexscan               | on
+ enable_material                | on
+ enable_mergejoin               | on
+ enable_nestloop                | on
+ enable_parallel_append         | on
+ enable_parallel_hash           | on
+ enable_partitionwise_aggregate | off
+ enable_partitionwise_join      | off
+ enable_seqscan                 | on
+ enable_sort                    | on
+ enable_tidscan                 | on
+(16 rows)
 
 -- Test that the pg_timezone_names and pg_timezone_abbrevs views are
 -- more-or-less working.  We can't test their contents in any great detail

src/test/regress/parallel_schedule

@@ -116,7 +116,7 @@ test: plancache limit plpgsql copy2 temp domain rangefuncs prepare without_oid c
 # ----------
 # Another group of parallel tests
 # ----------
-test: identity partition_join partition_prune reloptions hash_part indexing
+test: identity partition_join partition_prune reloptions hash_part indexing partition_aggregate
 
 # event triggers cannot run concurrently with any test that runs DDL
 test: event_trigger

src/test/regress/serial_schedule

@@ -185,5 +185,6 @@ test: partition_prune
 test: reloptions
 test: hash_part
 test: indexing
+test: partition_aggregate
 test: event_trigger
 test: stats

src/tools/pgindent/typedefs.list

@@ -884,6 +884,7 @@ GrantStmt
 GrantTargetType
 Group
 GroupPath
+GroupPathExtraData
 GroupState
 GroupVarInfo
 GroupingFunc
@@ -1597,6 +1598,7 @@ PartitionScheme
 PartitionSpec
 PartitionTupleRouting
 PartitionedChildRelInfo
+PartitionwiseAggregateType
 PasswordType
 Path
 PathClauseUsage