postgres_fdw: Push down partition-wise aggregation.

Since commit 7012b132d07c2b4ea15b0b3cb1ea9f3278801d98, postgres_fdw
has been able to push down the toplevel aggregation operation to the
remote server.  Commit e2f1eb0ee30d144628ab523432320f174a2c8966 made
it possible to break down the toplevel aggregation into one
aggregate per partition.  This commit lets postgres_fdw push down
aggregation in that case just as it does at the top level.

In order to make this work, this commit adds an additional argument
to the GetForeignUpperPaths FDW API.  A matching argument is added
to the signature for create_upper_paths_hook.  Third-party code using
either of these will need to be updated.

Also adjust create_foreignscan_plan() so that it picks up the correct
set of relids in this case.

Jeevan Chalke, reviewed by Ashutosh Bapat and by me and with some
adjustments by me.  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
Discussion: http://postgr.es/m/CAM2+6=XPWujjmj5zUaBTGDoB38CemwcPmjkRy0qOcsQj_V+2sQ@mail.gmail.com

1 files changed, 37 insertions, 16 deletions

diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index e8a0d5482a8..a15ce28a48b 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -352,7 +352,8 @@ static bool postgresRecheckForeignScan(ForeignScanState *node,
 static void postgresGetForeignUpperPaths(PlannerInfo *root,
 							 UpperRelationKind stage,
 							 RelOptInfo *input_rel,
-							 RelOptInfo *output_rel);
+							 RelOptInfo *output_rel,
+							 void *extra);
 
 /*
  * Helper functions
@@ -419,7 +420,8 @@ static void conversion_error_callback(void *arg);
 static bool foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel,
 				JoinType jointype, RelOptInfo *outerrel, RelOptInfo *innerrel,
 				JoinPathExtraData *extra);
-static bool foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel);
+static bool foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel,
+					Node *havingQual);
 static List *get_useful_pathkeys_for_relation(PlannerInfo *root,
 								 RelOptInfo *rel);
 static List *get_useful_ecs_for_relation(PlannerInfo *root, RelOptInfo *rel);
@@ -427,7 +429,8 @@ static void add_paths_with_pathkeys_for_rel(PlannerInfo *root, RelOptInfo *rel,
 								Path *epq_path);
 static void add_foreign_grouping_paths(PlannerInfo *root,
 						   RelOptInfo *input_rel,
-						   RelOptInfo *grouped_rel);
+						   RelOptInfo *grouped_rel,
+						   GroupPathExtraData *extra);
 static void apply_server_options(PgFdwRelationInfo *fpinfo);
 static void apply_table_options(PgFdwRelationInfo *fpinfo);
 static void merge_fdw_options(PgFdwRelationInfo *fpinfo,
@@ -2775,12 +2778,15 @@ estimate_path_cost_size(PlannerInfo *root,
 		else if (IS_UPPER_REL(foreignrel))
 		{
 			PgFdwRelationInfo *ofpinfo;
-			PathTarget *ptarget = root->upper_targets[UPPERREL_GROUP_AGG];
+			PathTarget *ptarget = foreignrel->reltarget;
 			AggClauseCosts aggcosts;
 			double		input_rows;
 			int			numGroupCols;
 			double		numGroups = 1;
 
+			/* Make sure the core code set the pathtarget. */
+			Assert(ptarget != NULL);
+
 			/*
 			 * This cost model is mixture of costing done for sorted and
 			 * hashed aggregates in cost_agg().  We are not sure which
@@ -2805,6 +2811,12 @@ estimate_path_cost_size(PlannerInfo *root,
 			{
 				get_agg_clause_costs(root, (Node *) fpinfo->grouped_tlist,
 									 AGGSPLIT_SIMPLE, &aggcosts);
+
+				/*
+				 * The cost of aggregates in the HAVING qual will be the same
+				 * for each child as it is for the parent, so there's no need
+				 * to use a translated version of havingQual.
+				 */
 				get_agg_clause_costs(root, (Node *) root->parse->havingQual,
 									 AGGSPLIT_SIMPLE, &aggcosts);
 			}
@@ -5017,11 +5029,12 @@ postgresGetForeignJoinPaths(PlannerInfo *root,
  * this function to PgFdwRelationInfo of the input relation.
  */
 static bool
-foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel)
+foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel,
+					Node *havingQual)
 {
 	Query	   *query = root->parse;
-	PathTarget *grouping_target = root->upper_targets[UPPERREL_GROUP_AGG];
 	PgFdwRelationInfo *fpinfo = (PgFdwRelationInfo *) grouped_rel->fdw_private;
+	PathTarget *grouping_target = grouped_rel->reltarget;
 	PgFdwRelationInfo *ofpinfo;
 	List	   *aggvars;
 	ListCell   *lc;
@@ -5131,11 +5144,11 @@ foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel)
 	 * Classify the pushable and non-pushable HAVING clauses and save them in
 	 * remote_conds and local_conds of the grouped rel's fpinfo.
 	 */
-	if (root->hasHavingQual && query->havingQual)
+	if (havingQual)
 	{
 		ListCell   *lc;
 
-		foreach(lc, (List *) query->havingQual)
+		foreach(lc, (List *) havingQual)
 		{
 			Expr	   *expr = (Expr *) lfirst(lc);
 			RestrictInfo *rinfo;
@@ -5232,7 +5245,8 @@ foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel)
  */
 static void
 postgresGetForeignUpperPaths(PlannerInfo *root, UpperRelationKind stage,
-							 RelOptInfo *input_rel, RelOptInfo *output_rel)
+							 RelOptInfo *input_rel, RelOptInfo *output_rel,
+							 void *extra)
 {
 	PgFdwRelationInfo *fpinfo;
 
@@ -5252,7 +5266,8 @@ postgresGetForeignUpperPaths(PlannerInfo *root, UpperRelationKind stage,
 	fpinfo->pushdown_safe = false;
 	output_rel->fdw_private = fpinfo;
 
-	add_foreign_grouping_paths(root, input_rel, output_rel);
+	add_foreign_grouping_paths(root, input_rel, output_rel,
+							   (GroupPathExtraData *) extra);
 }
 
 /*
@@ -5264,13 +5279,13 @@ postgresGetForeignUpperPaths(PlannerInfo *root, UpperRelationKind stage,
  */
 static void
 add_foreign_grouping_paths(PlannerInfo *root, RelOptInfo *input_rel,
-						   RelOptInfo *grouped_rel)
+						   RelOptInfo *grouped_rel,
+						   GroupPathExtraData *extra)
 {
 	Query	   *parse = root->parse;
 	PgFdwRelationInfo *ifpinfo = input_rel->fdw_private;
 	PgFdwRelationInfo *fpinfo = grouped_rel->fdw_private;
 	ForeignPath *grouppath;
-	PathTarget *grouping_target;
 	double		rows;
 	int			width;
 	Cost		startup_cost;
@@ -5281,7 +5296,8 @@ add_foreign_grouping_paths(PlannerInfo *root, RelOptInfo *input_rel,
 		!root->hasHavingQual)
 		return;
 
-	grouping_target = root->upper_targets[UPPERREL_GROUP_AGG];
+	Assert(extra->patype == PARTITIONWISE_AGGREGATE_NONE ||
+		   extra->patype == PARTITIONWISE_AGGREGATE_FULL);
 
 	/* save the input_rel as outerrel in fpinfo */
 	fpinfo->outerrel = input_rel;
@@ -5295,8 +5311,13 @@ add_foreign_grouping_paths(PlannerInfo *root, RelOptInfo *input_rel,
 	fpinfo->user = ifpinfo->user;
 	merge_fdw_options(fpinfo, ifpinfo, NULL);
 
-	/* Assess if it is safe to push down aggregation and grouping. */
-	if (!foreign_grouping_ok(root, grouped_rel))
+	/*
+	 * Assess if it is safe to push down aggregation and grouping.
+	 *
+	 * Use HAVING qual from extra. In case of child partition, it will have
+	 * translated Vars.
+	 */
+	if (!foreign_grouping_ok(root, grouped_rel, extra->havingQual))
 		return;
 
 	/* Estimate the cost of push down */
@@ -5312,7 +5333,7 @@ add_foreign_grouping_paths(PlannerInfo *root, RelOptInfo *input_rel,
 	/* Create and add foreign path to the grouping relation. */
 	grouppath = create_foreignscan_path(root,
 										grouped_rel,
-										grouping_target,
+										grouped_rel->reltarget,
 										rows,
 										startup_cost,
 										total_cost,