Offload entry root slice to QE

For some queries, Sort, (Finalized) Aggregate and Join are executed on
QD, which increase workload on a single point. This PR alleviates this
by offloading these operators to a QE. Specifically, it checks whether
the root slice (the slice of the Plan before it meets the first Motion)
contains these operators in should_offload_entry_to_qe_plan_walker, then
it checks whether the offloading can be performed safely in
safe_to_offload_entry_to_qe_rte_walker by walking the range table.
This implementation specifically does not rely on RelOptInfo or anything
postgres-optimizer-specific so that we can port this to Orca in the
future.

src/backend/optimizer/plan/createplan.c

@@ -46,6 +46,7 @@
 #include "optimizer/tlist.h"
 #include "parser/parse_clause.h"
 #include "parser/parsetree.h"
+#include "partitioning/partdesc.h"
 #include "partitioning/partprune.h"
 #include "utils/lsyscache.h"
 #include "utils/uri.h"
@@ -102,6 +103,11 @@ typedef struct
 	bool                  result;
 } contain_motion_walk_context;
 
+typedef struct
+{
+	bool computeOnSlice;    /* does root slice contain computation node (Sort, Join, Agg) */
+} offload_entry_to_qe_plan_walk_context;
+
 static Plan *create_scan_plan(PlannerInfo *root, Path *best_path,
 							  int flags);
 static List *build_path_tlist(PlannerInfo *root, Path *path);
@@ -9063,4 +9069,160 @@ push_locus_down_after_elide_motion(Plan* plan)
 				plan = plan->lefttree;
 		}
 	}
-}
+}
+
+/*
+ * Restore Entry locus to SingleQE in the root slice.
+ * This is simply a reverse of push_locus_down_after_elide_motion.
+ * The difference is that it's NOT used when creating a plan but rather
+ * after a plan gets created, it's used to modify the plan in offload_entry_to_qe.
+ */
+static void
+replace_entry_locus_with_singleqe(Plan *plan)
+{
+	while (plan && (plan->locustype == CdbLocusType_Entry))
+	{
+		plan->locustype = CdbLocusType_SingleQE;
+		switch (nodeTag(plan))
+		{
+			case T_Motion:
+				return;
+			case T_Append:
+			{
+				List *subplans = NIL;
+				ListCell *cell;
+				subplans = ((Append *) (plan))->appendplans;
+				foreach(cell, subplans)
+				{
+					replace_entry_locus_with_singleqe(lfirst(cell));
+				}
+				break;
+			}
+			case T_SubqueryScan:
+				plan = ((SubqueryScan *)(plan))->subplan;
+				break;
+			case T_NestLoop:
+			case T_MergeJoin:
+			case T_HashJoin:
+				replace_entry_locus_with_singleqe(plan->righttree);
+				/* FALLTHROUGH */
+			default:
+				plan = plan->lefttree;
+				break;
+		}
+	}
+}
+
+/*
+ * Check whether we can safely offload root slice on QD to a QE.
+ */
+static bool
+safe_to_offload_entry_to_qe_rte_walker(List *rtes)
+{
+	ListCell *lc;
+	foreach(lc, rtes)
+	{
+		RangeTblEntry *rte = lfirst_node(RangeTblEntry, lc);
+		if (rte->rtekind == RTE_RELATION)
+		{
+			// check if any partition of a partitioned table is a coordinator-only external/foreign table
+			if (rte->relkind == RELKIND_PARTITIONED_TABLE)
+			{
+				Relation rel;
+				PartitionDesc desc;
+
+				rel = relation_open(rte->relid, NoLock);
+				desc = RelationGetPartitionDesc(rel, true);
+				relation_close(rel, NoLock);
+				for (int i = 0; i < desc->nparts; i++)
+				{
+					if (GpPolicyIsEntry(GpPolicyFetch(desc->oids[i])))
+						return false;
+				}
+				return true;
+			}
+			else
+				return !GpPolicyIsEntry(GpPolicyFetch(rte->relid));
+		}
+		else if (rte->rtekind == RTE_SUBQUERY)
+		{
+			if (!safe_to_offload_entry_to_qe_rte_walker(rte->subquery->rtable))
+				return false;
+		}
+	}
+	return true;
+}
+
+/*
+ * Check if there are multiple Motion in which the root slice contains computation (Sort, Join or Aggregate).
+ */
+static bool
+should_offload_entry_to_qe_plan_walker(Plan *plan, offload_entry_to_qe_plan_walk_context *ctx)
+{
+	while (plan && plan->locustype == CdbLocusType_Entry)
+	{
+		switch (nodeTag(plan))
+		{
+			case T_Motion:
+				return ctx->computeOnSlice;
+			case T_SubqueryScan:
+				plan = ((SubqueryScan *) plan)->subplan;
+				break;
+			/* join */
+			case T_Join:
+			case T_MergeJoin:
+			case T_HashJoin:
+			case T_NestLoop:
+				ctx->computeOnSlice = true;
+				if (should_offload_entry_to_qe_plan_walker(plan->righttree, ctx))
+					return true;
+				plan = plan->lefttree;
+				break;
+			/* sort */
+			case T_Sort:
+			/* aggregates*/
+			case T_Agg:
+			case T_WindowAgg:
+				ctx->computeOnSlice = true;
+				/* FALLTHROUGH */
+			default:
+				plan = plan->lefttree;
+				break;
+		}
+	}
+	return false;
+}
+
+Plan *
+offload_entry_to_qe(PlannerInfo *root, Plan *plan, int sendslice_parallel)
+{
+	offload_entry_to_qe_plan_walk_context plan_walk_ctx;
+	plan_walk_ctx.computeOnSlice = false;
+
+	if (root->parse->commandType == CMD_SELECT &&
+		should_offload_entry_to_qe_plan_walker(plan, &plan_walk_ctx) &&
+		safe_to_offload_entry_to_qe_rte_walker(root->parse->rtable) &&
+		!contain_volatile_functions((Node *) root->parse))
+	{
+		CdbPathLocus entrylocus;
+		PlanSlice *sendSlice;
+		sendSlice = (PlanSlice *) palloc0(sizeof(PlanSlice));
+		sendSlice->gangType = GANGTYPE_SINGLETON_READER;
+		sendSlice->numsegments = 1;
+		sendSlice->sliceIndex = -1;
+		sendSlice->parallel_workers = sendslice_parallel;
+		sendSlice->segindex = gp_session_id % getgpsegmentCount();
+
+		replace_entry_locus_with_singleqe(plan);
+
+		plan = (Plan *) make_union_motion(plan);
+		((Motion *) plan)->senderSliceInfo = sendSlice;
+
+		plan->locustype = CdbLocusType_Entry;
+		CdbPathLocus_MakeEntry(&entrylocus);
+		if (plan->flow)
+			pfree(plan->flow);
+		plan->flow = cdbpathtoplan_create_flow(root, entrylocus);
+	}
+	return plan;
+}

src/backend/optimizer/plan/planner.c

@@ -594,6 +594,12 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
 	/* Decorate the top node of the plan with a Flow node. */
 	top_plan->flow = cdbpathtoplan_create_flow(root, best_path->locus);
 
+	/* Modifier: If root slice is executed on QD, try to offload it to a QE */
+	if (enable_offload_entry_to_qe && Gp_role == GP_ROLE_DISPATCH)
+	{
+		top_plan = offload_entry_to_qe(root, top_plan, best_path->locus.parallel_workers);
+	}
+
 	/*
 	 * If creating a plan for a scrollable cursor, make sure it can run
 	 * backwards on demand.  Add a Material node at the top at need.

src/backend/utils/misc/guc_gp.c

@@ -430,6 +430,7 @@ bool		gp_enable_global_deadlock_detector = false;
 bool gp_enable_predicate_pushdown;
 int  gp_predicate_pushdown_sample_rows;
 
+bool        enable_offload_entry_to_qe = false;
 bool enable_answer_query_using_materialized_views = false;
 
 static const struct config_enum_entry gp_log_format_options[] = {
@@ -2429,6 +2430,16 @@ struct config_bool ConfigureNamesBool_gp[] =
 		true,
 		NULL, NULL, NULL
 	},
+	{
+		{"enable_offload_entry_to_qe", PGC_USERSET, DEVELOPER_OPTIONS,
+		    gettext_noop("Enable plans with operations on coordinator to be offloaded to QEs."),
+	        NULL,
+		    GUC_NO_SHOW_ALL | GUC_NOT_IN_SAMPLE | GUC_GPDB_NO_SYNC
+		},
+		&enable_offload_entry_to_qe,
+		false,
+		NULL, NULL, NULL
+	},
 	{
 		{"optimizer_enable_gather_on_segment_for_dml", PGC_USERSET, DEVELOPER_OPTIONS,
 			gettext_noop("Enable DML optimization by enforcing a non-master gather in the optimizer."),

src/include/optimizer/planmain.h

@@ -80,6 +80,7 @@ extern Result *make_result(List *tlist, Node *resconstantqual, Plan *subplan);
 extern Plan *add_sort_cost(PlannerInfo *root, Plan *input, 
 						   double limit_tuples);
 extern Plan *plan_pushdown_tlist(PlannerInfo *root, Plan *plan, List *tlist);      /*CDB*/
+extern Plan *offload_entry_to_qe(PlannerInfo *root, Plan *plan, int sendslice_parallel); /* CBDB */
 
 /* External use of these functions is deprecated: */
 extern Sort *make_sort_from_sortclauses(List *sortcls,

src/include/utils/guc.h

@@ -301,6 +301,7 @@ extern bool enable_parallel;
 extern int  gp_appendonly_insert_files;
 extern int  gp_appendonly_insert_files_tuples_range;
 extern bool enable_answer_query_using_materialized_views;
+extern bool enable_offload_entry_to_qe;
 /*
  * gp_enable_multiphase_limit is not cost based.
  * When set to false, the planner will not use multi-phase limit.

src/include/utils/unsync_guc_name.h

@@ -150,6 +150,7 @@
 		"enable_seqscan",
 		"enable_sort",
 		"enable_tidscan",
+		"enable_offload_entry_to_qe",
 		"escape_string_warning",
 		"event_source",
 		"exit_on_error",