Add a Gather executor node.

A Gather executor node runs any number of copies of a plan in an equal
number of workers and merges all of the results into a single tuple
stream.  It can also run the plan itself, if the workers are
unavailable or haven't started up yet.  It is intended to work with
the Partial Seq Scan node which will be added in future commits.

It could also be used to implement parallel query of a different sort
by itself, without help from Partial Seq Scan, if the single_copy mode
is used.  In that mode, a worker executes the plan, and the parallel
leader does not, merely collecting the worker's results.  So, a Gather
node could be inserted into a plan to split the execution of that plan
across two processes.  Nested Gather nodes aren't currently supported,
but we might want to add support for that in the future.

There's nothing in the planner to actually generate Gather nodes yet,
so it's not quite time to break out the champagne.  But we're getting
close.

Amit Kapila.  Some designs suggestions were provided by me, and I also
reviewed the patch.  Single-copy mode, documentation, and other minor
changes also by me.

1 files changed, 57 insertions, 0 deletions

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 404c6f593d7..0ee7392bcce 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -60,6 +60,8 @@ static SeqScan *create_seqscan_plan(PlannerInfo *root, Path *best_path,
 					List *tlist, List *scan_clauses);
 static SampleScan *create_samplescan_plan(PlannerInfo *root, Path *best_path,
 					   List *tlist, List *scan_clauses);
+static Gather *create_gather_plan(PlannerInfo *root,
+				   GatherPath *best_path);
 static Scan *create_indexscan_plan(PlannerInfo *root, IndexPath *best_path,
 					  List *tlist, List *scan_clauses, bool indexonly);
 static BitmapHeapScan *create_bitmap_scan_plan(PlannerInfo *root,
@@ -104,6 +106,8 @@ static void copy_plan_costsize(Plan *dest, Plan *src);
 static SeqScan *make_seqscan(List *qptlist, List *qpqual, Index scanrelid);
 static SampleScan *make_samplescan(List *qptlist, List *qpqual, Index scanrelid,
 				TableSampleClause *tsc);
+static Gather *make_gather(List *qptlist, List *qpqual,
+			int nworkers, bool single_copy, Plan *subplan);
 static IndexScan *make_indexscan(List *qptlist, List *qpqual, Index scanrelid,
 			   Oid indexid, List *indexqual, List *indexqualorig,
 			   List *indexorderby, List *indexorderbyorig,
@@ -273,6 +277,10 @@ create_plan_recurse(PlannerInfo *root, Path *best_path)
 			plan = create_unique_plan(root,
 									  (UniquePath *) best_path);
 			break;
+		case T_Gather:
+			plan = (Plan *) create_gather_plan(root,
+											   (GatherPath *) best_path);
+			break;
 		default:
 			elog(ERROR, "unrecognized node type: %d",
 				 (int) best_path->pathtype);
@@ -1101,6 +1109,34 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path)
 	return plan;
 }
 
+/*
+ * create_gather_plan
+ *
+ *	  Create a Gather plan for 'best_path' and (recursively) plans
+ *	  for its subpaths.
+ */
+static Gather *
+create_gather_plan(PlannerInfo *root, GatherPath *best_path)
+{
+	Gather	   *gather_plan;
+	Plan	   *subplan;
+
+	subplan = create_plan_recurse(root, best_path->subpath);
+
+	gather_plan = make_gather(subplan->targetlist,
+							  NIL,
+							  best_path->num_workers,
+							  best_path->single_copy,
+							  subplan);
+
+	copy_path_costsize(&gather_plan->plan, &best_path->path);
+
+	/* use parallel mode for parallel plans. */
+	root->glob->parallelModeNeeded = true;
+
+	return gather_plan;
+}
+
 
 /*****************************************************************************
  *
@@ -4735,6 +4771,27 @@ make_unique(Plan *lefttree, List *distinctList)
 	return node;
 }
 
+static Gather *
+make_gather(List *qptlist,
+			List *qpqual,
+			int nworkers,
+			bool single_copy,
+			Plan *subplan)
+{
+	Gather	   *node = makeNode(Gather);
+	Plan	   *plan = &node->plan;
+
+	/* cost should be inserted by caller */
+	plan->targetlist = qptlist;
+	plan->qual = qpqual;
+	plan->lefttree = subplan;
+	plan->righttree = NULL;
+	node->num_workers = nworkers;
+	node->single_copy = single_copy;
+
+	return node;
+}
+
 /*
  * distinctList is a list of SortGroupClauses, identifying the targetlist
  * items that should be considered by the SetOp filter.  The input path must