Add optimizer and executor support for parallel index scans.

In combination with 569174f1be92be93f5366212cc46960d28a5c5cd, which
taught the btree AM how to perform parallel index scans, this allows
parallel index scan plans on btree indexes.  This infrastructure
should be general enough to support parallel index scans for other
index AMs as well, if someone updates them to support parallel
scans.

Amit Kapila, reviewed and tested by Anastasia Lubennikova, Tushar
Ahuja, and Haribabu Kommi, and me.

6 files changed, 124 insertions, 22 deletions

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 85505c57d36..eeacf815e35 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -127,8 +127,6 @@ 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 int compute_parallel_worker(RelOptInfo *rel, BlockNumber heap_pages,
-						BlockNumber index_pages);
 
 
 /*
@@ -2885,7 +2883,7 @@ remove_unused_subquery_outputs(Query *subquery, RelOptInfo *rel)
  * "heap_pages" is the number of pages from the table that we expect to scan.
  * "index_pages" is the number of pages from the index that we expect to scan.
  */
-static int
+int
 compute_parallel_worker(RelOptInfo *rel, BlockNumber heap_pages,
 						BlockNumber index_pages)
 {
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index a43daa744c7..d01630f8dba 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -391,7 +391,8 @@ cost_gather(GatherPath *path, PlannerInfo *root,
  * we have to fetch from the table, so they don't reduce the scan cost.
  */
 void
-cost_index(IndexPath *path, PlannerInfo *root, double loop_count)
+cost_index(IndexPath *path, PlannerInfo *root, double loop_count,
+		   bool partial_path)
 {
 	IndexOptInfo *index = path->indexinfo;
 	RelOptInfo *baserel = index->rel;
@@ -400,6 +401,7 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count)
 	List	   *qpquals;
 	Cost		startup_cost = 0;
 	Cost		run_cost = 0;
+	Cost		cpu_run_cost = 0;
 	Cost		indexStartupCost;
 	Cost		indexTotalCost;
 	Selectivity indexSelectivity;
@@ -413,6 +415,8 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count)
 	Cost		cpu_per_tuple;
 	double		tuples_fetched;
 	double		pages_fetched;
+	double		rand_heap_pages;
+	double		index_pages;
 
 	/* Should only be applied to base relations */
 	Assert(IsA(baserel, RelOptInfo) &&
@@ -459,7 +463,8 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count)
 	amcostestimate = (amcostestimate_function) index->amcostestimate;
 	amcostestimate(root, path, loop_count,
 				   &indexStartupCost, &indexTotalCost,
-				   &indexSelectivity, &indexCorrelation);
+				   &indexSelectivity, &indexCorrelation,
+				   &index_pages);
 
 	/*
 	 * Save amcostestimate's results for possible use in bitmap scan planning.
@@ -526,6 +531,8 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count)
 		if (indexonly)
 			pages_fetched = ceil(pages_fetched * (1.0 - baserel->allvisfrac));
 
+		rand_heap_pages = pages_fetched;
+
 		max_IO_cost = (pages_fetched * spc_random_page_cost) / loop_count;
 
 		/*
@@ -564,6 +571,8 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count)
 		if (indexonly)
 			pages_fetched = ceil(pages_fetched * (1.0 - baserel->allvisfrac));
 
+		rand_heap_pages = pages_fetched;
+
 		/* max_IO_cost is for the perfectly uncorrelated case (csquared=0) */
 		max_IO_cost = pages_fetched * spc_random_page_cost;
 
@@ -583,6 +592,29 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count)
 			min_IO_cost = 0;
 	}
 
+	if (partial_path)
+	{
+		/*
+		 * Estimate the number of parallel workers required to scan index. Use
+		 * the number of heap pages computed considering heap fetches won't be
+		 * sequential as for parallel scans the pages are accessed in random
+		 * order.
+		 */
+		path->path.parallel_workers = compute_parallel_worker(baserel,
+											   (BlockNumber) rand_heap_pages,
+												  (BlockNumber) index_pages);
+
+		/*
+		 * Fall out if workers can't be assigned for parallel scan, because in
+		 * such a case this path will be rejected.  So there is no benefit in
+		 * doing extra computation.
+		 */
+		if (path->path.parallel_workers <= 0)
+			return;
+
+		path->path.parallel_aware = true;
+	}
+
 	/*
 	 * Now interpolate based on estimated index order correlation to get total
 	 * disk I/O cost for main table accesses.
@@ -602,11 +634,24 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count)
 	startup_cost += qpqual_cost.startup;
 	cpu_per_tuple = cpu_tuple_cost + qpqual_cost.per_tuple;
 
-	run_cost += cpu_per_tuple * tuples_fetched;
+	cpu_run_cost += cpu_per_tuple * tuples_fetched;
 
 	/* tlist eval costs are paid per output row, not per tuple scanned */
 	startup_cost += path->path.pathtarget->cost.startup;
-	run_cost += path->path.pathtarget->cost.per_tuple * path->path.rows;
+	cpu_run_cost += path->path.pathtarget->cost.per_tuple * path->path.rows;
+
+	/* Adjust costing for parallelism, if used. */
+	if (path->path.parallel_workers > 0)
+	{
+		double		parallel_divisor = get_parallel_divisor(&path->path);
+
+		path->path.rows = clamp_row_est(path->path.rows / parallel_divisor);
+
+		/* The CPU cost is divided among all the workers. */
+		cpu_run_cost /= parallel_divisor;
+	}
+
+	run_cost += cpu_run_cost;
 
 	path->path.startup_cost = startup_cost;
 	path->path.total_cost = startup_cost + run_cost;
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 52834689881..56eccafd7b0 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -813,7 +813,7 @@ get_index_paths(PlannerInfo *root, RelOptInfo *rel,
 /*
  * build_index_paths
  *	  Given an index and a set of index clauses for it, construct zero
- *	  or more IndexPaths.
+ *	  or more IndexPaths. It also constructs zero or more partial IndexPaths.
  *
  * We return a list of paths because (1) this routine checks some cases
  * that should cause us to not generate any IndexPath, and (2) in some
@@ -1042,8 +1042,41 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 								  NoMovementScanDirection,
 								  index_only_scan,
 								  outer_relids,
-								  loop_count);
+								  loop_count,
+								  false);
 		result = lappend(result, ipath);
+
+		/*
+		 * If appropriate, consider parallel index scan.  We don't allow
+		 * parallel index scan for bitmap or index only scans.
+		 */
+		if (index->amcanparallel && !index_only_scan &&
+			rel->consider_parallel && outer_relids == NULL &&
+			scantype != ST_BITMAPSCAN)
+		{
+			ipath = create_index_path(root, index,
+									  index_clauses,
+									  clause_columns,
+									  orderbyclauses,
+									  orderbyclausecols,
+									  useful_pathkeys,
+									  index_is_ordered ?
+									  ForwardScanDirection :
+									  NoMovementScanDirection,
+									  index_only_scan,
+									  outer_relids,
+									  loop_count,
+									  true);
+
+			/*
+			 * if, after costing the path, we find that it's not worth
+			 * using parallel workers, just free it.
+			 */
+			if (ipath->path.parallel_workers > 0)
+				add_partial_path(rel, (Path *) ipath);
+			else
+				pfree(ipath);
+		}
 	}
 
 	/*
@@ -1066,8 +1099,36 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 									  BackwardScanDirection,
 									  index_only_scan,
 									  outer_relids,
-									  loop_count);
+									  loop_count,
+									  false);
 			result = lappend(result, ipath);
+
+			/* If appropriate, consider parallel index scan */
+			if (index->amcanparallel && !index_only_scan &&
+				rel->consider_parallel && outer_relids == NULL &&
+				scantype != ST_BITMAPSCAN)
+			{
+				ipath = create_index_path(root, index,
+										  index_clauses,
+										  clause_columns,
+										  NIL,
+										  NIL,
+										  useful_pathkeys,
+										  BackwardScanDirection,
+										  index_only_scan,
+										  outer_relids,
+										  loop_count,
+										  true);
+
+				/*
+				 * if, after costing the path, we find that it's not worth
+				 * using parallel workers, just free it.
+				 */
+				if (ipath->path.parallel_workers > 0)
+					add_partial_path(rel, (Path *) ipath);
+				else
+					pfree(ipath);
+			}
 		}
 	}
 
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index abb4f12cea1..3d33d469713 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -5333,7 +5333,7 @@ plan_cluster_use_sort(Oid tableOid, Oid indexOid)
 	indexScanPath = create_index_path(root, indexInfo,
 									  NIL, NIL, NIL, NIL, NIL,
 									  ForwardScanDirection, false,
-									  NULL, 1.0);
+									  NULL, 1.0, false);
 
 	return (seqScanAndSortPath.total_cost < indexScanPath->path.total_cost);
 }
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index f440875ceb1..324829690d2 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -744,10 +744,9 @@ add_path_precheck(RelOptInfo *parent_rel,
  *	  As with add_path, we pfree paths that are found to be dominated by
  *	  another partial path; this requires that there be no other references to
  *	  such paths yet.  Hence, GatherPaths must not be created for a rel until
- *	  we're done creating all partial paths for it.  We do not currently build
- *	  partial indexscan paths, so there is no need for an exception for
- *	  IndexPaths here; for safety, we instead Assert that a path to be freed
- *	  isn't an IndexPath.
+ *	  we're done creating all partial paths for it.  Unlike add_path, we don't
+ *	  take an exception for IndexPaths as partial index paths won't be
+ *	  referenced by partial BitmapHeapPaths.
  */
 void
 add_partial_path(RelOptInfo *parent_rel, Path *new_path)
@@ -826,8 +825,6 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
 		{
 			parent_rel->partial_pathlist =
 				list_delete_cell(parent_rel->partial_pathlist, p1, p1_prev);
-			/* we should not see IndexPaths here, so always safe to delete */
-			Assert(!IsA(old_path, IndexPath));
 			pfree(old_path);
 			/* p1_prev does not advance */
 		}
@@ -860,8 +857,6 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
 	}
 	else
 	{
-		/* we should not see IndexPaths here, so always safe to delete */
-		Assert(!IsA(new_path, IndexPath));
 		/* Reject and recycle the new path */
 		pfree(new_path);
 	}
@@ -1005,6 +1000,7 @@ create_samplescan_path(PlannerInfo *root, RelOptInfo *rel, Relids required_outer
  * 'required_outer' is the set of outer relids for a parameterized path.
  * 'loop_count' is the number of repetitions of the indexscan to factor into
  *		estimates of caching behavior.
+ * 'partial_path' is true if constructing a parallel index scan path.
  *
  * Returns the new path node.
  */
@@ -1019,7 +1015,8 @@ create_index_path(PlannerInfo *root,
 				  ScanDirection indexscandir,
 				  bool indexonly,
 				  Relids required_outer,
-				  double loop_count)
+				  double loop_count,
+				  bool partial_path)
 {
 	IndexPath  *pathnode = makeNode(IndexPath);
 	RelOptInfo *rel = index->rel;
@@ -1049,7 +1046,7 @@ create_index_path(PlannerInfo *root,
 	pathnode->indexorderbycols = indexorderbycols;
 	pathnode->indexscandir = indexscandir;
 
-	cost_index(pathnode, root, loop_count);
+	cost_index(pathnode, root, loop_count, partial_path);
 
 	return pathnode;
 }
@@ -3247,7 +3244,7 @@ reparameterize_path(PlannerInfo *root, Path *path,
 				memcpy(newpath, ipath, sizeof(IndexPath));
 				newpath->path.param_info =
 					get_baserel_parampathinfo(root, rel, required_outer);
-				cost_index(newpath, root, loop_count);
+				cost_index(newpath, root, loop_count, false);
 				return (Path *) newpath;
 			}
 		case T_BitmapHeapScan:
diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index 7836e6b3f85..4ed27054a11 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -241,6 +241,7 @@ get_relation_info(PlannerInfo *root, Oid relationObjectId, bool inhparent,
 			info->amoptionalkey = amroutine->amoptionalkey;
 			info->amsearcharray = amroutine->amsearcharray;
 			info->amsearchnulls = amroutine->amsearchnulls;
+			info->amcanparallel = amroutine->amcanparallel;
 			info->amhasgettuple = (amroutine->amgettuple != NULL);
 			info->amhasgetbitmap = (amroutine->amgetbitmap != NULL);
 			info->amcostestimate = amroutine->amcostestimate;