Factor out logic for computing number of parallel workers.

Forthcoming patches to allow other types of parallel scans will
need this logic, or something like it.

Dilip Kumar

src/backend/optimizer/path/allpaths.c

@@ -126,6 +126,7 @@ 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 pages);
 
 
 /*
@@ -678,49 +679,7 @@ create_plain_partial_paths(PlannerInfo *root, RelOptInfo *rel)
 {
 	int			parallel_workers;
 
-	/*
-	 * If the user has set the parallel_workers reloption, use that; otherwise
-	 * select a default number of workers.
-	 */
-	if (rel->rel_parallel_workers != -1)
-		parallel_workers = rel->rel_parallel_workers;
-	else
-	{
-		int			parallel_threshold;
-
-		/*
-		 * If this relation is too small to be worth a parallel scan, just
-		 * return without doing anything ... unless it's an inheritance child.
-		 * In that case, we want to generate a parallel path here anyway.  It
-		 * might not be worthwhile just for this relation, but when combined
-		 * with all of its inheritance siblings it may well pay off.
-		 */
-		if (rel->pages < (BlockNumber) min_parallel_relation_size &&
-			rel->reloptkind == RELOPT_BASEREL)
-			return;
-
-		/*
-		 * Select the number of workers based on the log of the size of the
-		 * relation.  This probably needs to be a good deal more
-		 * sophisticated, but we need something here for now.  Note that the
-		 * upper limit of the min_parallel_relation_size GUC is chosen to
-		 * prevent overflow here.
-		 */
-		parallel_workers = 1;
-		parallel_threshold = Max(min_parallel_relation_size, 1);
-		while (rel->pages >= (BlockNumber) (parallel_threshold * 3))
-		{
-			parallel_workers++;
-			parallel_threshold *= 3;
-			if (parallel_threshold > INT_MAX / 3)
-				break;			/* avoid overflow */
-		}
-	}
-
-	/*
-	 * In no case use more than max_parallel_workers_per_gather workers.
-	 */
-	parallel_workers = Min(parallel_workers, max_parallel_workers_per_gather);
+	parallel_workers = compute_parallel_worker(rel, rel->pages);
 
 	/* If any limit was set to zero, the user doesn't want a parallel scan. */
 	if (parallel_workers <= 0)
@@ -2908,6 +2867,64 @@ remove_unused_subquery_outputs(Query *subquery, RelOptInfo *rel)
 	}
 }
 
+/*
+ * Compute the number of parallel workers that should be used to scan a
+ * relation.  "pages" is the number of pages from the relation that we
+ * expect to scan.
+ */
+static int
+compute_parallel_worker(RelOptInfo *rel, BlockNumber pages)
+{
+	int			parallel_workers;
+
+	/*
+	 * If the user has set the parallel_workers reloption, use that; otherwise
+	 * select a default number of workers.
+	 */
+	if (rel->rel_parallel_workers != -1)
+		parallel_workers = rel->rel_parallel_workers;
+	else
+	{
+		int			parallel_threshold;
+
+		/*
+		 * If this relation is too small to be worth a parallel scan, just
+		 * return without doing anything ... unless it's an inheritance child.
+		 * In that case, we want to generate a parallel path here anyway.  It
+		 * might not be worthwhile just for this relation, but when combined
+		 * with all of its inheritance siblings it may well pay off.
+		 */
+		if (pages < (BlockNumber) min_parallel_relation_size &&
+			rel->reloptkind == RELOPT_BASEREL)
+			return 0;
+
+		/*
+		 * Select the number of workers based on the log of the size of the
+		 * relation.  This probably needs to be a good deal more
+		 * sophisticated, but we need something here for now.  Note that the
+		 * upper limit of the min_parallel_relation_size GUC is chosen to
+		 * prevent overflow here.
+		 */
+		parallel_workers = 1;
+		parallel_threshold = Max(min_parallel_relation_size, 1);
+		while (pages >= (BlockNumber) (parallel_threshold * 3))
+		{
+			parallel_workers++;
+			parallel_threshold *= 3;
+			if (parallel_threshold > INT_MAX / 3)
+				break;			/* avoid overflow */
+		}
+	}
+
+	/*
+	 * In no case use more than max_parallel_workers_per_gather workers.
+	 */
+	parallel_workers = Min(parallel_workers, max_parallel_workers_per_gather);
+
+	return parallel_workers;
+}
+
+
 /*****************************************************************************
  *			DEBUG SUPPORT
  *****************************************************************************/