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

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index da68d0d61fc42d6fc849c24914fa96d35a640299..96ca7d3bb0748da465b4452d2d821a23d37c98fe 100644 (file)
--- a/src/backend/optimizer/path/allpaths.c
+++ b/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
  *****************************************************************************/