Marginal tweaks to make sure that roundoff error won't cause us to make

a bad choice between sorted and hashed aggregation.

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 40621f9..54e47e2 100644 (file)
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -49,7 +49,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.105 2003/02/08 20:20:54 tgl Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.106 2003/02/15 21:39:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -601,6 +601,15 @@ cost_agg(Path *path, Query *root,
         *
         * We will produce a single output tuple if not grouping,
         * and a tuple per group otherwise.
+        *
+        * Note: in this cost model, AGG_SORTED and AGG_HASHED have exactly the
+        * same total CPU cost, but AGG_SORTED has lower startup cost.  If the
+        * input path is already sorted appropriately, AGG_SORTED should be
+        * preferred (since it has no risk of memory overflow).  This will happen
+        * as long as the computed total costs are indeed exactly equal --- but
+        * if there's roundoff error we might do the wrong thing.  So be sure
+        * that the computations below form the same intermediate values in the
+        * same order.
         */
        if (aggstrategy == AGG_PLAIN)
        {
@@ -614,15 +623,17 @@ cost_agg(Path *path, Query *root,
                /* Here we are able to deliver output on-the-fly */
                startup_cost = input_startup_cost;
                total_cost = input_total_cost;
-               total_cost += cpu_operator_cost * (input_tuples + numGroups) * numAggs;
+               /* calcs phrased this way to match HASHED case, see note above */
                total_cost += cpu_operator_cost * input_tuples * numGroupCols;
+               total_cost += cpu_operator_cost * input_tuples * numAggs;
+               total_cost += cpu_operator_cost * numGroups * numAggs;
        }
        else
        {
                /* must be AGG_HASHED */
                startup_cost = input_total_cost;
-               startup_cost += cpu_operator_cost * input_tuples * numAggs;
                startup_cost += cpu_operator_cost * input_tuples * numGroupCols;
+               startup_cost += cpu_operator_cost * input_tuples * numAggs;
                total_cost = startup_cost;
                total_cost += cpu_operator_cost * numGroups * numAggs;
        }

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 2b46b4b..76862c7 100644 (file)
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/optimizer/plan/planner.c,v 1.147 2003/02/15 20:12:40 tgl Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/optimizer/plan/planner.c,v 1.148 2003/02/15 21:39:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1003,7 +1003,7 @@ grouping_planner(Query *parse, double tuple_fraction)
                                                tuple_fraction /= dNumGroups;
 
                                        if (compare_fractional_path_costs(&hashed_p, &sorted_p,
-                                                                                                         tuple_fraction) <= 0)
+                                                                                                         tuple_fraction) < 0)
                                        {
                                                /* Hashed is cheaper, so use it */
                                                use_hashed_grouping = true;