Compute reasonable cost and output-row-count estimates for LIMIT plan

nodes.

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index bfb97bc..f872ca3 100644 (file)
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/optimizer/plan/createplan.c,v 1.101 2000/11/16 22:30:24 tgl Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/optimizer/plan/createplan.c,v 1.102 2000/12/23 18:49:41 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1733,6 +1733,49 @@ make_limit(List *tlist, Plan *lefttree,
 
        copy_plan_costsize(plan, lefttree);
 
+       /*
+        * If offset/count are constants, adjust the output rows count and costs
+        * accordingly.  This is only a cosmetic issue if we are at top level,
+        * but if we are building a subquery then it's important to report
+        * correct info to the outer planner.
+        */
+       if (limitOffset && IsA(limitOffset, Const))
+       {
+               Const      *limito = (Const *) limitOffset;
+               int32           offset = DatumGetInt32(limito->constvalue);
+
+               if (!limito->constisnull && offset > 0)
+               {
+                       if (offset > plan->plan_rows)
+                               offset = (int32) plan->plan_rows;
+                       if (plan->plan_rows > 0)
+                               plan->startup_cost +=
+                                       (plan->total_cost - plan->startup_cost)
+                                       * ((double) offset) / plan->plan_rows;
+                       plan->plan_rows -= offset;
+                       if (plan->plan_rows < 1)
+                               plan->plan_rows = 1;
+               }
+       }
+       if (limitCount && IsA(limitCount, Const))
+       {
+               Const      *limitc = (Const *) limitCount;
+               int32           count = DatumGetInt32(limitc->constvalue);
+
+               if (!limitc->constisnull && count >= 0)
+               {
+                       if (count > plan->plan_rows)
+                               count = (int32) plan->plan_rows;
+                       if (plan->plan_rows > 0)
+                               plan->total_cost = plan->startup_cost +
+                                       (plan->total_cost - plan->startup_cost)
+                                       * ((double) count) / plan->plan_rows;
+                       plan->plan_rows = count;
+                       if (plan->plan_rows < 1)
+                               plan->plan_rows = 1;
+               }
+       }
+
        plan->state = (EState *) NULL;
        plan->targetlist = tlist;
        plan->qual = NIL;