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 bfb97bc1eb522d7e3f83daf1d4709ca11eba6cc8..f872ca3fddae2631293aa05b70e9c549a5590fbf 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;