From 7df721af0ee61fa94eb973c52b9b1bff9bc45300 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sat, 23 Dec 2000 18:49:41 +0000
Subject: [PATCH] Compute reasonable cost and output-row-count estimates for
LIMIT plan nodes.
---
src/backend/optimizer/plan/createplan.c | 45 ++++++++++++++++++++++++-
1 file changed, 44 insertions(+), 1 deletion(-)
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index bfb97bc1eb5..f872ca3fdda 100644
--- 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;
--
GitLab