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

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This commit is contained in:
Tom Lane 2000-12-23 18:49:41 +00:00
parent 37c55f9849
commit 7df721af0e
1 changed files with 44 additions and 1 deletions
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45
src/backend/optimizer/plan/createplan.c
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@ -10,7 +10,7 @@
* *
* *
* IDENTIFICATION * 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); 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->state = (EState *) NULL;
plan->targetlist = tlist; plan->targetlist = tlist;
plan->qual = NIL; plan->qual = NIL;