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