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Reduce the rescan cost estimate for Materialize nodes to cpu_operator_cost per

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tuple, instead of the former cpu_tuple_cost.  It is sane to charge less than
cpu_tuple_cost because Materialize never does any qual-checking or projection,
so it's got less overhead than most plan node types.  In particular, we want
to have the same charge here as is charged for readout in cost_sort.  That
avoids the problem recently exhibited by Teodor wherein the planner prefers
a useless sort over a materialize step in a context where a lot of rescanning
will happen.  The rescan costs should be just about the same for both node
types, so make their estimates the same.

Not back-patching because all of the current logic for rescan cost estimates
is new in 9.0.  The old handling of rescans is sufficiently not-sane that
changing this in that structure is a bit pointless, and might indeed cause
regressions.
This commit is contained in:
Tom Lane
2010-02-19 21:49:10 +00:00
parent 2f6cf9192c
commit 3f56ca1d49
2 changed files with 49 additions and 18 deletions
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8
src/backend/optimizer/plan/createplan.c
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@ -10,7 +10,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.271 2010/02/12 17:33:20 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.272 2010/02/19 21:49:10 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -1703,11 +1703,11 @@ create_mergejoin_plan(PlannerInfo *root,
/*
* We assume the materialize will not spill to disk, and therefore
* charge just cpu_tuple_cost per tuple. (Keep this estimate in sync
* with cost_mergejoin.)
* charge just cpu_operator_cost per tuple. (Keep this estimate in
* sync with cost_mergejoin.)
*/
copy_plan_costsize(matplan, inner_plan);
matplan->total_cost += cpu_tuple_cost * matplan->plan_rows;
matplan->total_cost += cpu_operator_cost * matplan->plan_rows;
inner_plan = matplan;
}