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Fix an oversight in the 8.2 patch that improved mergejoin performance by

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inserting a materialize node above an inner-side sort node, when the sort is
expected to spill to disk.  (The materialize protects the sort from having
to support mark/restore, allowing it to do its final merge pass on-the-fly.)
We neglected to teach cost_mergejoin about that hack, so it was failing to
include the materialize's costs in the estimated cost of the mergejoin.
The materialize's costs are generally going to be pretty negligible in
comparison to the sort's, so this is only a small error and probably not
worth back-patching; but it's still wrong.

In the similar case where a materialize is inserted to protect an inner-side
node that can't do mark/restore at all, it's still true that the materialize
should not spill to disk, and so we should cost it cheaply rather than
expensively.

Noted while thinking about a question from Tom Raney.
This commit is contained in:
Tom Lane
2008-09-05 21:07:29 +00:00
parent 58829581f8
commit e540b97248
3 changed files with 50 additions and 12 deletions
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5
src/backend/optimizer/plan/createplan.c
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@ -10,7 +10,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.247 2008/08/28 23:09:46 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.248 2008/09/05 21:07:29 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -1545,7 +1545,8 @@ create_mergejoin_plan(PlannerInfo *root,
/*
* We assume the materialize will not spill to disk, and therefore
* charge just cpu_tuple_cost per tuple.
* charge just cpu_tuple_cost per tuple. (Keep this estimate in sync
* with similar ones in cost_mergejoin and create_mergejoin_path.)
*/
copy_plan_costsize(matplan, inner_plan);
matplan->total_cost += cpu_tuple_cost * matplan->plan_rows;