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Teach tuplesort.c about "top N" sorting, in which only the first N tuples

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need be returned.  We keep a heap of the current best N tuples and sift-up
new tuples into it as we scan the input.  For M input tuples this means
only about M*log(N) comparisons instead of M*log(M), not to mention a lot
less workspace when N is small --- avoiding spill-to-disk for large M
is actually the most attractive thing about it.  Patch includes planner
and executor support for invoking this facility in ORDER BY ... LIMIT
queries.  Greg Stark, with some editorialization by moi.
This commit is contained in:
Tom Lane
2007-05-04 01:13:45 +00:00
parent 0fef38da21
commit d26559dbf3
13 changed files with 465 additions and 59 deletions
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5
src/include/optimizer/cost.h
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@@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/optimizer/cost.h,v 1.85 2007/02/22 22:00:26 tgl Exp $
* $PostgreSQL: pgsql/src/include/optimizer/cost.h,v 1.86 2007/05/04 01:13:45 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -73,7 +73,8 @@ extern void cost_functionscan(Path *path, PlannerInfo *root,
extern void cost_valuesscan(Path *path, PlannerInfo *root,
RelOptInfo *baserel);
extern void cost_sort(Path *path, PlannerInfo *root,
List *pathkeys, Cost input_cost, double tuples, int width);
List *pathkeys, Cost input_cost, double tuples, int width,
double limit_tuples);
extern void cost_material(Path *path,
Cost input_cost, double tuples, int width);
extern void cost_agg(Path *path, PlannerInfo *root,