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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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12
src/backend/executor/nodeSort.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/executor/nodeSort.c,v 1.60 2007/01/09 02:14:11 tgl Exp $
* $PostgreSQL: pgsql/src/backend/executor/nodeSort.c,v 1.61 2007/05/04 01:13:44 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -89,6 +89,8 @@ ExecSort(SortState *node)
plannode->nullsFirst,
work_mem,
node->randomAccess);
if (node->bounded)
tuplesort_set_bound(tuplesortstate, node->bound);
node->tuplesortstate = (void *) tuplesortstate;
/*
@ -119,6 +121,8 @@ ExecSort(SortState *node)
* finally set the sorted flag to true
*/
node->sort_Done = true;
node->bounded_Done = node->bounded;
node->bound_Done = node->bound;
SO1_printf("ExecSort: %s\n", "sorting done");
}
@ -167,6 +171,7 @@ ExecInitSort(Sort *node, EState *estate, int eflags)
EXEC_FLAG_BACKWARD |
EXEC_FLAG_MARK)) != 0;
sortstate->bounded = false;
sortstate->sort_Done = false;
sortstate->tuplesortstate = NULL;
@ -307,11 +312,14 @@ ExecReScanSort(SortState *node, ExprContext *exprCtxt)
/*
* If subnode is to be rescanned then we forget previous sort results; we
* have to re-read the subplan and re-sort.
* have to re-read the subplan and re-sort. Also must re-sort if the
* bounded-sort parameters changed or we didn't select randomAccess.
*
* Otherwise we can just rewind and rescan the sorted output.
*/
if (((PlanState *) node)->lefttree->chgParam != NULL ||
node->bounded != node->bounded_Done ||
node->bound != node->bound_Done ||
!node->randomAccess)
{
node->sort_Done = false;