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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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25
src/backend/utils/misc/guc.c
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@@ -10,7 +10,7 @@
* Written by Peter Eisentraut <peter_e@gmx.net>.
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.389 2007/04/26 16:13:12 neilc Exp $
* $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.390 2007/05/04 01:13:44 tgl Exp $
*
*--------------------------------------------------------------------
*/
@@ -108,6 +108,9 @@ extern bool fullPageWrites;
#ifdef TRACE_SORT
extern bool trace_sort;
#endif
#ifdef DEBUG_BOUNDED_SORT
extern bool optimize_bounded_sort;
#endif
#ifdef USE_SSL
extern char *SSLCipherSuites;
@@ -966,6 +969,20 @@ static struct config_bool ConfigureNamesBool[] =
},
#endif
#ifdef DEBUG_BOUNDED_SORT
/* this is undocumented because not exposed in a standard build */
{
{
"optimize_bounded_sort", PGC_USERSET, QUERY_TUNING_METHOD,
gettext_noop("Enable bounded sorting using heap sort."),
NULL,
GUC_NOT_IN_SAMPLE
},
&optimize_bounded_sort,
true, NULL, NULL
},
#endif
#ifdef WAL_DEBUG
{
{"wal_debug", PGC_SUSET, DEVELOPER_OPTIONS,
@@ -1711,7 +1728,7 @@ static struct config_int ConfigureNamesInt[] =
&server_version_num,
PG_VERSION_NUM, PG_VERSION_NUM, PG_VERSION_NUM, NULL, NULL
},
{
{"log_temp_files", PGC_USERSET, LOGGING_WHAT,
gettext_noop("Log the use of temporary files larger than this number of kilobytes."),
@@ -2883,7 +2900,7 @@ InitializeGUCOptions(void)
PGC_S_DEFAULT))
elog(FATAL, "failed to initialize %s to %d",
conf->gen.name, conf->boot_val);
*conf->variable = conf->reset_val = conf->boot_val;
*conf->variable = conf->reset_val = conf->boot_val;
break;
}
case PGC_REAL:
@@ -2897,7 +2914,7 @@ InitializeGUCOptions(void)
PGC_S_DEFAULT))
elog(FATAL, "failed to initialize %s to %g",
conf->gen.name, conf->boot_val);
*conf->variable = conf->reset_val = conf->boot_val;
*conf->variable = conf->reset_val = conf->boot_val;
break;
}
case PGC_STRING: