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Commit6fbc323

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Further fallout from the MergeAppend patch.
Fix things so that top-N sorting can be used in child Sort nodes of aMergeAppend node, when there is a LIMIT and no intervening joins orgrouping. Actually doing this on the executor side isn't too bad,but it's a bit messier to get the planner to cost it properly.Per gripe from Robert Haas.In passing, fix an oversight in the original top-N-sorting patch:query_planner should not assume that a LIMIT can be used to make anexplicit sort cheaper when there will be grouping or aggregation inbetween. Possibly this should be back-patched, but I'm not sure themistake is serious enough to be a real problem in practice.
1 parent45768d1 commit6fbc323

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7 files changed

+103
-21
lines changed

7 files changed

+103
-21
lines changed

‎src/backend/executor/nodeLimit.c

Lines changed: 40 additions & 17 deletions
Original file line numberDiff line numberDiff line change
@@ -25,6 +25,7 @@
2525
#include"executor/nodeLimit.h"
2626

2727
staticvoidrecompute_limits(LimitState*node);
28+
staticvoidpass_down_bound(LimitState*node,PlanState*child_node);
2829

2930

3031
/* ----------------------------------------------------------------
@@ -293,26 +294,35 @@ recompute_limits(LimitState *node)
293294
/* Set state-machine state */
294295
node->lstate=LIMIT_RESCAN;
295296

296-
/*
297-
* If we have a COUNT, and our input is a Sort node, notify it that it can
298-
* use bounded sort.
299-
*
300-
* This is a bit of a kluge, but we don't have any more-abstract way of
301-
* communicating between the two nodes; and it doesn't seem worth trying
302-
* to invent one without some more examples of special communication
303-
* needs.
304-
*
305-
* Note: it is the responsibility of nodeSort.c to react properly to
306-
* changes of these parameters. If we ever do redesign this, it'd be a
307-
* good idea to integrate this signaling with the parameter-change
308-
* mechanism.
309-
*/
310-
if (IsA(outerPlanState(node),SortState))
297+
/* Notify child node about limit, if useful */
298+
pass_down_bound(node,outerPlanState(node));
299+
}
300+
301+
/*
302+
* If we have a COUNT, and our input is a Sort node, notify it that it can
303+
* use bounded sort. Also, if our input is a MergeAppend, we can apply the
304+
* same bound to any Sorts that are direct children of the MergeAppend,
305+
* since the MergeAppend surely need read no more than that many tuples from
306+
* any one input. We also have to be prepared to look through a Result,
307+
* since the planner might stick one atop MergeAppend for projection purposes.
308+
*
309+
* This is a bit of a kluge, but we don't have any more-abstract way of
310+
* communicating between the two nodes; and it doesn't seem worth trying
311+
* to invent one without some more examples of special communication needs.
312+
*
313+
* Note: it is the responsibility of nodeSort.c to react properly to
314+
* changes of these parameters. If we ever do redesign this, it'd be a
315+
* good idea to integrate this signaling with the parameter-change mechanism.
316+
*/
317+
staticvoid
318+
pass_down_bound(LimitState*node,PlanState*child_node)
319+
{
320+
if (IsA(child_node,SortState))
311321
{
312-
SortState*sortState= (SortState*)outerPlanState(node);
322+
SortState*sortState= (SortState*)child_node;
313323
int64tuples_needed=node->count+node->offset;
314324

315-
/* negative test checks for overflow */
325+
/* negative test checks for overflowin sum*/
316326
if (node->noCount||tuples_needed<0)
317327
{
318328
/* make sure flag gets reset if needed upon rescan */
@@ -324,6 +334,19 @@ recompute_limits(LimitState *node)
324334
sortState->bound=tuples_needed;
325335
}
326336
}
337+
elseif (IsA(child_node,MergeAppendState))
338+
{
339+
MergeAppendState*maState= (MergeAppendState*)child_node;
340+
inti;
341+
342+
for (i=0;i<maState->ms_nplans;i++)
343+
pass_down_bound(node,maState->mergeplans[i]);
344+
}
345+
elseif (IsA(child_node,ResultState))
346+
{
347+
if (outerPlanState(child_node))
348+
pass_down_bound(node,outerPlanState(child_node));
349+
}
327350
}
328351

329352
/* ----------------------------------------------------------------

‎src/backend/nodes/outfuncs.c

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1493,6 +1493,7 @@ _outMergeAppendPath(StringInfo str, MergeAppendPath *node)
14931493
_outPathInfo(str, (Path*)node);
14941494

14951495
WRITE_NODE_FIELD(subpaths);
1496+
WRITE_FLOAT_FIELD(limit_tuples,"%.0f");
14961497
}
14971498

14981499
staticvoid
@@ -1611,6 +1612,7 @@ _outPlannerInfo(StringInfo str, PlannerInfo *node)
16111612
WRITE_NODE_FIELD(minmax_aggs);
16121613
WRITE_FLOAT_FIELD(total_table_pages,"%.0f");
16131614
WRITE_FLOAT_FIELD(tuple_fraction,"%.4f");
1615+
WRITE_FLOAT_FIELD(limit_tuples,"%.0f");
16141616
WRITE_BOOL_FIELD(hasInheritedTarget);
16151617
WRITE_BOOL_FIELD(hasJoinRTEs);
16161618
WRITE_BOOL_FIELD(hasHavingQual);

‎src/backend/optimizer/plan/createplan.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -714,7 +714,7 @@ create_merge_append_plan(PlannerInfo *root, MergeAppendPath *best_path)
714714
if (!pathkeys_contained_in(pathkeys,subpath->pathkeys))
715715
subplan= (Plan*)make_sort(root,subplan,numsortkeys,
716716
sortColIdx,sortOperators,nullsFirst,
717-
-1.0);
717+
best_path->limit_tuples);
718718

719719
subplans=lappend(subplans,subplan);
720720
}

‎src/backend/optimizer/plan/planmain.c

Lines changed: 11 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -101,8 +101,9 @@ query_planner(PlannerInfo *root, List *tlist,
101101
ListCell*lc;
102102
doubletotal_pages;
103103

104-
/* Make tuple_fraction accessible to lower-level routines */
104+
/* Make tuple_fraction, limit_tuples accessible to lower-level routines */
105105
root->tuple_fraction=tuple_fraction;
106+
root->limit_tuples=limit_tuples;
106107

107108
*num_groups=1;/* default result */
108109

@@ -315,6 +316,9 @@ query_planner(PlannerInfo *root, List *tlist,
315316
!pathkeys_contained_in(root->distinct_pathkeys,root->group_pathkeys)||
316317
!pathkeys_contained_in(root->window_pathkeys,root->group_pathkeys))
317318
tuple_fraction=0.0;
319+
320+
/* In any case, limit_tuples shouldn't be specified here */
321+
Assert(limit_tuples<0);
318322
}
319323
elseif (parse->hasAggs||root->hasHavingQual)
320324
{
@@ -323,6 +327,9 @@ query_planner(PlannerInfo *root, List *tlist,
323327
* it will deliver a single result row (so leave *num_groups 1).
324328
*/
325329
tuple_fraction=0.0;
330+
331+
/* limit_tuples shouldn't be specified here */
332+
Assert(limit_tuples<0);
326333
}
327334
elseif (parse->distinctClause)
328335
{
@@ -347,6 +354,9 @@ query_planner(PlannerInfo *root, List *tlist,
347354
*/
348355
if (tuple_fraction >=1.0)
349356
tuple_fraction /=*num_groups;
357+
358+
/* limit_tuples shouldn't be specified here */
359+
Assert(limit_tuples<0);
350360
}
351361
else
352362
{

‎src/backend/optimizer/plan/planner.c

Lines changed: 17 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -968,6 +968,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
968968
{
969969
/* No set operations, do regular planning */
970970
List*sub_tlist;
971+
doublesub_limit_tuples;
971972
AttrNumber*groupColIdx=NULL;
972973
boolneed_tlist_eval= true;
973974
QualCosttlist_cost;
@@ -1119,13 +1120,28 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
11191120
else
11201121
root->query_pathkeys=NIL;
11211122

1123+
/*
1124+
* Figure out whether there's a hard limit on the number of rows that
1125+
* query_planner's result subplan needs to return. Even if we know a
1126+
* hard limit overall, it doesn't apply if the query has any
1127+
* grouping/aggregation operations.
1128+
*/
1129+
if (parse->groupClause||
1130+
parse->distinctClause||
1131+
parse->hasAggs||
1132+
parse->hasWindowFuncs||
1133+
root->hasHavingQual)
1134+
sub_limit_tuples=-1.0;
1135+
else
1136+
sub_limit_tuples=limit_tuples;
1137+
11221138
/*
11231139
* Generate the best unsorted and presorted paths for this Query (but
11241140
* note there may not be any presorted path). query_planner will also
11251141
* estimate the number of groups in the query, and canonicalize all
11261142
* the pathkeys.
11271143
*/
1128-
query_planner(root,sub_tlist,tuple_fraction,limit_tuples,
1144+
query_planner(root,sub_tlist,tuple_fraction,sub_limit_tuples,
11291145
&cheapest_path,&sorted_path,&dNumGroups);
11301146

11311147
/*

‎src/backend/optimizer/util/pathnode.c

Lines changed: 30 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -694,6 +694,35 @@ create_merge_append_path(PlannerInfo *root,
694694
pathnode->path.pathkeys=pathkeys;
695695
pathnode->subpaths=subpaths;
696696

697+
/*
698+
* Apply query-wide LIMIT if known and path is for sole base relation.
699+
* Finding out the latter at this low level is a bit klugy.
700+
*/
701+
pathnode->limit_tuples=root->limit_tuples;
702+
if (pathnode->limit_tuples >=0)
703+
{
704+
Indexrti;
705+
706+
for (rti=1;rti<root->simple_rel_array_size;rti++)
707+
{
708+
RelOptInfo*brel=root->simple_rel_array[rti];
709+
710+
if (brel==NULL)
711+
continue;
712+
713+
/* ignore RTEs that are "other rels" */
714+
if (brel->reloptkind!=RELOPT_BASEREL)
715+
continue;
716+
717+
if (brel!=rel)
718+
{
719+
/* Oops, it's a join query */
720+
pathnode->limit_tuples=-1.0;
721+
break;
722+
}
723+
}
724+
}
725+
697726
/* Add up all the costs of the input paths */
698727
input_startup_cost=0;
699728
input_total_cost=0;
@@ -720,7 +749,7 @@ create_merge_append_path(PlannerInfo *root,
720749
subpath->parent->width,
721750
0.0,
722751
work_mem,
723-
-1.0);
752+
pathnode->limit_tuples);
724753
input_startup_cost+=sort_path.startup_cost;
725754
input_total_cost+=sort_path.total_cost;
726755
}

‎src/include/nodes/relation.h

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -198,6 +198,7 @@ typedef struct PlannerInfo
198198
doubletotal_table_pages;/* # of pages in all tables of query */
199199

200200
doubletuple_fraction;/* tuple_fraction passed to query_planner */
201+
doublelimit_tuples;/* limit_tuples passed to query_planner */
201202

202203
boolhasInheritedTarget;/* true if parse->resultRelation is an
203204
* inheritance child rel */
@@ -766,6 +767,7 @@ typedef struct MergeAppendPath
766767
{
767768
Pathpath;
768769
List*subpaths;/* list of component Paths */
770+
doublelimit_tuples;/* hard limit on output tuples, or -1 */
769771
}MergeAppendPath;
770772

771773
/*

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