From 6fbc323c8042303a737028f9da7616896bccc517 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 18 Nov 2010 00:30:10 -0500
Subject: [PATCH] Further fallout from the MergeAppend patch.
Fix things so that top-N sorting can be used in child Sort nodes of a
MergeAppend node, when there is a LIMIT and no intervening joins or
grouping. 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 an
explicit sort cheaper when there will be grouping or aggregation in
between. Possibly this should be back-patched, but I'm not sure the
mistake is serious enough to be a real problem in practice.
---
src/backend/executor/nodeLimit.c | 57 +++++++++++++++++--------
src/backend/nodes/outfuncs.c | 2 +
src/backend/optimizer/plan/createplan.c | 2 +-
src/backend/optimizer/plan/planmain.c | 12 +++++-
src/backend/optimizer/plan/planner.c | 18 +++++++-
src/backend/optimizer/util/pathnode.c | 31 +++++++++++++-
src/include/nodes/relation.h | 2 +
7 files changed, 103 insertions(+), 21 deletions(-)
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index ce24f1b00af..5ed8664f66f 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -25,6 +25,7 @@
#include "executor/nodeLimit.h"
static void recompute_limits(LimitState *node);
+static void pass_down_bound(LimitState *node, PlanState *child_node);
/* ----------------------------------------------------------------
@@ -293,26 +294,35 @@ recompute_limits(LimitState *node)
/* Set state-machine state */
node->lstate = LIMIT_RESCAN;
- /*
- * If we have a COUNT, and our input is a Sort node, notify it that it can
- * use bounded sort.
- *
- * This is a bit of a kluge, but we don't have any more-abstract way of
- * communicating between the two nodes; and it doesn't seem worth trying
- * to invent one without some more examples of special communication
- * needs.
- *
- * Note: it is the responsibility of nodeSort.c to react properly to
- * changes of these parameters. If we ever do redesign this, it'd be a
- * good idea to integrate this signaling with the parameter-change
- * mechanism.
- */
- if (IsA(outerPlanState(node), SortState))
+ /* Notify child node about limit, if useful */
+ pass_down_bound(node, outerPlanState(node));
+}
+
+/*
+ * If we have a COUNT, and our input is a Sort node, notify it that it can
+ * use bounded sort. Also, if our input is a MergeAppend, we can apply the
+ * same bound to any Sorts that are direct children of the MergeAppend,
+ * since the MergeAppend surely need read no more than that many tuples from
+ * any one input. We also have to be prepared to look through a Result,
+ * since the planner might stick one atop MergeAppend for projection purposes.
+ *
+ * This is a bit of a kluge, but we don't have any more-abstract way of
+ * communicating between the two nodes; and it doesn't seem worth trying
+ * to invent one without some more examples of special communication needs.
+ *
+ * Note: it is the responsibility of nodeSort.c to react properly to
+ * changes of these parameters. If we ever do redesign this, it'd be a
+ * good idea to integrate this signaling with the parameter-change mechanism.
+ */
+static void
+pass_down_bound(LimitState *node, PlanState *child_node)
+{
+ if (IsA(child_node, SortState))
{
- SortState *sortState = (SortState *) outerPlanState(node);
+ SortState *sortState = (SortState *) child_node;
int64 tuples_needed = node->count + node->offset;
- /* negative test checks for overflow */
+ /* negative test checks for overflow in sum */
if (node->noCount || tuples_needed < 0)
{
/* make sure flag gets reset if needed upon rescan */
@@ -324,6 +334,19 @@ recompute_limits(LimitState *node)
sortState->bound = tuples_needed;
}
}
+ else if (IsA(child_node, MergeAppendState))
+ {
+ MergeAppendState *maState = (MergeAppendState *) child_node;
+ int i;
+
+ for (i = 0; i < maState->ms_nplans; i++)
+ pass_down_bound(node, maState->mergeplans[i]);
+ }
+ else if (IsA(child_node, ResultState))
+ {
+ if (outerPlanState(child_node))
+ pass_down_bound(node, outerPlanState(child_node));
+ }
}
/* ----------------------------------------------------------------
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 61aea612fd4..afbfccabda5 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -1493,6 +1493,7 @@ _outMergeAppendPath(StringInfo str, MergeAppendPath *node)
_outPathInfo(str, (Path *) node);
WRITE_NODE_FIELD(subpaths);
+ WRITE_FLOAT_FIELD(limit_tuples, "%.0f");
}
static void
@@ -1611,6 +1612,7 @@ _outPlannerInfo(StringInfo str, PlannerInfo *node)
WRITE_NODE_FIELD(minmax_aggs);
WRITE_FLOAT_FIELD(total_table_pages, "%.0f");
WRITE_FLOAT_FIELD(tuple_fraction, "%.4f");
+ WRITE_FLOAT_FIELD(limit_tuples, "%.0f");
WRITE_BOOL_FIELD(hasInheritedTarget);
WRITE_BOOL_FIELD(hasJoinRTEs);
WRITE_BOOL_FIELD(hasHavingQual);
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 7a84bd91239..41ad512a296 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -714,7 +714,7 @@ create_merge_append_plan(PlannerInfo *root, MergeAppendPath *best_path)
if (!pathkeys_contained_in(pathkeys, subpath->pathkeys))
subplan = (Plan *) make_sort(root, subplan, numsortkeys,
sortColIdx, sortOperators, nullsFirst,
- -1.0);
+ best_path->limit_tuples);
subplans = lappend(subplans, subplan);
}
diff --git a/src/backend/optimizer/plan/planmain.c b/src/backend/optimizer/plan/planmain.c
index cab6e9e25ad..9e6b0b724c1 100644
--- a/src/backend/optimizer/plan/planmain.c
+++ b/src/backend/optimizer/plan/planmain.c
@@ -101,8 +101,9 @@ query_planner(PlannerInfo *root, List *tlist,
ListCell *lc;
double total_pages;
- /* Make tuple_fraction accessible to lower-level routines */
+ /* Make tuple_fraction, limit_tuples accessible to lower-level routines */
root->tuple_fraction = tuple_fraction;
+ root->limit_tuples = limit_tuples;
*num_groups = 1; /* default result */
@@ -315,6 +316,9 @@ query_planner(PlannerInfo *root, List *tlist,
!pathkeys_contained_in(root->distinct_pathkeys, root->group_pathkeys) ||
!pathkeys_contained_in(root->window_pathkeys, root->group_pathkeys))
tuple_fraction = 0.0;
+
+ /* In any case, limit_tuples shouldn't be specified here */
+ Assert(limit_tuples < 0);
}
else if (parse->hasAggs || root->hasHavingQual)
{
@@ -323,6 +327,9 @@ query_planner(PlannerInfo *root, List *tlist,
* it will deliver a single result row (so leave *num_groups 1).
*/
tuple_fraction = 0.0;
+
+ /* limit_tuples shouldn't be specified here */
+ Assert(limit_tuples < 0);
}
else if (parse->distinctClause)
{
@@ -347,6 +354,9 @@ query_planner(PlannerInfo *root, List *tlist,
*/
if (tuple_fraction >= 1.0)
tuple_fraction /= *num_groups;
+
+ /* limit_tuples shouldn't be specified here */
+ Assert(limit_tuples < 0);
}
else
{
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 620888cbb86..6324bce2403 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -968,6 +968,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
{
/* No set operations, do regular planning */
List *sub_tlist;
+ double sub_limit_tuples;
AttrNumber *groupColIdx = NULL;
bool need_tlist_eval = true;
QualCost tlist_cost;
@@ -1119,13 +1120,28 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
else
root->query_pathkeys = NIL;
+ /*
+ * Figure out whether there's a hard limit on the number of rows that
+ * query_planner's result subplan needs to return. Even if we know a
+ * hard limit overall, it doesn't apply if the query has any
+ * grouping/aggregation operations.
+ */
+ if (parse->groupClause ||
+ parse->distinctClause ||
+ parse->hasAggs ||
+ parse->hasWindowFuncs ||
+ root->hasHavingQual)
+ sub_limit_tuples = -1.0;
+ else
+ sub_limit_tuples = limit_tuples;
+
/*
* Generate the best unsorted and presorted paths for this Query (but
* note there may not be any presorted path). query_planner will also
* estimate the number of groups in the query, and canonicalize all
* the pathkeys.
*/
- query_planner(root, sub_tlist, tuple_fraction, limit_tuples,
+ query_planner(root, sub_tlist, tuple_fraction, sub_limit_tuples,
&cheapest_path, &sorted_path, &dNumGroups);
/*
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 20d8074bce4..231d221b21e 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -694,6 +694,35 @@ create_merge_append_path(PlannerInfo *root,
pathnode->path.pathkeys = pathkeys;
pathnode->subpaths = subpaths;
+ /*
+ * Apply query-wide LIMIT if known and path is for sole base relation.
+ * Finding out the latter at this low level is a bit klugy.
+ */
+ pathnode->limit_tuples = root->limit_tuples;
+ if (pathnode->limit_tuples >= 0)
+ {
+ Index rti;
+
+ for (rti = 1; rti < root->simple_rel_array_size; rti++)
+ {
+ RelOptInfo *brel = root->simple_rel_array[rti];
+
+ if (brel == NULL)
+ continue;
+
+ /* ignore RTEs that are "other rels" */
+ if (brel->reloptkind != RELOPT_BASEREL)
+ continue;
+
+ if (brel != rel)
+ {
+ /* Oops, it's a join query */
+ pathnode->limit_tuples = -1.0;
+ break;
+ }
+ }
+ }
+
/* Add up all the costs of the input paths */
input_startup_cost = 0;
input_total_cost = 0;
@@ -720,7 +749,7 @@ create_merge_append_path(PlannerInfo *root,
subpath->parent->width,
0.0,
work_mem,
- -1.0);
+ pathnode->limit_tuples);
input_startup_cost += sort_path.startup_cost;
input_total_cost += sort_path.total_cost;
}
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index f885f5a0c46..81126a23665 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -198,6 +198,7 @@ typedef struct PlannerInfo
double total_table_pages; /* # of pages in all tables of query */
double tuple_fraction; /* tuple_fraction passed to query_planner */
+ double limit_tuples; /* limit_tuples passed to query_planner */
bool hasInheritedTarget; /* true if parse->resultRelation is an
* inheritance child rel */
@@ -766,6 +767,7 @@ typedef struct MergeAppendPath
{
Path path;
List *subpaths; /* list of component Paths */
+ double limit_tuples; /* hard limit on output tuples, or -1 */
} MergeAppendPath;
/*
--
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