From e46eb01ac1834b82dd765c0106f2e7db8081fa8d Mon Sep 17 00:00:00 2001 From: Ilia Evdokimov Date: Wed, 29 Jul 2026 13:04:32 +0300 Subject: [PATCH v3] Show estimated number of groups for IncrementalSort in EXPLAIN IncrementalSort cost heavily depends on the estimated number of groups with equal presorted key values. A bad estimate can cause the planner to choose IncrementalSort over Sort even when it would actually be slower, with no visible indication of why. --- doc/src/sgml/perform.sgml | 1 + src/backend/commands/explain.c | 5 +++++ src/backend/optimizer/path/costsize.c | 15 ++++++++++++--- src/backend/optimizer/plan/createplan.c | 4 +++- src/backend/optimizer/util/pathnode.c | 6 ++++-- src/include/nodes/pathnodes.h | 1 + src/include/nodes/plannodes.h | 2 ++ src/include/optimizer/cost.h | 3 ++- 8 files changed, 30 insertions(+), 7 deletions(-) diff --git a/doc/src/sgml/perform.sgml b/doc/src/sgml/perform.sgml index 604e8578a8d..ea8da01b779 100644 --- a/doc/src/sgml/perform.sgml +++ b/doc/src/sgml/perform.sgml @@ -325,6 +325,7 @@ EXPLAIN SELECT * FROM tenk1 ORDER BY hundred, ten LIMIT 100; -> Incremental Sort (cost=19.35..2033.39 rows=10000 width=244) Sort Key: hundred, ten Presorted Key: hundred + Estimated Groups: 100 -> Index Scan using tenk1_hundred on tenk1 (cost=0.29..1574.20 rows=10000 width=244) diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c index a40d03d35f3..f223c12f294 100644 --- a/src/backend/commands/explain.c +++ b/src/backend/commands/explain.c @@ -3306,11 +3306,16 @@ static void show_incremental_sort_info(IncrementalSortState *incrsortstate, ExplainState *es) { + IncrementalSort *plan = (IncrementalSort *) incrsortstate->ss.ps.plan; IncrementalSortGroupInfo *fullsortGroupInfo; IncrementalSortGroupInfo *prefixsortGroupInfo; fullsortGroupInfo = &incrsortstate->incsort_info.fullsortGroupInfo; + if (es->costs) + ExplainPropertyFloat("Estimated Groups", NULL, + plan->numGroups, 0, es); + if (!es->analyze) return; diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c index ac523ecf9a8..f01b2c93c9d 100644 --- a/src/backend/optimizer/path/costsize.c +++ b/src/backend/optimizer/path/costsize.c @@ -2046,6 +2046,9 @@ cost_tuplesort(Cost *startup_cost, Cost *run_cost, * 'presorted_keys' is the number of leading pathkeys by which the input path * is sorted. * + * If p_numGroups is not NULL, *p_numGroups is set to the estimated number of + * sort groups (groups of tuples sharing equal presorted-key values). + * * We estimate the number of groups into which the relation is divided by the * leading pathkeys, and then calculate the cost of sorting a single group * with tuplesort using cost_tuplesort(). @@ -2056,7 +2059,8 @@ cost_incremental_sort(Path *path, int input_disabled_nodes, Cost input_startup_cost, Cost input_total_cost, double input_tuples, int width, Cost comparison_cost, int sort_mem, - double limit_tuples) + double limit_tuples, + Cardinality *p_numGroups) { Cost startup_cost, run_cost, @@ -2172,6 +2176,9 @@ cost_incremental_sort(Path *path, */ run_cost += 2.0 * cpu_tuple_cost * input_groups; + if (p_numGroups) + *p_numGroups = input_groups; + path->rows = input_tuples; /* @@ -2407,7 +2414,8 @@ cost_append(AppendPath *apath, PlannerInfo *root) subpath->pathtarget->width, 0.0, work_mem, - apath->limit_tuples); + apath->limit_tuples, + NULL); } else { @@ -3850,7 +3858,8 @@ initial_cost_mergejoin(PlannerInfo *root, JoinCostWorkspace *workspace, outer_path->pathtarget->width, 0.0, work_mem, - -1.0); + -1.0, + NULL); } else { diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c index 2c696ea0268..3029efe9872 100644 --- a/src/backend/optimizer/plan/createplan.c +++ b/src/backend/optimizer/plan/createplan.c @@ -2073,6 +2073,7 @@ create_incrementalsort_plan(PlannerInfo *root, IncrementalSortPath *best_path, best_path->nPresortedCols); copy_generic_path_info(&plan->sort.plan, (Path *) best_path); + plan->numGroups = best_path->numGroups; return plan; } @@ -5468,7 +5469,8 @@ label_incrementalsort_with_costsize(PlannerInfo *root, IncrementalSort *plan, lefttree->plan_width, 0.0, work_mem, - limit_tuples); + limit_tuples, + &plan->numGroups); plan->sort.plan.startup_cost = sort_path.startup_cost; plan->sort.plan.total_cost = sort_path.total_cost; plan->sort.plan.plan_rows = lefttree->plan_rows; diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c index 3875e3dd571..a8dcad72958 100644 --- a/src/backend/optimizer/util/pathnode.c +++ b/src/backend/optimizer/util/pathnode.c @@ -1605,7 +1605,8 @@ create_merge_append_path(PlannerInfo *root, subpath->pathtarget->width, 0.0, work_mem, - pathnode->limit_tuples); + pathnode->limit_tuples, + NULL); } else { @@ -2883,7 +2884,8 @@ create_incremental_sort_path(PlannerInfo *root, subpath->rows, subpath->pathtarget->width, 0.0, /* XXX comparison_cost shouldn't be 0? */ - work_mem, limit_tuples); + work_mem, limit_tuples, + &sort->numGroups); sort->nPresortedCols = presorted_keys; diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h index 27a2c6815b7..185e2fb8761 100644 --- a/src/include/nodes/pathnodes.h +++ b/src/include/nodes/pathnodes.h @@ -2548,6 +2548,7 @@ typedef struct IncrementalSortPath { SortPath spath; int nPresortedCols; /* number of presorted columns */ + Cardinality numGroups; /* estimated number of groups */ } IncrementalSortPath; /* diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h index b880ce0f4be..781651693eb 100644 --- a/src/include/nodes/plannodes.h +++ b/src/include/nodes/plannodes.h @@ -1171,6 +1171,8 @@ typedef struct IncrementalSort Sort sort; /* number of presorted columns */ int nPresortedCols; + /* estimated number of groups (groups with equal presorted keys) */ + Cardinality numGroups; } IncrementalSort; /* --------------- diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h index bda3f1690c0..9d571e513c6 100644 --- a/src/include/optimizer/cost.h +++ b/src/include/optimizer/cost.h @@ -118,7 +118,8 @@ extern void cost_incremental_sort(Path *path, int input_disabled_nodes, Cost input_startup_cost, Cost input_total_cost, double input_tuples, int width, Cost comparison_cost, int sort_mem, - double limit_tuples); + double limit_tuples, + Cardinality *p_numGroups); extern void cost_append(AppendPath *apath, PlannerInfo *root); extern void cost_merge_append(Path *path, PlannerInfo *root, List *pathkeys, int n_streams, -- 2.34.1