Hi!
On 14.10.2024 22:59, Kirill Reshke wrote:
The patch in the attachment is a simplistic attempt to differentiate
these sortings by the number of columns. It is not ideal because
comparisons depend on the duplicates in each column, but it may be the
subject of further work.
Even such a trivial model allows us to resolve the case like the below:
CREATE INDEX ON test (x1,x2,x3,x8);
EXPLAIN (ANALYZE) SELECT * FROM test WHERE x1<9E4 ORDER BY x1,x2,x3,x8;
The current master will choose SeqScan for four columns as well as for a
single column. With the model, the optimiser will understand that
sorting four columns costs more than sorting a single column and switch
to IndexScan.
--
regards, Andrei Lepikhov
Also, this patch needs to be rebased, again.
I also noticed that the code needs to be updated, so I rebased the code
to master during the review and attached a patch.
I have a question about estimating the cost of an Append node with your
sort cost model.
I see that if pathkeys is not a subset of subpath pathkeys, then we
calculate the cost taking pathkeys into account. However, I didn't
notice the estimation like that for the opposite situation.
I see that the cost of Append is the sum of the subpath costs, but we
don't take pathkeys into account for subpaths, right?
--
Regards,
Alena Rybakina
Postgres Professional
From 9e9c5ff8aa977ec339c20433a427d98e15f8ed9d Mon Sep 17 00:00:00 2001
From: Alena Rybakina <a.rybak...@postgrespro.ru>
Date: Tue, 15 Oct 2024 01:05:38 +0300
Subject: [PATCH] Introduce the number of columns in the cost-sort model.
The sort node calls the comparison operator for each pair of attributes for
each couple of tuples. However, the current cost model uses
a '2.0*cpu_operator_cost' multiplier, which performs some sort of averaging.
This technique can lead to incorrect estimations when sorting on three, four,
or more columns, quite common in practice.
Moreover, further elaboration of the optimiser forms more strict requirements
for the balance of sortings, as caused by IncrementalSort, MergeAppend, and
MergeJoin.
In this patch, the multiplier is a linear function of a number of columns.
Member 1.0 needs to smooth the fact that dependence on the number of columns is
weaker than linear.
It is an extreme formula. The number of comparisons depends on the distinct
values in each column. As a TODO, we can natively elaborate this model by the
next step, involving distinct statistics to make estimations more precise.
---
.../postgres_fdw/expected/postgres_fdw.out | 15 +++--
contrib/postgres_fdw/postgres_fdw.c | 2 +-
src/backend/optimizer/path/costsize.c | 34 ++++++----
src/backend/optimizer/plan/createplan.c | 2 +-
src/backend/optimizer/plan/planner.c | 2 +-
src/backend/optimizer/prep/prepunion.c | 2 +-
src/backend/optimizer/util/pathnode.c | 8 +--
src/include/optimizer/cost.h | 2 +-
src/test/regress/expected/aggregates.out | 13 ++--
src/test/regress/expected/join.out | 20 +++---
src/test/regress/expected/partition_join.out | 8 ++-
src/test/regress/expected/union.out | 66 +++++++++----------
12 files changed, 95 insertions(+), 79 deletions(-)
diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out
b/contrib/postgres_fdw/expected/postgres_fdw.out
index f2bcd6aa98c..5da33ab69ec 100644
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -9995,13 +9995,16 @@ SELECT t1.a,t2.b,t3.c FROM fprt1 t1 INNER JOIN fprt2 t2
ON (t1.a = t2.b) INNER J
-- left outer join + nullable clause
EXPLAIN (VERBOSE, COSTS OFF)
SELECT t1.a,t2.b,t2.c FROM fprt1 t1 LEFT JOIN (SELECT * FROM fprt2 WHERE a <
10) t2 ON (t1.a = t2.b and t1.b = t2.a) WHERE t1.a < 10 ORDER BY 1,2,3;
-
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
- Foreign Scan
+
QUERY PLAN
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Sort
Output: t1.a, fprt2.b, fprt2.c
- Relations: (public.ftprt1_p1 t1) LEFT JOIN (public.ftprt2_p1 fprt2)
- Remote SQL: SELECT r5.a, r6.b, r6.c FROM (public.fprt1_p1 r5 LEFT JOIN
public.fprt2_p1 r6 ON (((r5.a = r6.b)) AND ((r5.b = r6.a)) AND ((r6.a < 10))))
WHERE ((r5.a < 10)) ORDER BY r5.a ASC NULLS LAST, r6.b ASC NULLS LAST, r6.c ASC
NULLS LAST
-(4 rows)
+ Sort Key: t1.a, fprt2.b, fprt2.c
+ -> Foreign Scan
+ Output: t1.a, fprt2.b, fprt2.c
+ Relations: (public.ftprt1_p1 t1) LEFT JOIN (public.ftprt2_p1 fprt2)
+ Remote SQL: SELECT r5.a, r6.b, r6.c FROM (public.fprt1_p1 r5 LEFT
JOIN public.fprt2_p1 r6 ON (((r5.a = r6.b)) AND ((r5.b = r6.a)) AND ((r6.a <
10)))) WHERE ((r5.a < 10))
+(7 rows)
SELECT t1.a,t2.b,t2.c FROM fprt1 t1 LEFT JOIN (SELECT * FROM fprt2 WHERE a <
10) t2 ON (t1.a = t2.b and t1.b = t2.a) WHERE t1.a < 10 ORDER BY 1,2,3;
a | b | c
diff --git a/contrib/postgres_fdw/postgres_fdw.c
b/contrib/postgres_fdw/postgres_fdw.c
index adc62576d1f..f9a2e88a17f 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3667,7 +3667,7 @@ adjust_foreign_grouping_path_cost(PlannerInfo *root,
cost_sort(&sort_path,
root,
- pathkeys,
+ list_length(pathkeys),
0,
*p_startup_cost + *p_run_cost,
retrieved_rows,
diff --git a/src/backend/optimizer/path/costsize.c
b/src/backend/optimizer/path/costsize.c
index 2bb6db1df77..7c4e39065b7 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -493,6 +493,10 @@ cost_gather_merge(GatherMergePath *path, PlannerInfo *root,
Cost comparison_cost;
double N;
double logN;
+ int npathkeys = list_length(((Path *)
path)->pathkeys);
+ int cmpMultiplier = npathkeys;
+
+ Assert(npathkeys > 0);
/* Mark the path with the correct row estimate */
if (rows)
@@ -512,7 +516,7 @@ cost_gather_merge(GatherMergePath *path, PlannerInfo *root,
logN = LOG2(N);
/* Assumed cost per tuple comparison */
- comparison_cost = 2.0 * cpu_operator_cost;
+ comparison_cost = (1.0 + cmpMultiplier) * cpu_operator_cost;
/* Heap creation cost */
startup_cost += comparison_cost * N * logN;
@@ -1896,7 +1900,7 @@ cost_recursive_union(Path *runion, Path *nrterm, Path
*rterm)
*/
static void
cost_tuplesort(Cost *startup_cost, Cost *run_cost,
- double tuples, int width,
+ double tuples, int width, int cmpMultiplier,
Cost comparison_cost, int sort_mem,
double limit_tuples)
{
@@ -1913,7 +1917,7 @@ cost_tuplesort(Cost *startup_cost, Cost *run_cost,
tuples = 2.0;
/* Include the default cost-per-comparison */
- comparison_cost += 2.0 * cpu_operator_cost;
+ comparison_cost += (1.0 + cmpMultiplier) * cpu_operator_cost;
/* Do we have a useful LIMIT? */
if (limit_tuples > 0 && limit_tuples < tuples)
@@ -2086,7 +2090,9 @@ cost_incremental_sort(Path *path,
* are equal.
*/
cost_tuplesort(&group_startup_cost, &group_run_cost,
- group_tuples, width, comparison_cost,
sort_mem,
+ group_tuples, width,
+ list_length(pathkeys),
+ comparison_cost, sort_mem,
limit_tuples);
/*
@@ -2110,7 +2116,7 @@ cost_incremental_sort(Path *path,
* detect the sort groups. This is roughly equal to one extra copy and
* comparison per tuple.
*/
- run_cost += (cpu_tuple_cost + comparison_cost) * input_tuples;
+ run_cost += (cpu_tuple_cost + (list_length(pathkeys) + 1) *
comparison_cost) * input_tuples;
/*
* Additionally, we charge double cpu_tuple_cost for each input group to
@@ -2142,7 +2148,7 @@ cost_incremental_sort(Path *path,
*/
void
cost_sort(Path *path, PlannerInfo *root,
- List *pathkeys, int input_disabled_nodes,
+ int npathkeys, int input_disabled_nodes,
Cost input_cost, double tuples, int width,
Cost comparison_cost, int sort_mem,
double limit_tuples)
@@ -2150,9 +2156,12 @@ cost_sort(Path *path, PlannerInfo *root,
{
Cost startup_cost;
Cost run_cost;
+ int cmpMultiplier = npathkeys;
+
+ Assert(npathkeys > 0);
cost_tuplesort(&startup_cost, &run_cost,
- tuples, width,
+ tuples, width, cmpMultiplier,
comparison_cost, sort_mem,
limit_tuples);
@@ -2321,7 +2330,7 @@ cost_append(AppendPath *apath)
*/
cost_sort(&sort_path,
NULL, /* doesn't
currently need root */
- pathkeys,
+ list_length(pathkeys),
subpath->disabled_nodes,
subpath->total_cost,
subpath->rows,
@@ -2440,6 +2449,9 @@ cost_merge_append(Path *path, PlannerInfo *root,
Cost comparison_cost;
double N;
double logN;
+ int cmpMultiplier = list_length(pathkeys);
+
+ Assert(pathkeys != NIL);
/*
* Avoid log(0)...
@@ -2448,7 +2460,7 @@ cost_merge_append(Path *path, PlannerInfo *root,
logN = LOG2(N);
/* Assumed cost per tuple comparison */
- comparison_cost = 2.0 * cpu_operator_cost;
+ comparison_cost = (1.0 + cmpMultiplier) * cpu_operator_cost;
/* Heap creation cost */
startup_cost += comparison_cost * N * logN;
@@ -3708,7 +3720,7 @@ initial_cost_mergejoin(PlannerInfo *root,
JoinCostWorkspace *workspace,
{
cost_sort(&sort_path,
root,
- outersortkeys,
+ list_length(outersortkeys),
outer_path->disabled_nodes,
outer_path->total_cost,
outer_path_rows,
@@ -3758,7 +3770,7 @@ initial_cost_mergejoin(PlannerInfo *root,
JoinCostWorkspace *workspace,
cost_sort(&sort_path,
root,
- innersortkeys,
+ list_length(innersortkeys),
inner_path->disabled_nodes,
inner_path->total_cost,
inner_path_rows,
diff --git a/src/backend/optimizer/plan/createplan.c
b/src/backend/optimizer/plan/createplan.c
index f2ed0d81f61..fbb64e4cf3e 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -5496,7 +5496,7 @@ label_sort_with_costsize(PlannerInfo *root, Sort *plan,
double limit_tuples)
Assert(IsA(plan, Sort));
- cost_sort(&sort_path, root, NIL,
+ cost_sort(&sort_path, root, list_length(lefttree->targetlist),
plan->plan.disabled_nodes,
lefttree->total_cost,
lefttree->plan_rows,
diff --git a/src/backend/optimizer/plan/planner.c
b/src/backend/optimizer/plan/planner.c
index 0f423e96847..f326e1c34b9 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -6855,7 +6855,7 @@ plan_cluster_use_sort(Oid tableOid, Oid indexOid)
/* Estimate the cost of seq scan + sort */
seqScanPath = create_seqscan_path(root, rel, NULL, 0);
- cost_sort(&seqScanAndSortPath, root, NIL,
+ cost_sort(&seqScanAndSortPath, root, rel->max_attr,
seqScanPath->disabled_nodes,
seqScanPath->total_cost, rel->tuples,
rel->reltarget->width,
comparisonCost, maintenance_work_mem, -1.0);
diff --git a/src/backend/optimizer/prep/prepunion.c
b/src/backend/optimizer/prep/prepunion.c
index a0baf6d4a18..0d51e1f6127 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -1358,7 +1358,7 @@ choose_hashed_setop(PlannerInfo *root, List *groupClauses,
sorted_p.startup_cost = input_path->startup_cost;
sorted_p.total_cost = input_path->total_cost;
/* XXX cost_sort doesn't actually look at pathkeys, so just pass NIL */
- cost_sort(&sorted_p, root, NIL, sorted_p.disabled_nodes,
+ cost_sort(&sorted_p, root, list_length(input_path->pathtarget->exprs),
sorted_p.disabled_nodes,
sorted_p.total_cost,
input_path->rows, input_path->pathtarget->width,
0.0, work_mem, -1.0);
diff --git a/src/backend/optimizer/util/pathnode.c
b/src/backend/optimizer/util/pathnode.c
index fc97bf6ee26..393f9931767 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1537,7 +1537,7 @@ create_merge_append_path(PlannerInfo *root,
cost_sort(&sort_path,
root,
- pathkeys,
+ list_length(pathkeys),
subpath->disabled_nodes,
subpath->total_cost,
subpath->rows,
@@ -1866,7 +1866,7 @@ create_unique_path(PlannerInfo *root, RelOptInfo *rel,
Path *subpath,
/*
* Estimate cost for sort+unique implementation
*/
- cost_sort(&sort_path, root, NIL,
+ cost_sort(&sort_path, root,
list_length(subpath->pathtarget->exprs),
subpath->disabled_nodes,
subpath->total_cost,
rel->rows,
@@ -3101,7 +3101,7 @@ create_sort_path(PlannerInfo *root,
pathnode->subpath = subpath;
- cost_sort(&pathnode->path, root, pathkeys,
+ cost_sort(&pathnode->path, root, list_length(pathkeys),
subpath->disabled_nodes,
subpath->total_cost,
subpath->rows,
@@ -3436,7 +3436,7 @@ create_groupingsets_path(PlannerInfo *root,
else
{
/* Account for cost of sort, but don't charge
input cost again */
- cost_sort(&sort_path, root, NIL, 0,
+ cost_sort(&sort_path, root,
list_length(subpath->pathtarget->exprs), 0,
0.0,
subpath->rows,
subpath->pathtarget->width,
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index 854a782944a..b8113b87486 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -108,7 +108,7 @@ extern void cost_resultscan(Path *path, PlannerInfo *root,
RelOptInfo *baserel,
ParamPathInfo *param_info);
extern void cost_recursive_union(Path *runion, Path *nrterm, Path *rterm);
extern void cost_sort(Path *path, PlannerInfo *root,
- List *pathkeys, int disabled_nodes,
+ int npathkeys, int disabled_nodes,
Cost input_cost, double tuples, int
width,
Cost comparison_cost, int sort_mem,
double limit_tuples);
diff --git a/src/test/regress/expected/aggregates.out
b/src/test/regress/expected/aggregates.out
index 1e682565d13..7546c795d0f 100644
--- a/src/test/regress/expected/aggregates.out
+++ b/src/test/regress/expected/aggregates.out
@@ -3031,17 +3031,18 @@ ANALYZE agg_sort_order;
EXPLAIN (COSTS OFF)
SELECT array_agg(c1 ORDER BY c2),c2
FROM agg_sort_order WHERE c2 < 100 GROUP BY c1 ORDER BY 2;
- QUERY PLAN
-----------------------------------------------------------------------------
+ QUERY PLAN
+--------------------------------------------------------------------------
Sort
Sort Key: c2
-> GroupAggregate
Group Key: c1
- -> Sort
+ -> Incremental Sort
Sort Key: c1, c2
- -> Index Scan using agg_sort_order_c2_idx on agg_sort_order
- Index Cond: (c2 < 100)
-(8 rows)
+ Presorted Key: c1
+ -> Index Scan using agg_sort_order_pkey on agg_sort_order
+ Filter: (c2 < 100)
+(9 rows)
DROP TABLE agg_sort_order CASCADE;
DROP TABLE btg;
diff --git a/src/test/regress/expected/join.out
b/src/test/regress/expected/join.out
index 756c2e24965..f543e416f3e 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -5736,18 +5736,20 @@ select d.* from d left join (select * from b group by
b.id, b.c_id) s
explain (costs off)
select d.* from d left join (select distinct * from b) s
on d.a = s.id;
- QUERY PLAN
---------------------------------------
- Merge Right Join
- Merge Cond: (b.id = d.a)
- -> Unique
- -> Sort
- Sort Key: b.id, b.c_id
- -> Seq Scan on b
+ QUERY PLAN
+---------------------------------------------
+ Merge Left Join
+ Merge Cond: (d.a = s.id)
-> Sort
Sort Key: d.a
-> Seq Scan on d
-(9 rows)
+ -> Sort
+ Sort Key: s.id
+ -> Subquery Scan on s
+ -> HashAggregate
+ Group Key: b.id, b.c_id
+ -> Seq Scan on b
+(11 rows)
-- join removal is not possible here
explain (costs off)
diff --git a/src/test/regress/expected/partition_join.out
b/src/test/regress/expected/partition_join.out
index 108f9ecb445..886dc559a50 100644
--- a/src/test/regress/expected/partition_join.out
+++ b/src/test/regress/expected/partition_join.out
@@ -1274,9 +1274,11 @@ EXPLAIN (COSTS OFF)
SELECT t1.a, t1.c, t2.b, t2.c, t3.a + t3.b, t3.c FROM (prt1 t1 LEFT JOIN prt2
t2 ON t1.a = t2.b) RIGHT JOIN prt1_e t3 ON (t1.a = (t3.a + t3.b)/2) WHERE t3.c
= 0 ORDER BY t1.a, t2.b, t3.a + t3.b;
QUERY PLAN
----------------------------------------------------------------------------
- Sort
+ Incremental Sort
Sort Key: t1.a, t2.b, ((t3.a + t3.b))
- -> Append
+ Presorted Key: t1.a
+ -> Merge Append
+ Sort Key: t1.a
-> Merge Left Join
Merge Cond: (t1_1.a = t2_1.b)
-> Sort
@@ -1325,7 +1327,7 @@ SELECT t1.a, t1.c, t2.b, t2.c, t3.a + t3.b, t3.c FROM
(prt1 t1 LEFT JOIN prt2 t2
-> Sort
Sort Key: t2_3.b
-> Seq Scan on prt2_p3 t2_3
-(51 rows)
+(53 rows)
SELECT t1.a, t1.c, t2.b, t2.c, t3.a + t3.b, t3.c FROM (prt1 t1 LEFT JOIN prt2
t2 ON t1.a = t2.b) RIGHT JOIN prt1_e t3 ON (t1.a = (t3.a + t3.b)/2) WHERE t3.c
= 0 ORDER BY t1.a, t2.b, t3.a + t3.b;
a | c | b | c | ?column? | c
diff --git a/src/test/regress/expected/union.out
b/src/test/regress/expected/union.out
index c73631a9a1d..f1c0af1c8d6 100644
--- a/src/test/regress/expected/union.out
+++ b/src/test/regress/expected/union.out
@@ -1225,18 +1225,17 @@ SELECT * FROM
SELECT 2 AS t, 4 AS x) ss
WHERE x < 4
ORDER BY x;
- QUERY PLAN
---------------------------------------------------
+ QUERY PLAN
+--------------------------------------------
Sort
Sort Key: (2)
- -> Unique
- -> Sort
- Sort Key: (1), (2)
- -> Append
- -> Result
- -> Result
- One-Time Filter: false
-(9 rows)
+ -> HashAggregate
+ Group Key: (1), (2)
+ -> Append
+ -> Result
+ -> Result
+ One-Time Filter: false
+(8 rows)
SELECT * FROM
(SELECT 1 AS t, 2 AS x
@@ -1290,19 +1289,18 @@ SELECT * FROM
SELECT 2 AS t, 4 AS x) ss
WHERE x > 3
ORDER BY x;
- QUERY PLAN
-------------------------------------------------------------------------------------
+ QUERY PLAN
+-------------------------------------------------------------------------------
Sort
Sort Key: ss.x
-> Subquery Scan on ss
Filter: (ss.x > 3)
- -> Unique
- -> Sort
- Sort Key: (1), (((random() * '3'::double
precision))::integer)
- -> Append
- -> Result
- -> Result
-(10 rows)
+ -> HashAggregate
+ Group Key: (1), (((random() * '3'::double precision))::integer)
+ -> Append
+ -> Result
+ -> Result
+(9 rows)
SELECT * FROM
(SELECT 1 AS t, (random()*3)::int AS x
@@ -1323,24 +1321,22 @@ select distinct q1 from
union all
select distinct * from int8_tbl i82) ss
where q2 = q2;
- QUERY PLAN
-----------------------------------------------------------
- Unique
- -> Merge Append
- Sort Key: "*SELECT* 1".q1
+ QUERY PLAN
+----------------------------------------------------
+ HashAggregate
+ Group Key: "*SELECT* 1".q1
+ -> Append
-> Subquery Scan on "*SELECT* 1"
- -> Unique
- -> Sort
- Sort Key: i81.q1, i81.q2
- -> Seq Scan on int8_tbl i81
- Filter: (q2 IS NOT NULL)
+ -> HashAggregate
+ Group Key: i81.q1, i81.q2
+ -> Seq Scan on int8_tbl i81
+ Filter: (q2 IS NOT NULL)
-> Subquery Scan on "*SELECT* 2"
- -> Unique
- -> Sort
- Sort Key: i82.q1, i82.q2
- -> Seq Scan on int8_tbl i82
- Filter: (q2 IS NOT NULL)
-(15 rows)
+ -> HashAggregate
+ Group Key: i82.q1, i82.q2
+ -> Seq Scan on int8_tbl i82
+ Filter: (q2 IS NOT NULL)
+(13 rows)
select distinct q1 from
(select distinct * from int8_tbl i81
--
2.34.1
