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Commit598aa19

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Still another try at fixing scanjoin_target insertion into parallel plans.
The previous code neglected the fact that the scanjoin_target mightcarry sortgroupref labelings that we need to absorb. Instead, docreate_projection_path() unconditionally, and tweak the path's costestimate after the fact. (I'm now convinced that we ought to refactorthe way we account for sometimes not needing a separate projection step,but right now is not the time for that sort of cleanup.)Problem identified by Amit Kapila, patch by me.
1 parent7e81a18 commit598aa19

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3 files changed

+43
-20
lines changed

3 files changed

+43
-20
lines changed

‎src/backend/optimizer/plan/planner.c

Lines changed: 21 additions & 18 deletions
Original file line numberDiff line numberDiff line change
@@ -1788,36 +1788,39 @@ grouping_planner(PlannerInfo *root, bool inheritance_update,
17881788
Path*subpath= (Path*)lfirst(lc);
17891789
Path*newpath;
17901790

1791+
/* Shouldn't have any parameterized paths anymore */
1792+
Assert(subpath->param_info==NULL);
1793+
17911794
/*
17921795
* We can't use apply_projection_to_path() here, because there
17931796
* could already be pointers to these paths, and therefore we
1794-
* cannot modify them in place. Instead, we must use
1795-
* create_projection_path(). The good news is this won't
1796-
* actually insert a Result node into the final plan unless
1797-
* it's needed, but the bad news is that it will charge for
1798-
* the node whether it's needed or not. Therefore, if the
1799-
* target list is already what we need it to be, just leave
1800-
* this partial path alone.
1797+
* dare not modify them in place. Instead, we must use
1798+
* create_projection_path() unconditionally.
18011799
*/
1802-
if (equal(scanjoin_target->exprs,subpath->pathtarget->exprs))
1803-
continue;
1804-
1805-
Assert(subpath->param_info==NULL);
18061800
newpath= (Path*)create_projection_path(root,
18071801
current_rel,
18081802
subpath,
18091803
scanjoin_target);
1810-
if (is_projection_capable_path(subpath))
1804+
1805+
/*
1806+
* Although create_projection_path() inserts a ProjectionPath
1807+
* unconditionally, create_projection_plan() will only insert
1808+
* a Result node if the subpath is not projection-capable, so
1809+
* we should discount the cost of that node if it will not
1810+
* actually get inserted. (This is pretty grotty but we can
1811+
* improve it later if it seems important.)
1812+
*/
1813+
if (equal(scanjoin_target->exprs,subpath->pathtarget->exprs))
18111814
{
1812-
/*
1813-
* Since the target lists differ, a projection path is
1814-
* essential, but it will disappear at plan creation time
1815-
* because the subpath is projection-capable. So avoid
1816-
* charging anything for the disappearing node.
1817-
*/
1815+
/* at most we need a relabeling of the subpath */
18181816
newpath->startup_cost=subpath->startup_cost;
18191817
newpath->total_cost=subpath->total_cost;
18201818
}
1819+
elseif (is_projection_capable_path(subpath))
1820+
{
1821+
/* need to project, but we don't need a Result */
1822+
newpath->total_cost-=cpu_tuple_cost*subpath->rows;
1823+
}
18211824

18221825
lfirst(lc)=newpath;
18231826
}

‎src/test/regress/expected/select_parallel.out

Lines changed: 17 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -6,9 +6,10 @@ create or replace function parallel_restricted(int) returns int as
66
-- Serializable isolation would disable parallel query, so explicitly use an
77
-- arbitrary other level.
88
begin isolation level repeatable read;
9-
--setupparalleltest
9+
--encourage use ofparallelplans
1010
set parallel_setup_cost=0;
1111
set parallel_tuple_cost=0;
12+
set min_parallel_relation_size=0;
1213
set max_parallel_workers_per_gather=4;
1314
explain (costs off)
1415
select count(*) from a_star;
@@ -71,6 +72,21 @@ select length(stringu1) from tenk1 group by length(stringu1);
7172
6
7273
(1 row)
7374

75+
explain (costs off)
76+
select stringu1, count(*) from tenk1 group by stringu1 order by stringu1;
77+
QUERY PLAN
78+
----------------------------------------------------
79+
Sort
80+
Sort Key: stringu1
81+
-> Finalize HashAggregate
82+
Group Key: stringu1
83+
-> Gather
84+
Workers Planned: 4
85+
-> Partial HashAggregate
86+
Group Key: stringu1
87+
-> Parallel Seq Scan on tenk1
88+
(9 rows)
89+
7490
-- test that parallel plan for aggregates is not selected when
7591
-- target list contains parallel restricted clause.
7692
explain (costs off)

‎src/test/regress/sql/select_parallel.sql

Lines changed: 5 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -9,9 +9,10 @@ create or replace function parallel_restricted(int) returns int as
99
-- arbitrary other level.
1010
begin isolation level repeatable read;
1111

12-
--setupparalleltest
12+
--encourage use ofparallelplans
1313
set parallel_setup_cost=0;
1414
set parallel_tuple_cost=0;
15+
set min_parallel_relation_size=0;
1516
set max_parallel_workers_per_gather=4;
1617

1718
explain (costs off)
@@ -29,6 +30,9 @@ explain (costs off)
2930
select length(stringu1)from tenk1group by length(stringu1);
3031
select length(stringu1)from tenk1group by length(stringu1);
3132

33+
explain (costs off)
34+
select stringu1,count(*)from tenk1group by stringu1order by stringu1;
35+
3236
-- test that parallel plan for aggregates is not selected when
3337
-- target list contains parallel restricted clause.
3438
explain (costs off)

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