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Commitc4c2885

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Fix UNION/INTERSECT/EXCEPT over no columns.
Since 9.4, we've allowed the syntax "select union select" and variantsof that. However, the planner wasn't expecting a no-column set operationand ended up treating the set operation as if it were UNION ALL.Turns out it's trivial to fix in v10 and later; we just need to be carefulabout not generating a Sort node with no sort keys. However, since a weirdcorner case like this is never going to be exercised by developers, we'dbetter have thorough regression tests if we want to consider it supported.Per report from Victor Yegorov.Discussion:https://postgr.es/m/CAGnEbojGJrRSOgJwNGM7JSJZpVAf8xXcVPbVrGdhbVEHZ-BUMw@mail.gmail.com
1 parent854823f commitc4c2885

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4 files changed

+168
-17
lines changed

4 files changed

+168
-17
lines changed

‎src/backend/optimizer/plan/createplan.c‎

Lines changed: 0 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -6326,7 +6326,6 @@ make_setop(SetOpCmd cmd, SetOpStrategy strategy, Plan *lefttree,
63266326
* convert SortGroupClause list into arrays of attr indexes and equality
63276327
* operators, as wanted by executor
63286328
*/
6329-
Assert(numCols>0);
63306329
dupColIdx= (AttrNumber*)palloc(sizeof(AttrNumber)*numCols);
63316330
dupOperators= (Oid*)palloc(sizeof(Oid)*numCols);
63326331

‎src/backend/optimizer/prep/prepunion.c‎

Lines changed: 10 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -711,10 +711,6 @@ generate_nonunion_path(SetOperationStmt *op, PlannerInfo *root,
711711
/* Identify the grouping semantics */
712712
groupList=generate_setop_grouplist(op,tlist);
713713

714-
/* punt if nothing to group on (can this happen?) */
715-
if (groupList==NIL)
716-
returnpath;
717-
718714
/*
719715
* Estimate number of distinct groups that we'll need hashtable entries
720716
* for; this is the size of the left-hand input for EXCEPT, or the smaller
@@ -741,7 +737,7 @@ generate_nonunion_path(SetOperationStmt *op, PlannerInfo *root,
741737
dNumGroups,dNumOutputRows,
742738
(op->op==SETOP_INTERSECT) ?"INTERSECT" :"EXCEPT");
743739

744-
if (!use_hash)
740+
if (groupList&&!use_hash)
745741
path= (Path*)create_sort_path(root,
746742
result_rel,
747743
path,
@@ -864,10 +860,6 @@ make_union_unique(SetOperationStmt *op, Path *path, List *tlist,
864860
/* Identify the grouping semantics */
865861
groupList=generate_setop_grouplist(op,tlist);
866862

867-
/* punt if nothing to group on (can this happen?) */
868-
if (groupList==NIL)
869-
returnpath;
870-
871863
/*
872864
* XXX for the moment, take the number of distinct groups as equal to the
873865
* total input size, ie, the worst case. This is too conservative, but we
@@ -898,13 +890,15 @@ make_union_unique(SetOperationStmt *op, Path *path, List *tlist,
898890
else
899891
{
900892
/* Sort and Unique */
901-
path= (Path*)create_sort_path(root,
902-
result_rel,
903-
path,
904-
make_pathkeys_for_sortclauses(root,
905-
groupList,
906-
tlist),
907-
-1.0);
893+
if (groupList)
894+
path= (Path*)
895+
create_sort_path(root,
896+
result_rel,
897+
path,
898+
make_pathkeys_for_sortclauses(root,
899+
groupList,
900+
tlist),
901+
-1.0);
908902
/* We have to manually jam the right tlist into the path; ick */
909903
path->pathtarget=create_pathtarget(root,tlist);
910904
path= (Path*)create_upper_unique_path(root,

‎src/test/regress/expected/union.out‎

Lines changed: 115 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -552,6 +552,121 @@ SELECT q1 FROM int8_tbl EXCEPT (((SELECT q2 FROM int8_tbl ORDER BY q2 LIMIT 1)))
552552
4567890123456789 | -4567890123456789
553553
(5 rows)
554554

555+
--
556+
-- Check behavior with empty select list (allowed since 9.4)
557+
--
558+
select union select;
559+
--
560+
(1 row)
561+
562+
select intersect select;
563+
--
564+
(1 row)
565+
566+
select except select;
567+
--
568+
(0 rows)
569+
570+
-- check hashed implementation
571+
set enable_hashagg = true;
572+
set enable_sort = false;
573+
explain (costs off)
574+
select from generate_series(1,5) union select from generate_series(1,3);
575+
QUERY PLAN
576+
----------------------------------------------------------------
577+
HashAggregate
578+
-> Append
579+
-> Function Scan on generate_series
580+
-> Function Scan on generate_series generate_series_1
581+
(4 rows)
582+
583+
explain (costs off)
584+
select from generate_series(1,5) intersect select from generate_series(1,3);
585+
QUERY PLAN
586+
----------------------------------------------------------------------
587+
HashSetOp Intersect
588+
-> Append
589+
-> Subquery Scan on "*SELECT* 1"
590+
-> Function Scan on generate_series
591+
-> Subquery Scan on "*SELECT* 2"
592+
-> Function Scan on generate_series generate_series_1
593+
(6 rows)
594+
595+
select from generate_series(1,5) union select from generate_series(1,3);
596+
--
597+
(1 row)
598+
599+
select from generate_series(1,5) union all select from generate_series(1,3);
600+
--
601+
(8 rows)
602+
603+
select from generate_series(1,5) intersect select from generate_series(1,3);
604+
--
605+
(1 row)
606+
607+
select from generate_series(1,5) intersect all select from generate_series(1,3);
608+
--
609+
(3 rows)
610+
611+
select from generate_series(1,5) except select from generate_series(1,3);
612+
--
613+
(0 rows)
614+
615+
select from generate_series(1,5) except all select from generate_series(1,3);
616+
--
617+
(2 rows)
618+
619+
-- check sorted implementation
620+
set enable_hashagg = false;
621+
set enable_sort = true;
622+
explain (costs off)
623+
select from generate_series(1,5) union select from generate_series(1,3);
624+
QUERY PLAN
625+
----------------------------------------------------------------
626+
Unique
627+
-> Append
628+
-> Function Scan on generate_series
629+
-> Function Scan on generate_series generate_series_1
630+
(4 rows)
631+
632+
explain (costs off)
633+
select from generate_series(1,5) intersect select from generate_series(1,3);
634+
QUERY PLAN
635+
----------------------------------------------------------------------
636+
SetOp Intersect
637+
-> Append
638+
-> Subquery Scan on "*SELECT* 1"
639+
-> Function Scan on generate_series
640+
-> Subquery Scan on "*SELECT* 2"
641+
-> Function Scan on generate_series generate_series_1
642+
(6 rows)
643+
644+
select from generate_series(1,5) union select from generate_series(1,3);
645+
--
646+
(1 row)
647+
648+
select from generate_series(1,5) union all select from generate_series(1,3);
649+
--
650+
(8 rows)
651+
652+
select from generate_series(1,5) intersect select from generate_series(1,3);
653+
--
654+
(1 row)
655+
656+
select from generate_series(1,5) intersect all select from generate_series(1,3);
657+
--
658+
(3 rows)
659+
660+
select from generate_series(1,5) except select from generate_series(1,3);
661+
--
662+
(0 rows)
663+
664+
select from generate_series(1,5) except all select from generate_series(1,3);
665+
--
666+
(2 rows)
667+
668+
reset enable_hashagg;
669+
reset enable_sort;
555670
--
556671
-- Check handling of a case with unknown constants. We don't guarantee
557672
-- an undecorated constant will work in all cases, but historically this

‎src/test/regress/sql/union.sql‎

Lines changed: 43 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -190,6 +190,49 @@ SELECT q1 FROM int8_tbl EXCEPT (((SELECT q2 FROM int8_tbl ORDER BY q2 LIMIT 1)))
190190

191191
(((((select*from int8_tbl)))));
192192

193+
--
194+
-- Check behavior with empty select list (allowed since 9.4)
195+
--
196+
197+
selectunionselect;
198+
select intersectselect;
199+
select exceptselect;
200+
201+
-- check hashed implementation
202+
set enable_hashagg= true;
203+
set enable_sort= false;
204+
205+
explain (costs off)
206+
selectfrom generate_series(1,5)unionselectfrom generate_series(1,3);
207+
explain (costs off)
208+
selectfrom generate_series(1,5) intersectselectfrom generate_series(1,3);
209+
210+
selectfrom generate_series(1,5)unionselectfrom generate_series(1,3);
211+
selectfrom generate_series(1,5)union allselectfrom generate_series(1,3);
212+
selectfrom generate_series(1,5) intersectselectfrom generate_series(1,3);
213+
selectfrom generate_series(1,5) intersect allselectfrom generate_series(1,3);
214+
selectfrom generate_series(1,5) exceptselectfrom generate_series(1,3);
215+
selectfrom generate_series(1,5) except allselectfrom generate_series(1,3);
216+
217+
-- check sorted implementation
218+
set enable_hashagg= false;
219+
set enable_sort= true;
220+
221+
explain (costs off)
222+
selectfrom generate_series(1,5)unionselectfrom generate_series(1,3);
223+
explain (costs off)
224+
selectfrom generate_series(1,5) intersectselectfrom generate_series(1,3);
225+
226+
selectfrom generate_series(1,5)unionselectfrom generate_series(1,3);
227+
selectfrom generate_series(1,5)union allselectfrom generate_series(1,3);
228+
selectfrom generate_series(1,5) intersectselectfrom generate_series(1,3);
229+
selectfrom generate_series(1,5) intersect allselectfrom generate_series(1,3);
230+
selectfrom generate_series(1,5) exceptselectfrom generate_series(1,3);
231+
selectfrom generate_series(1,5) except allselectfrom generate_series(1,3);
232+
233+
reset enable_hashagg;
234+
reset enable_sort;
235+
193236
--
194237
-- Check handling of a case with unknown constants. We don't guarantee
195238
-- an undecorated constant will work in all cases, but historically this

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