Merge #44156

44156: opt: fix incorrect empty key for full outer join r=RaduBerinde a=RaduBerinde

When joining two single-row expressions, the cross product has an
empty key, but the outer join does not. We need to remove the key in
this case.

Fixes #44029.

Release note (bug fix): fixed planning bug related to FULL joins
between single-row relations.

Co-authored-by: Radu Berinde <radu@cockroachlabs.com>

pkg/sql/logictest/testdata/logic_test/join

@@ -1074,3 +1074,11 @@ SELECT * FROM foo JOIN bar ON generate_series(0, 1) < 2
 
 query error aggregate functions are not allowed in ON
 SELECT * FROM foo JOIN bar ON max(foo.c) < 2
+
+# Regression test for #44029 (outer join on two single-row clauses, with two
+# results).
+query IIII
+SELECT * FROM (VALUES (1, 2)) a(a1,a2) FULL JOIN (VALUES (3, 4)) b(b1,b2) ON a1=b1 ORDER BY a2
+----
+NULL  NULL  3     4
+1     2     NULL  NULL

pkg/sql/opt/memo/logical_props_builder.go

@@ -1883,6 +1883,10 @@ func (h *joinPropsHelper) setFuncDeps(rel *props.Relational) {
 			inputCols := h.leftProps.OutputCols.Union(h.rightProps.OutputCols)
 			if !inputCols.Intersects(notNullInputCols) {
 				rel.FuncDeps.DowngradeKey()
+			} else if key, ok := rel.FuncDeps.StrictKey(); ok && key.Empty() {
+				// The cross-product has an empty key when both sides have an empty key;
+				// but the outer join can have two rows so the empty key doesn't hold.
+				rel.FuncDeps.DowngradeKey()
 			}
 			rel.FuncDeps.MakeOuter(h.leftProps.OutputCols, notNullInputCols)
 			rel.FuncDeps.MakeOuter(h.rightProps.OutputCols, notNullInputCols)

pkg/sql/opt/memo/testdata/logprops/join

@@ -886,6 +886,38 @@ full-join (cross)
  └── filters
       └── true [type=bool]
 
+# Calculate full-join cardinality when both sides have an empty key (#44029).
+build
+SELECT * FROM (VALUES (1, 2)) a(a1,a2) FULL JOIN (VALUES (3, 4)) b(b1,b2) ON a1=b1
+----
+full-join (hash)
+ ├── columns: a1:1(int) a2:2(int) b1:3(int) b2:4(int)
+ ├── cardinality: [1 - 2]
+ ├── prune: (2,4)
+ ├── reject-nulls: (1-4)
+ ├── values
+ │    ├── columns: column1:1(int!null) column2:2(int!null)
+ │    ├── cardinality: [1 - 1]
+ │    ├── key: ()
+ │    ├── fd: ()-->(1,2)
+ │    ├── prune: (1,2)
+ │    └── tuple [type=tuple{int, int}]
+ │         ├── const: 1 [type=int]
+ │         └── const: 2 [type=int]
+ ├── values
+ │    ├── columns: column1:3(int!null) column2:4(int!null)
+ │    ├── cardinality: [1 - 1]
+ │    ├── key: ()
+ │    ├── fd: ()-->(3,4)
+ │    ├── prune: (3,4)
+ │    └── tuple [type=tuple{int, int}]
+ │         ├── const: 3 [type=int]
+ │         └── const: 4 [type=int]
+ └── filters
+      └── eq [type=bool, outer=(1,3), constraints=(/1: (/NULL - ]; /3: (/NULL - ]), fd=(1)==(3), (3)==(1)]
+           ├── variable: column1 [type=int]
+           └── variable: column1 [type=int]
+
 # Calculate full-join cardinality with false filter.
 build
 SELECT * FROM (VALUES (NULL), (NULL)) a FULL JOIN (VALUES (NULL), (NULL)) b ON a.column1=b.column1
@@ -949,9 +981,7 @@ SELECT * FROM (SELECT * FROM xysd LIMIT 1) FULL JOIN (SELECT * FROM xysd LIMIT 1
 ----
 full-join (cross)
  ├── columns: x:1(int) y:2(int) s:3(string) d:4(decimal) x:5(int) y:6(int) s:7(string) d:8(decimal)
- ├── cardinality: [0 - 1]
- ├── key: ()
- ├── fd: ()-->(1-8)
+ ├── cardinality: [0 - 2]
  ├── prune: (1-8)
  ├── reject-nulls: (1-8)
  ├── interesting orderings: (+1) (-3,+4,+1) (+5) (-7,+8,+5)