From efe0689c56dbfe24fe57929aba516a1c23aa7134 Mon Sep 17 00:00:00 2001
From: Rebecca Taft <becca@cockroachlabs.com>
Date: Fri, 30 Sep 2022 17:54:56 +0000
Subject: [PATCH] opt: fix min cardinality calculation of EXCEPT

Prior to this commit, we assumed that the minimum cardinality of
EXCEPT could never be lower than the minimum cardinality of the
left side minus the maximum cardinality of the right side.
However, this assumption is only true for EXCEPT ALL, not EXCEPT.

For example, VALUES (1), (1) EXCEPT VALUES (1) produces 0 rows,
even though the left side produces more rows than the right.
Because of this invalid assumption, the optimizer could make some
invalid transformations, resulting in incorrect results.

Fixes #89101

Release note (bug fix): Fixed a bug that has existed since v2.1.0
where queries containing a subquery with EXCEPT could produce
incorrect results. This could happen if the optimizer could guarantee
that the left side of the EXCEPT always returned more rows than the
right side. In this case, the optimizer made a faulty assupmtion that
the EXCEPT subquery always returned at least one row, which could cause
the optimizer to perform an invalid transformation, possibly causing
the full query to return incorrect results.
---
 pkg/sql/opt/memo/logical_props_builder.go |  7 ++-
 pkg/sql/opt/memo/testdata/logprops/set    | 54 +++++++++++++++++++++++
 2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/pkg/sql/opt/memo/logical_props_builder.go b/pkg/sql/opt/memo/logical_props_builder.go
index 5c37632045df..56e218f802a2 100644
--- a/pkg/sql/opt/memo/logical_props_builder.go
+++ b/pkg/sql/opt/memo/logical_props_builder.go
@@ -1894,9 +1894,14 @@ func (b *logicalPropsBuilder) makeSetCardinality(
 		card = props.Cardinality{Min: 0, Max: left.Max}
 		card = card.Limit(right.Max)
 
-	case opt.ExceptOp, opt.ExceptAllOp:
+	case opt.ExceptOp:
 		// Use left Max cardinality.
 		card = props.Cardinality{Min: 0, Max: left.Max}
+
+	case opt.ExceptAllOp:
+		// Use left Max cardinality. Cardinality cannot be less than left Min minus
+		// right Max.
+		card = props.Cardinality{Min: 0, Max: left.Max}
 		if left.Min > right.Max {
 			card.Min = left.Min - right.Max
 		}
diff --git a/pkg/sql/opt/memo/testdata/logprops/set b/pkg/sql/opt/memo/testdata/logprops/set
index 948970af0e93..9ae1125d2cb2 100644
--- a/pkg/sql/opt/memo/testdata/logprops/set
+++ b/pkg/sql/opt/memo/testdata/logprops/set
@@ -585,3 +585,57 @@ except-all
            └── eq [type=bool, outer=(6,7), constraints=(/6: (/NULL - ]; /7: (/NULL - ]), fd=(6)==(7), (7)==(6)]
                 ├── variable: a:6 [type=int]
                 └── variable: b:7 [type=int]
+
+# Regression test for #89101. EXCEPT can have cardinality 0 even if left side
+# has more rows than the right.
+norm
+VALUES (1), (1) EXCEPT VALUES (1)
+----
+except
+ ├── columns: column1:1(int!null)
+ ├── left columns: column1:1(int!null)
+ ├── right columns: column1:2(int)
+ ├── cardinality: [0 - 2]
+ ├── key: (1)
+ ├── values
+ │    ├── columns: column1:1(int!null)
+ │    ├── cardinality: [2 - 2]
+ │    ├── prune: (1)
+ │    ├── tuple [type=tuple{int}]
+ │    │    └── const: 1 [type=int]
+ │    └── tuple [type=tuple{int}]
+ │         └── const: 1 [type=int]
+ └── values
+      ├── columns: column1:2(int!null)
+      ├── cardinality: [1 - 1]
+      ├── key: ()
+      ├── fd: ()-->(2)
+      ├── prune: (2)
+      └── tuple [type=tuple{int}]
+           └── const: 1 [type=int]
+
+# EXCEPT ALL cannot have cardinality lower than min left - max right.
+norm
+VALUES (1), (1) EXCEPT ALL VALUES (1)
+----
+except-all
+ ├── columns: column1:1(int!null)
+ ├── left columns: column1:1(int!null)
+ ├── right columns: column1:2(int)
+ ├── cardinality: [1 - 2]
+ ├── values
+ │    ├── columns: column1:1(int!null)
+ │    ├── cardinality: [2 - 2]
+ │    ├── prune: (1)
+ │    ├── tuple [type=tuple{int}]
+ │    │    └── const: 1 [type=int]
+ │    └── tuple [type=tuple{int}]
+ │         └── const: 1 [type=int]
+ └── values
+      ├── columns: column1:2(int!null)
+      ├── cardinality: [1 - 1]
+      ├── key: ()
+      ├── fd: ()-->(2)
+      ├── prune: (2)
+      └── tuple [type=tuple{int}]
+           └── const: 1 [type=int]