Revert "opt: disallow mutations under union"

This reverts commit a34d705.

Release justification: recently merge fix no longer needed (thanks
to #41307).

Release note (sql change): Mutations under UNION or UNION ALL are
allowed again.

pkg/sql/logictest/testdata/logic_test/union

@@ -286,28 +286,3 @@ SELECT a FROM (SELECT a FROM c UNION ALL SELECT a FROM a) WHERE a > 0 AND a < 3
 ----
 1
 1
-
-# Ensure that mutations under a UNION or UNION ALL error out (#40853).
-statement error mutations not supported under UNION
-SELECT * FROM uniontest
-UNION SELECT * FROM [INSERT INTO uniontest VALUES (1), (1) RETURNING k, v]
-
-statement error mutations not supported under UNION
-SELECT * FROM [INSERT INTO uniontest VALUES (1), (1) RETURNING k, v]
-UNION ALL SELECT * FROM uniontest
-
-statement error mutations not supported under UNION
-SELECT * FROM uniontest
-UNION SELECT * FROM (
-  WITH cte AS (INSERT INTO uniontest VALUES (1), (1) RETURNING k, v) SELECT * FROM cte
-)
-
-# The right way to run such a query is to use WITH above the union.
-statement ok
-WITH cte AS (INSERT INTO uniontest VALUES (1), (1) RETURNING k, v)
-(SELECT * FROM cte UNION SELECT * FROM uniontest)
-
-# Other set ops are allowed.
-statement ok
-SELECT * FROM uniontest
-EXCEPT SELECT * FROM [INSERT INTO uniontest VALUES (1), (1) RETURNING k, v]

pkg/sql/logictest/testdata/logic_test/upsert

@@ -374,13 +374,12 @@ upsert INTO upsert_returning VALUES (1, 5, 4, 3), (6, 5, 4, 3) RETURNING *
 statement ok
 COMMIT
 
-# TODO(justin): reenable when we fix #35060 / #40853.
-## For #22300. Test UPSERT ... RETURNING with UNION.
-#query I rowsort
-#SELECT a FROM [UPSERT INTO upsert_returning VALUES (7) RETURNING a] UNION VALUES (8)
-#----
-#7
-#8
+# For #22300. Test UPSERT ... RETURNING with UNION.
+query I rowsort
+SELECT a FROM [UPSERT INTO upsert_returning VALUES (7) RETURNING a] UNION VALUES (8)
+----
+7
+8
 
 # For #6710. Add an unused column to disable the fast path which doesn't have this bug.
 statement ok

pkg/sql/opt/exec/execbuilder/relational.go

@@ -1110,13 +1110,6 @@ func (b *Builder) buildSetOp(set memo.RelExpr) (execPlan, error) {
 	default:
 		panic(errors.AssertionFailedf("invalid operator %s", log.Safe(set.Op())))
 	}
-	// TODO(justin): We cannot execute mutations under a UNION or UNION ALL
-	// (because mutations can't run in parallel). Once we pull up all mutations
-	// into top-level With expressions, this case will not be possible anymore.
-	if typ == tree.UnionOp && (leftExpr.Relational().CanMutate || rightExpr.Relational().CanMutate) {
-		err := unimplemented.NewWithIssuef(40853, "mutations not supported under UNION; use WITH instead")
-		return execPlan{}, err
-	}
 
 	node, err := b.factory.ConstructSetOp(typ, all, left.root, right.root)
 	if err != nil {