Additional set operation parentheses fix (#36124)

Follow-up for 246b2d2

Author: roji

src/EFCore.Relational/Query/QuerySqlGenerator.cs

@@ -1368,10 +1368,13 @@ static string GetSetOperation(SetOperationBase operation)
     protected virtual void GenerateSetOperationOperand(SetOperationBase setOperation, SelectExpression operand)
     {
         // INTERSECT has higher precedence over UNION and EXCEPT, but otherwise evaluation is left-to-right.
-        // To preserve evaluation order, add parentheses whenever a set operation is nested within a different set operation.
+        // To preserve evaluation order, add parentheses whenever a set operation is nested within a different set operation
+        // - including different distinctness.
         // In addition, EXCEPT is non-commutative (unlike UNION/INTERSECT), so add parentheses for that case too (see #36105).
         if (TryUnwrapBareSetOperation(operand, out var nestedSetOperation)
-            && (nestedSetOperation is ExceptExpression || nestedSetOperation.GetType() != setOperation.GetType()))
+            && (nestedSetOperation is ExceptExpression
+                || nestedSetOperation.GetType() != setOperation.GetType()
+                || nestedSetOperation.IsDistinct != setOperation.IsDistinct))
         {
             _relationalCommandBuilder.AppendLine("(");
 

src/EFCore.Sqlite.Core/Query/Internal/SqliteQuerySqlGenerator.cs

@@ -104,10 +104,13 @@ protected override void GenerateSetOperationOperand(SetOperationBase setOperatio
         // The following is a copy-paste of the base implementation from QuerySqlGenerator, adding the SELECT.
 
         // INTERSECT has higher precedence over UNION and EXCEPT, but otherwise evaluation is left-to-right.
-        // To preserve evaluation order, add parentheses whenever a set operation is nested within a different set operation.
+        // To preserve evaluation order, add parentheses whenever a set operation is nested within a different set operation
+        // - including different distinctness.
         // In addition, EXCEPT is non-commutative (unlike UNION/INTERSECT), so add parentheses for that case too (see #36105).
         if (TryUnwrapBareSetOperation(operand, out var nestedSetOperation)
-            && (nestedSetOperation is ExceptExpression || nestedSetOperation.GetType() != setOperation.GetType()))
+            && (nestedSetOperation is ExceptExpression
+                || nestedSetOperation.GetType() != setOperation.GetType()
+                || nestedSetOperation.IsDistinct != setOperation.IsDistinct))
         {
             Sql.AppendLine("SELECT * FROM (");
 

test/EFCore.Specification.Tests/Query/NorthwindSetOperationsQueryTestBase.cs

@@ -231,6 +231,17 @@ public virtual Task Union_Intersect(bool async)
                 .Union(ss.Set<Customer>().Where(c => c.City == "London"))
                 .Intersect(ss.Set<Customer>().Where(c => c.ContactName.Contains("Thomas"))));
 
+    // The evaluation order of Concat and Union can matter: A UNION ALL (B UNION C) can be different from (A UNION ALL B) UNION C.
+    // Make sure parentheses are added.
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual Task Union_inside_Concat(bool async)
+        => AssertQuery(
+            async,
+            ss => ss.Set<Customer>().Where(c => c.City == "Berlin")
+                .Concat(ss.Set<Customer>().Where(c => c.City == "London")
+                    .Union(ss.Set<Customer>().Where(c => c.City == "Berlin"))));
+
     [ConditionalTheory]
     [MemberData(nameof(IsAsyncData))]
     public virtual Task Union_Take_Union_Take(bool async)

test/EFCore.SqlServer.FunctionalTests/Query/NorthwindSetOperationsQuerySqlServerTest.cs

@@ -200,6 +200,28 @@ WHERE [c1].[ContactName] LIKE N'%Thomas%'
 """);
     }
 
+    public override async Task Union_inside_Concat(bool async)
+    {
+        await base.Union_inside_Concat(async);
+
+AssertSql(
+"""
+SELECT [c].[CustomerID], [c].[Address], [c].[City], [c].[CompanyName], [c].[ContactName], [c].[ContactTitle], [c].[Country], [c].[Fax], [c].[Phone], [c].[PostalCode], [c].[Region]
+FROM [Customers] AS [c]
+WHERE [c].[City] = N'Berlin'
+UNION ALL
+(
+    SELECT [c0].[CustomerID], [c0].[Address], [c0].[City], [c0].[CompanyName], [c0].[ContactName], [c0].[ContactTitle], [c0].[Country], [c0].[Fax], [c0].[Phone], [c0].[PostalCode], [c0].[Region]
+    FROM [Customers] AS [c0]
+    WHERE [c0].[City] = N'London'
+    UNION
+    SELECT [c1].[CustomerID], [c1].[Address], [c1].[City], [c1].[CompanyName], [c1].[ContactName], [c1].[ContactTitle], [c1].[Country], [c1].[Fax], [c1].[Phone], [c1].[PostalCode], [c1].[Region]
+    FROM [Customers] AS [c1]
+    WHERE [c1].[City] = N'Berlin'
+)
+""");
+    }
+
     public override async Task Union_Take_Union_Take(bool async)
     {
         await base.Union_Take_Union_Take(async);