Stop pushing down to subquery for non-Concat set operations without l…

…imit/offset

Closes #30684

src/EFCore.Relational/Query/SqlExpressions/SelectExpression.cs

@@ -2280,25 +2280,26 @@ private void ApplySetOperation(SetOperationType setOperationType, SelectExpressi
         var entityProjectionValueComparers = new List<ValueComparer>();
         var otherExpressions = new List<SqlExpression>();
 
-        if (select1.Orderings.Count != 0
-            || select1.Limit != null
-            || select1.Offset != null)
+        // Push down into a subquery if limit/offset are defined. If not, any orderings can be discarded as set operations don't preserve
+        // them.
+        // Note that in some databases it may be possible to preserve the internal ordering of the set operands for Concat, but we don't
+        // currently support that.
+        if (select1.Limit != null || select1.Offset != null)
         {
             // If we are pushing down here, we need to make sure to assign unique alias to subquery also.
             var subqueryAlias = GenerateUniqueAlias(_usedAliases, "t");
             select1.PushdownIntoSubquery();
             select1._tables[0].Alias = subqueryAlias;
             select1._tableReferences[0].Alias = subqueryAlias;
-            select1.ClearOrdering();
         }
+        select1.ClearOrdering();
 
-        if (select2.Orderings.Count != 0
-            || select2.Limit != null
-            || select2.Offset != null)
+        // Do the same for the other side of the set operation
+        if (select2.Limit != null || select2.Offset != null)
         {
             select2.PushdownIntoSubquery();
-            select2.ClearOrdering();
         }
+        select2.ClearOrdering();
 
         if (_clientProjections.Count > 0
             || select2._clientProjections.Count > 0)

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

@@ -761,6 +761,38 @@ public virtual Task Union_over_scalarsubquery_scalarsubquery(bool async)
             ss => ss.Set<Order>().Select(o => o.OrderDetails.Count())
                 .Union(ss.Set<Order>().Select(o => o.OrderDetails.Count())));
 
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual Task Union_over_OrderBy_Take1(bool async)
+        => AssertQueryScalar(
+            async,
+            ss => ss.Set<Order>().OrderBy(o => o.OrderID).Take(5).Select(o => o.OrderDate)
+                .Union(ss.Set<Order>().Select(o => o.OrderDate)));
+
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual Task Union_over_OrderBy_without_Skip_Take1(bool async)
+        => AssertQueryScalar(
+            async,
+            ss => ss.Set<Order>().OrderBy(o => o.OrderID).Select(o => o.OrderDate)
+                .Union(ss.Set<Order>().Select(o => o.OrderDate)));
+
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual Task Union_over_OrderBy_Take2(bool async)
+        => AssertQueryScalar(
+            async,
+            ss => ss.Set<Order>().Select(o => o.OrderDate)
+                .Union(ss.Set<Order>().OrderBy(o => o.OrderID).Take(5).Select(o => o.OrderDate)));
+
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual Task Union_over_OrderBy_without_Skip_Take2(bool async)
+        => AssertQueryScalar(
+            async,
+            ss => ss.Set<Order>().Select(o => o.OrderDate)
+                .Union(ss.Set<Order>().OrderBy(o => o.OrderID).Select(o => o.OrderDate)));
+
     [ConditionalTheory]
     [MemberData(nameof(IsAsyncData))]
     public virtual Task OrderBy_Take_Union(bool async)

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

@@ -1020,6 +1020,74 @@ SELECT COUNT(*)
 """);
     }
 
+    public override async Task Union_over_OrderBy_Take1(bool async)
+    {
+        await base.Union_over_OrderBy_Take1(async);
+
+        AssertSql(
+"""
+@__p_0='5'
+
+SELECT [t].[OrderDate]
+FROM (
+    SELECT TOP(@__p_0) [o].[OrderDate], [o].[OrderID]
+    FROM [Orders] AS [o]
+    ORDER BY [o].[OrderID]
+) AS [t]
+UNION
+SELECT [o0].[OrderDate]
+FROM [Orders] AS [o0]
+""");
+    }
+
+    public override async Task Union_over_OrderBy_without_Skip_Take1(bool async)
+    {
+        await base.Union_over_OrderBy_without_Skip_Take1(async);
+
+        AssertSql(
+"""
+SELECT [o].[OrderDate]
+FROM [Orders] AS [o]
+UNION
+SELECT [o0].[OrderDate]
+FROM [Orders] AS [o0]
+""");
+    }
+
+    public override async Task Union_over_OrderBy_Take2(bool async)
+    {
+        await base.Union_over_OrderBy_Take2(async);
+
+        AssertSql(
+"""
+@__p_0='5'
+
+SELECT [o].[OrderDate]
+FROM [Orders] AS [o]
+UNION
+SELECT [t0].[OrderDate]
+FROM (
+    SELECT TOP(@__p_0) [o0].[OrderDate], [o0].[OrderID]
+    FROM [Orders] AS [o0]
+    ORDER BY [o0].[OrderID]
+) AS [t0]
+""");
+    }
+
+    public override async Task Union_over_OrderBy_without_Skip_Take2(bool async)
+    {
+        await base.Union_over_OrderBy_without_Skip_Take2(async);
+
+        AssertSql(
+"""
+SELECT [o].[OrderDate]
+FROM [Orders] AS [o]
+UNION
+SELECT [o0].[OrderDate]
+FROM [Orders] AS [o0]
+""");
+    }
+
     public override async Task OrderBy_Take_Union(bool async)
     {
         await base.OrderBy_Take_Union(async);