Fix to #29667 - Count after Take throws "No column name was specified…

… for column 1 of 't'."

Problem was that in some cases (e.g. count over keyless entity that has been pushed down) we now generate empty projection in the subquery, where before we were projecting some columns.
SQL Server doesn't allow unaliased projection in the subquery, so the fix is to simply add an alias to the empty projection that we generate.

Fixes #29667

src/EFCore.Relational/Query/QuerySqlGenerator.cs

@@ -250,7 +250,7 @@ protected override Expression VisitSelect(SelectExpression selectExpression)
             }
             else
             {
-                _relationalCommandBuilder.Append("1");
+                GenerateEmptyProjection(selectExpression);
             }
 
             if (selectExpression.Tables.Any())
@@ -309,6 +309,15 @@ protected virtual void GeneratePseudoFromClause()
     {
     }
 
+    /// <summary>
+    ///     Generates empty projection for a SelectExpression.
+    /// </summary>
+    /// <param name="selectExpression">SelectExpression for which the empty projection will be generated.</param>
+    protected virtual void GenerateEmptyProjection(SelectExpression selectExpression)
+    {
+        _relationalCommandBuilder.Append("1");
+    }
+
     /// <inheritdoc />
     protected override Expression VisitProjection(ProjectionExpression projectionExpression)
     {

src/EFCore.SqlServer/Query/Internal/SqlServerQuerySqlGenerator.cs

@@ -16,6 +16,9 @@ namespace Microsoft.EntityFrameworkCore.SqlServer.Query.Internal;
 /// </summary>
 public class SqlServerQuerySqlGenerator : QuerySqlGenerator
 {
+    private static readonly bool UseOldBehavior29667
+        = AppContext.TryGetSwitch("Microsoft.EntityFrameworkCore.Issue29667", out var enabled29667) && enabled29667;
+
     private readonly IRelationalTypeMappingSource _typeMappingSource;
 
     /// <summary>
@@ -72,6 +75,21 @@ protected override Expression VisitDelete(DeleteExpression deleteExpression)
             RelationalStrings.ExecuteOperationWithUnsupportedOperatorInSqlGeneration(nameof(RelationalQueryableExtensions.ExecuteDelete)));
     }
 
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    protected override void GenerateEmptyProjection(SelectExpression selectExpression)
+    {
+        base.GenerateEmptyProjection(selectExpression);
+        if (!UseOldBehavior29667 && selectExpression.Alias != null)
+        {
+            Sql.Append(" AS empty");
+        }
+    }
+
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
     ///     the same compatibility standards as public APIs. It may be changed or removed without notice in

test/EFCore.Cosmos.FunctionalTests/Query/NorthwindKeylessEntitiesQueryCosmosTest.cs

@@ -171,6 +171,30 @@ SELECT c
 """);
     }
 
+    public override async Task Count_over_keyless_entity(bool async)
+    {
+        await base.Count_over_keyless_entity(async);
+
+        AssertSql(
+"""
+SELECT COUNT(1) AS c
+FROM root c
+WHERE (c["Discriminator"] = "Customer")
+""");
+    }
+
+    public override async Task Count_over_keyless_entity_with_pushdown(bool async)
+    {
+        // Cosmos client evaluation. Issue #17246.
+        await AssertTranslationFailed(() => base.Count_over_keyless_entity_with_pushdown(async));
+    }
+
+    public override async Task Count_over_keyless_entity_with_pushdown_empty_projection(bool async)
+    {
+        // Cosmos client evaluation. Issue #17246.
+        await AssertTranslationFailed(() => base.Count_over_keyless_entity_with_pushdown_empty_projection(async));
+    }
+
     private void AssertSql(params string[] expected)
         => Fixture.TestSqlLoggerFactory.AssertBaseline(expected);
 

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

@@ -167,4 +167,25 @@ public virtual Task Collection_correlated_with_keyless_entity_in_predicate_works
                 .Select(pv => new { pv.City, pv.ContactName })
                 .OrderBy(x => x.ContactName)
                 .Take(2));
+
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual Task Count_over_keyless_entity(bool async)
+        => AssertCount(
+            async,
+            ss => ss.Set<CustomerQuery>());
+
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual Task Count_over_keyless_entity_with_pushdown(bool async)
+        => AssertCount(
+            async,
+            ss => ss.Set<CustomerQuery>().OrderBy(x => x.ContactTitle).Take(10));
+
+    [ConditionalTheory]
+    [MemberData(nameof(IsAsyncData))]
+    public virtual Task Count_over_keyless_entity_with_pushdown_empty_projection(bool async)
+        => AssertCount(
+            async,
+            ss => ss.Set<CustomerQuery>().Take(10));
 }

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

@@ -232,6 +232,54 @@ public override async Task KeylessEntity_with_included_nav(bool async)
 """);
     }
 
+    public override async Task Count_over_keyless_entity(bool async)
+    {
+        await base.Count_over_keyless_entity(async);
+
+        AssertSql(
+"""
+SELECT COUNT(*)
+FROM (
+    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]
+) AS [m]
+""");
+    }
+
+    public override async Task Count_over_keyless_entity_with_pushdown(bool async)
+    {
+        await base.Count_over_keyless_entity_with_pushdown(async);
+
+        AssertSql(
+"""
+@__p_0='10'
+SELECT COUNT(*)
+FROM (
+    SELECT TOP(@__p_0) [m].[ContactTitle]
+    FROM (
+        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]
+    ) AS [m]
+    ORDER BY [m].[ContactTitle]
+) AS [t]
+""");
+    }
+
+    public override async Task Count_over_keyless_entity_with_pushdown_empty_projection(bool async)
+    {
+        await base.Count_over_keyless_entity_with_pushdown_empty_projection(async);
+
+        AssertSql(
+"""
+@__p_0='10'
+SELECT COUNT(*)
+FROM (
+    SELECT TOP(@__p_0) 1 AS empty
+    FROM (
+        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]
+    ) AS [m]
+) AS [t]
+""");
+    }
+
     private void AssertSql(params string[] expected)
         => Fixture.TestSqlLoggerFactory.AssertBaseline(expected);