Optimize DISTINCT over singleton collections

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

@@ -269,6 +269,11 @@ public void ApplyDistinct()
             throw new InvalidOperationException(RelationalStrings.DistinctOnCollectionNotSupported);
         }
 
+        if (Limit is SqlConstantExpression { Value: 1 })
+        {
+            return;
+        }
+
         if (Limit != null
             || Offset != null)
         {
@@ -1915,6 +1920,11 @@ public void ApplyLimit(SqlExpression sqlExpression)
         }
 
         Limit = sqlExpression;
+
+        if (Offset is null && Limit is SqlConstantExpression { Value: 1 })
+        {
+            IsDistinct = false;
+        }
     }
 
     /// <summary>

test/EFCore.SqlServer.FunctionalTests/Query/ComplexNavigationsQuerySqlServer160Test.cs

@@ -69,7 +69,7 @@ where l1.Id < 3
 SELECT (
     SELECT TOP(1) [l1].[Name]
     FROM (
-        SELECT DISTINCT TOP(1) [l0].[Id], [l0].[Level2_Optional_Id], [l0].[Level2_Required_Id], [l0].[Name], [l0].[OneToMany_Optional_Inverse3Id], [l0].[OneToMany_Optional_Self_Inverse3Id], [l0].[OneToMany_Required_Inverse3Id], [l0].[OneToMany_Required_Self_Inverse3Id], [l0].[OneToOne_Optional_PK_Inverse3Id], [l0].[OneToOne_Optional_Self3Id]
+        SELECT TOP(1) [l0].[Id], [l0].[Name]
         FROM [LevelThree] AS [l0]
     ) AS [l1]
     ORDER BY [l1].[Id])

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

@@ -69,7 +69,7 @@ where l1.Id < 3
 SELECT (
     SELECT TOP(1) [l1].[Name]
     FROM (
-        SELECT DISTINCT TOP(1) [l0].[Id], [l0].[Level2_Optional_Id], [l0].[Level2_Required_Id], [l0].[Name], [l0].[OneToMany_Optional_Inverse3Id], [l0].[OneToMany_Optional_Self_Inverse3Id], [l0].[OneToMany_Required_Inverse3Id], [l0].[OneToMany_Required_Self_Inverse3Id], [l0].[OneToOne_Optional_PK_Inverse3Id], [l0].[OneToOne_Optional_Self3Id]
+        SELECT TOP(1) [l0].[Id], [l0].[Name]
         FROM [LevelThree] AS [l0]
     ) AS [l1]
     ORDER BY [l1].[Id])

test/EFCore.SqlServer.FunctionalTests/Query/ComplexNavigationsSharedTypeQuerySqlServer160Test.cs

@@ -87,7 +87,7 @@ where l1.Id < 3
 SELECT (
     SELECT TOP(1) [s].[Level3_Name]
     FROM (
-        SELECT DISTINCT TOP(1) [l4].[Id], [l4].[Level2_Optional_Id], [l4].[Level2_Required_Id], [l4].[Level3_Name], [l4].[OneToMany_Optional_Inverse3Id], [l4].[OneToMany_Required_Inverse3Id], [l4].[OneToOne_Optional_PK_Inverse3Id]
+        SELECT TOP(1) [l4].[Id], [l4].[Level2_Required_Id], [l4].[Level3_Name], [l4].[OneToMany_Required_Inverse3Id]
         FROM [Level1] AS [l0]
         LEFT JOIN (
             SELECT [l1].[Id], [l1].[OneToOne_Required_PK_Date], [l1].[Level1_Required_Id], [l1].[OneToMany_Required_Inverse2Id]
@@ -97,7 +97,7 @@ WHERE [l1].[OneToOne_Required_PK_Date] IS NOT NULL AND [l1].[Level1_Required_Id]
             WHEN [l2].[OneToOne_Required_PK_Date] IS NOT NULL AND [l2].[Level1_Required_Id] IS NOT NULL AND [l2].[OneToMany_Required_Inverse2Id] IS NOT NULL THEN [l2].[Id]
         END
         LEFT JOIN (
-            SELECT [l3].[Id], [l3].[Level2_Optional_Id], [l3].[Level2_Required_Id], [l3].[Level3_Name], [l3].[OneToMany_Optional_Inverse3Id], [l3].[OneToMany_Required_Inverse3Id], [l3].[OneToOne_Optional_PK_Inverse3Id]
+            SELECT [l3].[Id], [l3].[Level2_Required_Id], [l3].[Level3_Name], [l3].[OneToMany_Required_Inverse3Id]
             FROM [Level1] AS [l3]
             WHERE [l3].[Level2_Required_Id] IS NOT NULL AND [l3].[OneToMany_Required_Inverse3Id] IS NOT NULL
         ) AS [l4] ON CASE

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

@@ -89,7 +89,7 @@ where l1.Id < 3
 SELECT (
     SELECT TOP(1) [s].[Level3_Name]
     FROM (
-        SELECT DISTINCT TOP(1) [l4].[Id], [l4].[Level2_Optional_Id], [l4].[Level2_Required_Id], [l4].[Level3_Name], [l4].[OneToMany_Optional_Inverse3Id], [l4].[OneToMany_Required_Inverse3Id], [l4].[OneToOne_Optional_PK_Inverse3Id]
+        SELECT TOP(1) [l4].[Id], [l4].[Level2_Required_Id], [l4].[Level3_Name], [l4].[OneToMany_Required_Inverse3Id]
         FROM [Level1] AS [l0]
         LEFT JOIN (
             SELECT [l1].[Id], [l1].[OneToOne_Required_PK_Date], [l1].[Level1_Required_Id], [l1].[OneToMany_Required_Inverse2Id]
@@ -99,7 +99,7 @@ WHERE [l1].[OneToOne_Required_PK_Date] IS NOT NULL AND [l1].[Level1_Required_Id]
             WHEN [l2].[OneToOne_Required_PK_Date] IS NOT NULL AND [l2].[Level1_Required_Id] IS NOT NULL AND [l2].[OneToMany_Required_Inverse2Id] IS NOT NULL THEN [l2].[Id]
         END
         LEFT JOIN (
-            SELECT [l3].[Id], [l3].[Level2_Optional_Id], [l3].[Level2_Required_Id], [l3].[Level3_Name], [l3].[OneToMany_Optional_Inverse3Id], [l3].[OneToMany_Required_Inverse3Id], [l3].[OneToOne_Optional_PK_Inverse3Id]
+            SELECT [l3].[Id], [l3].[Level2_Required_Id], [l3].[Level3_Name], [l3].[OneToMany_Required_Inverse3Id]
             FROM [Level1] AS [l3]
             WHERE [l3].[Level2_Required_Id] IS NOT NULL AND [l3].[OneToMany_Required_Inverse3Id] IS NOT NULL
         ) AS [l4] ON CASE

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

@@ -1562,7 +1562,7 @@ public override async Task Where_subquery_distinct_singleordefault_boolean2(bool
 SELECT [g].[Nickname], [g].[SquadId], [g].[AssignedCityName], [g].[CityOfBirthName], [g].[Discriminator], [g].[FullName], [g].[HasSoulPatch], [g].[LeaderNickname], [g].[LeaderSquadId], [g].[Rank]
 FROM [Gears] AS [g]
 WHERE [g].[HasSoulPatch] = CAST(1 AS bit) AND COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] AS [w]
     WHERE [g].[FullName] = [w].[OwnerFullName] AND [w].[Name] LIKE N'%Lancer%'), CAST(0 AS bit)) = CAST(1 AS bit)
 ORDER BY [g].[Nickname]
@@ -5838,7 +5838,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean2(boo
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] AS [w]
     WHERE [g].[FullName] = [w].[OwnerFullName] AND [w].[Name] LIKE N'%Lancer%'), CAST(0 AS bit))
 FROM [Gears] AS [g]
@@ -5889,7 +5889,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean_empt
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] AS [w]
     WHERE [g].[FullName] = [w].[OwnerFullName] AND [w].[Name] = N'BFG'), CAST(0 AS bit))
 FROM [Gears] AS [g]

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

@@ -795,7 +795,7 @@ public override async Task Project_single_element_from_collection_with_OrderBy_D
         AssertSql(
             """
 SELECT (
-    SELECT DISTINCT TOP(1) [o].[CustomerID]
+    SELECT TOP(1) [o].[CustomerID]
     FROM [Orders] AS [o]
     WHERE [c].[CustomerID] = [o].[CustomerID])
 FROM [Customers] AS [c]

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

@@ -2230,7 +2230,7 @@ UNION ALL
     FROM [Officers] AS [o]
 ) AS [u]
 WHERE [u].[HasSoulPatch] = CAST(1 AS bit) AND COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] AS [w]
     WHERE [u].[FullName] = [w].[OwnerFullName] AND [w].[Name] LIKE N'%Lancer%'), CAST(0 AS bit)) = CAST(1 AS bit)
 ORDER BY [u].[Nickname]
@@ -7957,7 +7957,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean2(boo
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] AS [w]
     WHERE [u].[FullName] = [w].[OwnerFullName] AND [w].[Name] LIKE N'%Lancer%'), CAST(0 AS bit))
 FROM (
@@ -8026,7 +8026,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean_empt
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] AS [w]
     WHERE [u].[FullName] = [w].[OwnerFullName] AND [w].[Name] = N'BFG'), CAST(0 AS bit))
 FROM (

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

@@ -1859,7 +1859,7 @@ END AS [Discriminator]
 FROM [Gears] AS [g]
 LEFT JOIN [Officers] AS [o] ON [g].[Nickname] = [o].[Nickname] AND [g].[SquadId] = [o].[SquadId]
 WHERE [g].[HasSoulPatch] = CAST(1 AS bit) AND COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] AS [w]
     WHERE [g].[FullName] = [w].[OwnerFullName] AND [w].[Name] LIKE N'%Lancer%'), CAST(0 AS bit)) = CAST(1 AS bit)
 ORDER BY [g].[Nickname]
@@ -6678,7 +6678,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean2(boo
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] AS [w]
     WHERE [g].[FullName] = [w].[OwnerFullName] AND [w].[Name] LIKE N'%Lancer%'), CAST(0 AS bit))
 FROM [Gears] AS [g]
@@ -6729,7 +6729,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean_empt
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] AS [w]
     WHERE [g].[FullName] = [w].[OwnerFullName] AND [w].[Name] = N'BFG'), CAST(0 AS bit))
 FROM [Gears] AS [g]

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

@@ -1536,7 +1536,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean2(boo
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] FOR SYSTEM_TIME AS OF '2010-01-01T00:00:00.0000000' AS [w]
     WHERE [g].[FullName] = [w].[OwnerFullName] AND [w].[Name] LIKE N'%Lancer%'), CAST(0 AS bit))
 FROM [Gears] FOR SYSTEM_TIME AS OF '2010-01-01T00:00:00.0000000' AS [g]
@@ -2395,7 +2395,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean_empt
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] FOR SYSTEM_TIME AS OF '2010-01-01T00:00:00.0000000' AS [w]
     WHERE [g].[FullName] = [w].[OwnerFullName] AND [w].[Name] = N'BFG'), CAST(0 AS bit))
 FROM [Gears] FOR SYSTEM_TIME AS OF '2010-01-01T00:00:00.0000000' AS [g]
@@ -6607,7 +6607,7 @@ public override async Task Where_subquery_distinct_singleordefault_boolean2(bool
 SELECT [g].[Nickname], [g].[SquadId], [g].[AssignedCityName], [g].[CityOfBirthName], [g].[Discriminator], [g].[FullName], [g].[HasSoulPatch], [g].[LeaderNickname], [g].[LeaderSquadId], [g].[PeriodEnd], [g].[PeriodStart], [g].[Rank]
 FROM [Gears] FOR SYSTEM_TIME AS OF '2010-01-01T00:00:00.0000000' AS [g]
 WHERE [g].[HasSoulPatch] = CAST(1 AS bit) AND COALESCE((
-    SELECT DISTINCT TOP(1) [w].[IsAutomatic]
+    SELECT TOP(1) [w].[IsAutomatic]
     FROM [Weapons] FOR SYSTEM_TIME AS OF '2010-01-01T00:00:00.0000000' AS [w]
     WHERE [g].[FullName] = [w].[OwnerFullName] AND [w].[Name] LIKE N'%Lancer%'), CAST(0 AS bit)) = CAST(1 AS bit)
 ORDER BY [g].[Nickname]

test/EFCore.Sqlite.FunctionalTests/Query/GearsOfWarQuerySqliteTest.cs

@@ -2335,7 +2335,7 @@ public override async Task Where_subquery_distinct_singleordefault_boolean2(bool
 SELECT "g"."Nickname", "g"."SquadId", "g"."AssignedCityName", "g"."CityOfBirthName", "g"."Discriminator", "g"."FullName", "g"."HasSoulPatch", "g"."LeaderNickname", "g"."LeaderSquadId", "g"."Rank"
 FROM "Gears" AS "g"
 WHERE "g"."HasSoulPatch" AND COALESCE((
-    SELECT DISTINCT "w"."IsAutomatic"
+    SELECT "w"."IsAutomatic"
     FROM "Weapons" AS "w"
     WHERE "g"."FullName" = "w"."OwnerFullName" AND instr("w"."Name", 'Lancer') > 0
     LIMIT 1), 0)
@@ -4852,7 +4852,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean2(boo
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT "w"."IsAutomatic"
+    SELECT "w"."IsAutomatic"
     FROM "Weapons" AS "w"
     WHERE "g"."FullName" = "w"."OwnerFullName" AND instr("w"."Name", 'Lancer') > 0
     LIMIT 1), 0)
@@ -7320,7 +7320,7 @@ public override async Task Select_subquery_distinct_singleordefault_boolean_empt
         AssertSql(
             """
 SELECT COALESCE((
-    SELECT DISTINCT "w"."IsAutomatic"
+    SELECT "w"."IsAutomatic"
     FROM "Weapons" AS "w"
     WHERE "g"."FullName" = "w"."OwnerFullName" AND "w"."Name" = 'BFG'
     LIMIT 1), 0)