patternsqlMajor

In SQL Server, can I guarantee an order without an explicit ORDER BY clause when an index seek is forced on a table with only a clustered index?

Submitted by: @import:stackexchange-dba·Mar 10, 2026·

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clusteredcanorderguaranteewithoutseeksqlwithexplicitforced

Problem

Update 2014-12-18

With the overwhelming response to the main question being "No", the more interesting responses have focused on part 2, how to solve the performance puzzle with an explicit ORDER BY. Although I've marked an answer already, I wouldn't be surprised if there were an even better performing solution.

Original

This question arose because the only extremely fast solution I could find to a particular problem only works without an ORDER BY clause. Below is the full T-SQL needed to produce the problem, along with my proposed solution (I am using SQL Server 2008 R2, if that matters.)

```
--Create Orders table
IF OBJECT_ID('tempdb..#Orders') IS NOT NULL DROP TABLE #Orders
CREATE TABLE #Orders
(
OrderID INT NOT NULL IDENTITY(1,1)
, CustID INT NOT NULL
, StoreID INT NOT NULL
, Amount FLOAT NOT NULL
)
CREATE CLUSTERED INDEX IX ON #Orders (StoreID, Amount DESC, CustID)

--Add 1 million rows w/ 100K Customers each of whom had 10 orders
;WITH
Cte0 AS (SELECT 1 AS C UNION ALL SELECT 1), --2 rows
Cte1 AS (SELECT 1 AS C FROM Cte0 AS A, Cte0 AS B),--4 rows
Cte2 AS (SELECT 1 AS C FROM Cte1 AS A ,Cte1 AS B),--16 rows
Cte3 AS (SELECT 1 AS C FROM Cte2 AS A ,Cte2 AS B),--256 rows
Cte4 AS (SELECT 1 AS C FROM Cte3 AS A ,Cte3 AS B),--65536 rows
Cte5 AS (SELECT 1 AS C FROM Cte4 AS A ,Cte2 AS B),--1048576 rows
FinalCte AS (SELECT ROW_NUMBER() OVER (ORDER BY C) AS Number FROM Cte5)
INSERT INTO #Orders (CustID, StoreID, Amount)
SELECT CustID = Number / 10
, StoreID = Number % 4
, Amount = 1000 * RAND(Number)
FROM FinalCte
WHERE Number <= 1000000

SET STATISTICS IO ON
SET STATISTICS TIME ON

--For StoreID = 1, find the top 500 customers ordered by their most expensive purchase (Amount)

--Solution A: Without ORDER BY
DECLARE @Top INT = 500
SELECT DISTINCT TOP (@Top) CustID
FROM #Orders WITH(FORCESEEK)
WHERE StoreID = 1
OPTION(OPTIMIZE FOR (@Top = 1), FAST 1);
--9 logical

Solution

Am I right that it will guarantee the order in this case without an order by clause?

No. A Flow Distinct that preserves order (allowing ORDER BY without a sort) is not implemented in SQL Server today. It is possible to do in principle, but then many things are possible if we are allowed to change the SQL Server source code. If you can make a good case for this development work, you could suggest it to Microsoft.

If not, is there another method to force a plan that is as fast as Solution A, preferably one that avoids sorts?

Yes. (Table & query hints only required when using the pre-2014 cardinality estimator):

-- Additional index
CREATE UNIQUE NONCLUSTERED INDEX i 
ON #Orders (StoreID, CustID, Amount, OrderID);

-- Query
SELECT TOP (500) 
    O.CustID, 
    O.Amount
FROM #Orders AS O
    WITH (FORCESEEK(IX (StoreID)))
WHERE O.StoreID = 1
AND NOT EXISTS
(
    SELECT NULL
    FROM #Orders AS O2
        WITH (FORCESEEK(i (StoreID, CustID, Amount)))
    WHERE 
        O2.StoreID = O.StoreID
        AND O2.CustID = O.CustID
        AND O2.Amount >= O.Amount
        AND
        (
            O2.Amount > O.Amount
            OR
            (
                O2.Amount = O.Amount
                AND O2.OrderID > O.OrderID
            )
        )
)
ORDER BY
    O.Amount DESC
OPTION (MAXDOP 1);

(500 row(s) affected)

 SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 4 ms.

SQL CLR solution

The following script shows using a SQL CLR table-valued function to meet the stated requirements. I am not a C# expert, so the code may bear improvement:

```
USE Sandpit;
GO
-- Ensure SQLCLR is enabled
EXECUTE sys.sp_configure
@configname = 'clr enabled',
@configvalue = 1;
RECONFIGURE;
GO
-- Lazy, but effective to allow EXTERNAL_ACCESS
ALTER DATABASE Sandpit
SET TRUSTWORTHY ON;
GO
-- The CLR assembly
CREATE ASSEMBLY FlowDistinctOrder
AUTHORIZATION dbo
FROM 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

Code Snippets

-- Additional index
CREATE UNIQUE NONCLUSTERED INDEX i 
ON #Orders (StoreID, CustID, Amount, OrderID);

-- Query
SELECT TOP (500) 
    O.CustID, 
    O.Amount
FROM #Orders AS O
    WITH (FORCESEEK(IX (StoreID)))
WHERE O.StoreID = 1
AND NOT EXISTS
(
    SELECT NULL
    FROM #Orders AS O2
        WITH (FORCESEEK(i (StoreID, CustID, Amount)))
    WHERE 
        O2.StoreID = O.StoreID
        AND O2.CustID = O.CustID
        AND O2.Amount >= O.Amount
        AND
        (
            O2.Amount > O.Amount
            OR
            (
                O2.Amount = O.Amount
                AND O2.OrderID > O.OrderID
            )
        )
)
ORDER BY
    O.Amount DESC
OPTION (MAXDOP 1);

USE Sandpit;
GO
-- Ensure SQLCLR is enabled
EXECUTE sys.sp_configure
    @configname = 'clr enabled',
    @configvalue = 1;
RECONFIGURE;
GO
-- Lazy, but effective to allow EXTERNAL_ACCESS
ALTER DATABASE Sandpit
SET TRUSTWORTHY ON;
GO
-- The CLR assembly
CREATE ASSEMBLY FlowDistinctOrder
AUTHORIZATION dbo
FROM 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

-- Test table
CREATE TABLE dbo.Orders
(  
    OrderID    integer  NOT NULL IDENTITY(1,1),
    CustID     integer  NOT NULL,
    StoreID    integer  NOT NULL,
    Amount     float    NOT NULL
);
GO
-- Sample data
WITH  
    Cte0 AS (SELECT 1 AS C UNION ALL SELECT 1), --2 rows  
    Cte1 AS (SELECT 1 AS C FROM Cte0 AS A, Cte0 AS B),--4 rows  
    Cte2 AS (SELECT 1 AS C FROM Cte1 AS A ,Cte1 AS B),--16 rows 
    Cte3 AS (SELECT 1 AS C FROM Cte2 AS A ,Cte2 AS B),--256 rows 
    Cte4 AS (SELECT 1 AS C FROM Cte3 AS A ,Cte3 AS B),--65536 rows 
    Cte5 AS (SELECT 1 AS C FROM Cte4 AS A ,Cte2 AS B),--1048576 rows 
    FinalCte AS (SELECT  ROW_NUMBER() OVER (ORDER BY C) AS Number FROM   Cte5)
INSERT dbo.Orders 
    (CustID, StoreID, Amount)
SELECT 
    CustID  = Number / 10,
    StoreID = Number % 4,
    Amount  = 1000 * RAND(Number)
FROM FinalCte
WHERE 
    Number <= 1000000;
GO
-- Index
CREATE CLUSTERED INDEX IX 
ON dbo.Orders 
    (StoreID ASC, Amount DESC, CustID ASC);

-- Test the function
-- Run several times to ensure connection is cached
-- and CLR code fully compiled
DECLARE @Start datetime2 = SYSUTCDATETIME();

SELECT TOP (500) 
    FDO.CustID
FROM dbo.FlowDistinctOrder
(
    @@SERVERNAME,   -- For external connection
    DB_NAME(),      -- For external connection
    500             -- Number of rows to return
) AS FDO 
ORDER BY 
    FDO.Amount DESC;

SELECT DATEDIFF(MILLISECOND, @Start, SYSUTCDATETIME());

Context

StackExchange Database Administrators Q#86596, answer score: 24

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