在SQL Server中，当在仅具有聚集索引的表上强制执行索引查找时，是否可以保证没有显式ORDER BY子句的订单？

更新2014-12-18

对于主要问题“否”的压倒性回答，更有趣的回答集中在第2部分，即如何用显式解决性能难题ORDER BY。尽管我已经标记了答案，但是如果有一个性能更好的解决方案，我也不会感到惊讶。

原版的

之所以出现这个问题，是因为我可以找到的解决特定问题的唯一极其快速的解决方案，只有在没有ORDER BY子句的情况下才能起作用。以下是产生问题所需的完整T-SQL，以及我提出的解决方案（如果有问题，我正在使用SQL Server 2008 R2。）

--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 reads, CPU Time = 0 ms, elapsed time = 1 ms
GO
--Solution B: With ORDER BY
DECLARE @Top INT = 500
SELECT TOP (@Top) CustID
FROM #Orders
WHERE StoreID = 1
GROUP BY CustID
ORDER BY MAX(Amount) DESC
OPTION(MAXDOP 1)
--745 logical reads, CPU Time = 141 ms, elapsed time = 145 ms
--Uses Sort operator

GO

以下分别是解决方案A和B的执行计划：

解决方案A提供了我需要的性能，但是在添加任何种类的ORDER BY子句时（例如，参见解决方案B），我无法使其具有相同的性能。当然，解决方案A似乎必须按顺序传递其结果，因为1）该表仅具有一个索引，2）强制执行查找，因此消除了使用基于IAM页的分配顺序扫描的可能性。

所以我的问题是：

1. 我是否可以在没有order by子句的情况下保证顺序？

2. 如果不是，是否存在另一种方法来强制执行与解决方案A一样快的计划，最好是避免排序的方法？请注意，这将必须解决完全相同的问题（例如，针对StoreID = 1，按最贵的购买金额找到订购的前500名客户）。它还必须仍然使用该#Orders表，但是可以使用不同的索引方案。

1. 我是否可以在没有order by子句的情况下保证顺序？

否。今天，SQL Server尚未实现保留顺序（不允许排序）的Flow DistinctORDER BY。原则上可以这样做，但是如果允许我们更改SQL Server源代码，那么很多事情都是可能的。如果您可以为这项开发工作提供充分的理由，则可以向Microsoft建议。

2. 如果不是，是否存在另一种方法来强制执行与解决方案A一样快的计划，最好是避免排序的方法？

是。（仅在使用2014年之前的基数估算器时才需要表和查询提示）：

-- 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解决方案

以下脚本显示了如何使用SQL CLR表值函数来满足规定的要求。我不是C＃专家，因此代码可能需要改进：

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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WITH PERMISSION_SET = EXTERNAL_ACCESS;
GO
-- The CLR TVF with order guarantee
CREATE FUNCTION dbo.FlowDistinctOrder 
(
    @ServerName nvarchar(128), 
    @DatabaseName nvarchar(128), 
    @MaxRows bigint
)
RETURNS TABLE 
(
    CustID integer NULL, 
    Amount float NULL
)
ORDER (Amount DESC)
AS EXTERNAL NAME FlowDistinctOrder.UserDefinedFunctions.FlowDistinctOrder;

测试表和来自问题的样本数据：

-- 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);

功能测试：

-- 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());

执行计划（注意ORDER担保的确认）：

在我的笔记本电脑上，这通常会在80-100毫秒内执行。这远没有上面的T-SQL重写快，但面对不同的数据分布，它应该显示出良好的性能稳定性。

源代码：

using Microsoft.SqlServer.Server;
using System.Collections;
using System.Collections.Generic;
using System.Data.SqlClient;

public partial class UserDefinedFunctions
{
    private sealed class ReverseComparer<T> : IComparer<T>
    {
        private readonly IComparer<T> original;

        public ReverseComparer(IComparer<T> original)
        {
            this.original = original;
        }

        public int Compare(T left, T right)
        {
            return original.Compare(right, left);
        }
    }

    [SqlFunction
        (
        DataAccess = DataAccessKind.Read,
        SystemDataAccess = SystemDataAccessKind.None,
        FillRowMethodName = "FillRow",
        TableDefinition = "CustID integer NULL, Amount float NULL"
        )
    ]
    public static IEnumerable FlowDistinctOrder
        (
        [SqlFacet (MaxSize=128)]string ServerName, 
        [SqlFacet (MaxSize=128)]string DatabaseName,
        long MaxRows
        )
    {
        var list = new SortedDictionary<double, int>
            (new ReverseComparer<double>(Comparer<double>.Default));

        var csb = new SqlConnectionStringBuilder();
        csb.ConnectTimeout = 10;
        csb.DataSource = ServerName;
        csb.Enlist = false;
        csb.InitialCatalog = DatabaseName;
        csb.IntegratedSecurity = true;

        using (var conn = new SqlConnection(csb.ConnectionString))
        {
            conn.Open();
            using (var cmd = conn.CreateCommand())
            {
                cmd.CommandText =
                    @"
                    SELECT
                        O.CustID, 
                        O.Amount
                    FROM dbo.Orders AS O
                    WHERE 
                        O.StoreID = 1 
                    ORDER BY 
                        O.Amount DESC";

                int custid;
                double amount;

                using (var rdr = cmd.ExecuteReader())
                {
                    while (rdr.Read())
                    {
                        custid = rdr.GetInt32(0);
                        amount = rdr.GetDouble(1);

                        if (!list.ContainsKey(amount))
                        {
                            list.Add(amount, custid);
                            if (list.Count == MaxRows)
                            {
                                break;
                            }
                        }
                    }
                }
            }
        }
        return list;
    }

    public static void FillRow(object obj, out int CustID, out double Amount)
    {
        var v = (KeyValuePair<double, int>)obj;
        CustID = v.Value;
        Amount = v.Key;
    }
}

没有ORDER BY很多事情，可能会出错。您已经排除了我能想到的所有可能的问题，但这并不意味着没有问题，将来的版本中也不会存在任何问题。

这应该工作：

循环从表中提取500行的批次，并在获得500个不同的客户ID时停止。提取查询如下所示：

select TOP (500) Amount, CustID
into #fetchedOrders
from Orders
where StoreID = 1234 and Amount <= @lastAmountFetched
order by Amount DESC

这将对索引执行有序范围扫描。该Amount <= @lastAmountFetched谓词是有逐步拉更多的记录。每个查询只会实际触及500条记录。这意味着它是O（1）。您进入索引的距离越远，它的成本就越高。

您必须维护该变量@lastAmountFetched以减小到在该语句中获取的最小值。

这样，您将以有序方式逐步扫描索引。您最多可以阅读（500-1）行，而不是最佳行数。

这将比特定商店总共汇总100000个左右的订单要快得多。大概只需要进行500行的几次迭代。

本质上，这是一个手动编码的流区分运算符。

或者，使用光标来获取尽可能少的行。这会慢很多，因为执行500个单行查询通常比执行500行的查询慢。

另外，也可以简单地查询所有行，而无需DISTINCT按有序方式进行操作，并在返回足够的行后使客户端应用程序终止查询（使用SqlCommand.Cancel）。