在 SQL Server 中，当对只有聚集索引的表强制执行索引查找时，我能否保证没有显式 ORDER BY 子句的顺序？
更新 2014-12-18
由于对主要问题的压倒性回答是“否”，更有趣的回答集中在第 2 部分，如何使用明确的ORDER BY. 尽管我已经标记了一个答案，但如果有性能更好的解决方案，我也不会感到惊讶。
原来的
出现这个问题是因为我能找到的针对特定问题的唯一极快的解决方案只能在没有ORDER BY子句的情况下起作用。下面是产生问题所需的完整 T-SQL，以及我提出的解决方案（我使用的是 SQL Server 2008 R2，如果这很重要。）
--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
下面分别是解决方案 A 和 B 的执行计划：
解决方案 A 提供了我需要的性能，但是在添加任何类型的 ORDER BY 子句时，我无法让它以相同的性能工作（例如，参见解决方案 B）。而且似乎解决方案 A 必须按顺序交付其结果，因为 1) 表上只有一个索引，2) 强制执行查找，从而消除了使用基于 IAM 页面的分配顺序扫描的可能性.
所以我的问题是：
在没有 order by 条款的情况下，它会保证订单，我是对的吗？
如果不是，是否有另一种方法可以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，它必须解决完全相同的问题（对于
StoreID = 1，查找按最昂贵的购买金额订购的前 500 位客户）。它还必须仍然使用该#Orders表，但不同的索引方案也可以。
Pau*_*ite 24
- 在没有 order by 条款的情况下，它会保证订单，我是对的吗？
第一个流鲜明，保留顺序（允许ORDER BY无排序）没有在SQL Server今天实现。原则上是可以做到的，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。如果你能为这项开发工作提出一个很好的案例，你可以向 Microsoft 推荐它。
- 如果不是，是否有另一种方法可以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？
是的。（仅在使用 2014 年之前的基数估计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);
-- 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);
SQL CLR 解决方案
以下脚本显示使用 SQL CLR 表值函数来满足规定的要求。我不是 C# 专家，所以代码可能需要改进：
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;
来自问题的测试表和样本数据：
-- 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（注意验证 ORDER保证）：
在我的笔记本电脑上，这通常在 80-100 毫秒内执行。这远不及上面的 T-SQL 重写快，但它在面对不同的数据分布时应该表现出良好的性能稳定性。
源代码：
(500 row(s) affected)
SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 4 ms.
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，将来的版本也不会有问题。
这应该有效：
在循环中从表中提取 500 行的批次，并在您有 500 个不同的客户 ID 时停止。获取查询可能如下所示：
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。每个查询只会物理接触 500 条记录。这意味着它是 O(1)。您进入索引越远，它不会变得越贵。
您必须维护该变量@lastAmountFetched以减少到您在该语句中获取的最小值。
这样，您将以有序的方式增量扫描索引。您最多会读取 (500 - 1) 行比最佳数量多。
这将比总是为特定商店汇总 100000 个左右的订单要快得多。可能只需要几次迭代，每次迭代 500 行。
本质上，这是一个手动编码的流不同运算符。
或者，使用游标获取尽可能少的行。这会慢很多，因为执行 500 个单行查询通常比执行一批 500 行要慢。
或者，只需不DISTINCT以有序方式查询所有行，并让客户端应用程序在返回足够多的行后终止查询（使用SqlCommand.Cancel）。
- @JeroenMostert - `IGNORE_DUP_KEY` 可以做到这一点。 (2认同)