Question

我遇到了一些有效排序表的麻烦（该表包含大量行，因此任何优化都会产生很大的不同）。

我目前拥有的正确结果（11）是下面的代码。

BEGIN TRANSACTION

    CREATE TABLE #tPallets
    (
      PalletNumber bigint,
      Placement nvarchar(4)
    )

    INSERT INTO #tPallets  VALUES(100000, 'B')   
    INSERT INTO #tPallets  VALUES(100001, 'M1')  
    INSERT INTO #tPallets  VALUES(100002, 'M2')  
    INSERT INTO #tPallets  VALUES(100003, 'M3')  
    INSERT INTO #tPallets  VALUES(100004, 'M4')  
    INSERT INTO #tPallets  VALUES(100005, 'M5')  
    INSERT INTO #tPallets  VALUES(100006, 'M6')  
    INSERT INTO #tPallets  VALUES(100007, 'M7')  
    INSERT INTO #tPallets  VALUES(100008, 'M8')  
    INSERT INTO #tPallets  VALUES(100009, 'M9')  
    INSERT INTO #tPallets  VALUES(100010, 'M10')  
    INSERT INTO #tPallets  VALUES(100011, 'M11')  

    SELECT 
        TOP 1 CASE 
            WHEN Placement LIKE 'M%' THEN CONVERT(int, SUBSTRING(Placement,2, len(Placement)-1)) 
            ELSE 0 
        END AS PALLET_PLACEMENT 
    FROM 
        #tPallets 
    ORDER BY 
        1 DESC

ROLLBACK TRANSACTION

所以我正在寻找一种方法来使选择更快，如果在这种特定情况下可能的话。

我不认为这是答案的重复。因为线程中没有答案会使得执行时间更快而不在C＃中进行排序（而不是在TSql中进行排序）。而且我很难相信在TSql本身没有更快的方法。

Answer 1

您提供的详细信息，请尝试这种方式。

CREATE TABLE #tPallets
    (
      PalletNumber bigint,
      Placement nvarchar(4),
      OrderBy as CASE 
            WHEN Placement LIKE 'M%' THEN CONVERT(int, SUBSTRING(Placement,2, len(Placement)-1)) 
            ELSE 0 END
    )

    INSERT INTO #tPallets  VALUES(100000, 'B')   
    INSERT INTO #tPallets  VALUES(100001, 'M1')  
    INSERT INTO #tPallets  VALUES(100002, 'M2')  
    INSERT INTO #tPallets  VALUES(100003, 'M3')  
    INSERT INTO #tPallets  VALUES(100004, 'M4')  
    INSERT INTO #tPallets  VALUES(100005, 'M5')  
    INSERT INTO #tPallets  VALUES(100006, 'M6')  
    INSERT INTO #tPallets  VALUES(100007, 'M7')  
    INSERT INTO #tPallets  VALUES(100008, 'M8')  
    INSERT INTO #tPallets  VALUES(100009, 'M9')  
    INSERT INTO #tPallets  VALUES(100010, 'M10')  
    INSERT INTO #tPallets  VALUES(100011, 'M11')  

    select * from #tPallets
    order by OrderBy

或者尝试这个但不确定。

select * from #tPallets
order by BINARY_CHECKSUM(Placement) desc

Answer 2

计算列https://docs.microsoft.com/en-us/sql/relational-databases/indexes/indexes-on-computed-columns

的索引

或者，在您的情况下可能更好地结合使用过滤索引

https://docs.microsoft.com/en-us/sql/relational-databases/indexes/create-filtered-indexes

摆脱那些讨厌的0。（顺便说一下placement不是like %M时，您对订单的期望是什么？）

Answer 3

使用计算列 加快速度的方法是让列持久化，而不是在列上创建索引。请记住，这当然会对 DML（插入/更新/删除）造成性能损失

DROP TABLE IF EXISTS #tPallets
CREATE TABLE #tPallets
(
    PalletNumber bigint,
    Placement nvarchar(4),
    PALLET_PLACEMENT AS (CASE 
        WHEN Placement LIKE 'M%' THEN CONVERT(int, SUBSTRING(Placement,2, len(Placement)-1)) 
        ELSE 0 
    END) PERSISTED /*Will save the calculation in the table*/
)

INSERT INTO #tPallets
VALUES(100000, 'B')   
,(100001, 'M1')  
,(100002, 'M2')  
,(100003, 'M3')  
,(100004, 'M4')  
,(100005, 'M5')  
,(100006, 'M6')  
,(100007, 'M7')  
,(100008, 'M8')  
,(100009, 'M9')  
,(100010, 'M10')  
,(100011, 'M11')  

/*Can create this to speed up the query*/
CREATE INDEX ix_test on #tPallets (PALLET_PLACEMENT) INCLUDE (PalletNumber,Placement)

SELECT *
FROM  #tPallets 
ORDER BY PALLET_PLACEMENT DESC

如何有效地排序包含数字和字母的nvarchar？

3 个答案: