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SQL Server的Descending Indexes降序索引

SQL Server的Descending Indexes降序索引

 

 

1、建立测试环境

测试环境:SQL Server 2012

 

表结构如下

USE [test]
GO


CREATE TABLE [dbo].[tt8](
    [id] INT IDENTITY(1,1) NOT NULL,
    [win_num] [int] NOT NULL DEFAULT ((0)),
    [lost_num] [int] NOT NULL   DEFAULT ((0)),
    [draw_num] [int] NOT NULL  DEFAULT ((0)),
    [offline_num] [int] NOT NULL   DEFAULT ((0)),
    [login_key] [nvarchar](50) NULL
 CONSTRAINT [PK_user_T] PRIMARY KEY CLUSTERED 
(
    [id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

 

插入测试数据

DECLARE @i INT;
DECLARE @sql NVARCHAR(MAX);
SET @i = 1;
WHILE @i <= 9999
    BEGIN
        SET @sql = 'INSERT  INTO [dbo].[tt8]
        ( 
          [win_num] ,
          [lost_num] ,
          [draw_num] ,
          [offline_num] ,
          [login_key] 
         
        )
VALUES  ( 
        ''' + CAST(@i AS NVARCHAR(3000)) + ''' ,
        ''' + CAST(@i AS NVARCHAR(3000)) + ''' ,
        ''' + CAST(@i AS NVARCHAR(3000)) + ''' ,
            ''' + CAST(@i AS NVARCHAR(3000)) + ''' ,
            ''' + CAST(@i AS NVARCHAR(3000)) + ''' 


        );';
        EXEC ( @sql );
        SET @i = @i + 1;

    END;

 


 

2、构建查询语句

查询语句如下,可以看到这个是组合字段排序,要求是:按照draw_num值正序,对于相同的draw_num值,按照win_num值倒序

select top 10 * from [dbo].[tt8] order by  [draw_num] asc,[win_num] desc

 

 

 根据查询语句建一个非聚集索引

CREATE NONCLUSTERED INDEX [IX_tt8_draw_numwin_num] ON [dbo].[tt8]
(
    [draw_num] ASC,
    [win_num] ASC

)WITH (online= ON) ON [PRIMARY]
GO

 

 

 建了索引之后,执行计划如下,可以看到无法用到刚才建的索引,因为建索引时候,两个字段的排序顺序都是单向遍历的,统一升序或统一降序

 

 


 

 

3、建索引

那么,建索引时候能不能按照查询语句的顺序,[draw_num] 升序,[win_num] 降序呢?

答案是可以的,删除刚才建的索引,再建一个新索引

DROP INDEX [IX_tt8_draw_numwin_num]    ON [tt8]

CREATE NONCLUSTERED INDEX [IX_tt8_draw_numwin_num]   ON [dbo].[tt8]
(
    [draw_num] ASC ,
    [win_num] DESC 
)
WITH ( ONLINE = ON ) ON [PRIMARY] GO

 

建了索引之后,索引大概是这样,第一个字段升序,第二个字段降序

 

再查询一次,执行计划如下,可以看到这次利用到索引

 

通过这个例子说明,对于组合字段排序的语句,当多个字段排序顺序不一致的时候,只建单个字段的索引是无法利用到索引的,例如下面只建一个[draw_num] 字段的索引是无法利用到[IX_tt8_draw_num]索引的

CREATE NONCLUSTERED INDEX [IX_tt8_draw_num]   ON [dbo].[tt8]
(
    [draw_num] ASC 
)  WITH ( ONLINE = ON ) ON [PRIMARY]
GO

 

必须要建排序字段的组合索引,并且索引字段的排序要跟查询语句一致,这个索引在Oracle里面叫Descending Indexes

 

 

总结

Descending Indexes这个特性在SQL Server和Oracle的早期版本已经支持,在MySQL里面只有MySQL8.0才支持

所以有时候,还是商业数据库比较强大

 

参考文章:

https://www.mssqltips.com/sqlservertip/1337/building-sql-server-indexes-in-ascending-vs-descending-order/

 

 

如有不对的地方,欢迎大家拍砖o(∩_∩)o 

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