添加组合索引时，做相等运算字段应该放在最前面

有一个通常的误解，觉得应该把选择性高的字段放在最前面，这通常只是针对一个字段的索引，对于组合索引，常常要把做等式运算的字段放在最前面，看看测试

USE AdventureWorks
GO

CREATE TABLE demo1 
( 
    id INT identity(1,1)  PRIMARY KEY,
    gender char(1) NOT NULL ,
    age int NOT NULL,
    DESCRIPTION varchar(1000) default(replicate('a',1000))  
)


--填充数字辅助表
--DROP TABLE  #num

SELECT row_number() over( order BY customerid ) AS  number 
INTO #num  
FROM adventureworks.sales.Individual

--插入测试数据
INSERT demo1 (gender,age)
SELECT CASE 
         WHEN number%2=0 THEN 'f'
         ELSE 'm'
        END,
        abs(checksum(newid()))%80
FROM #num 

创建如下的索引：

CREATE INDEX ix_age_sex ON demo1(age,gender) include(description) WITH (online=on)

查询如下的语句：

DBCC FREEPROCCACHE
CHECKPOINT
DBCC DROPCLEANBUFFERS

SELECT age,gender,description FROM demo1  
WHERE gender='f' 
AND age BETWEEN 30 AND 40

逻辑读为：

(1236 row(s) affected)
Table 'demo1'. Scan count 1, logical reads 350, physical reads 3, read-ahead reads 346, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

现在删除原来的索引，创建低选择性在前的索引

DROP INDEX ix_age_sex ON demo1 
CREATE INDEX ix_age_sex ON demo1(gender,age)INCLUDE (description) WITH (online=on)

在来查询相同的语句：

DBCC FREEPROCCACHE
CHECKPOINT
DBCC DROPCLEANBUFFERS

SELECT age,gender,description FROM demo1  
WHERE gender='f' 
AND age BETWEEN 30 AND 40

逻辑读为：

(1236 row(s) affected)
Table 'demo1'. Scan count 1, logical reads 181, physical reads 3, read-ahead reads 178, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 

可以看到低选择性的字段放在前面逻辑读有350减少到了181，由此产生性能提升

 

总结：1：做等式运算的字段放在最前面，如果有多个等式运算，才要考虑等式运算字段的选择性