验证联合索引使用的情况

索引是一个排序的结构，用于快速检索和加速排序

MySQL表结构

index_test | CREATE TABLE `index_test` (

`c1` char(10) NOT NULL,

`c2` char(10) NOT NULL,

`c3` char(10) NOT NULL,

`c4` char(10) NOT NULL,

`c5` char(10) NOT NULL,

KEY `index_c1c2c3c4` (`c1`,`c2`,`c3`,`c4`)

) ENGINE=MyISAM DEFAULT CHARSET=utf8

(1).

mysql> explain select * from index_test where c1=‘c1‘ and c2=‘c2‘ and c4>‘c4‘ and c3=‘c3‘\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: index_test

type: range

possible_keys: index_c1c2c3c4

key: index_c1c2c3c4

key_len: 120

ref: NULL

rows: 1

Extra: Using index condition

由key_len: 120可知，四个索引都用到了(utf8，一个字符占用3个字节)；

(2).

explain select * from index_test where c1=‘c1‘ and c2=‘c2‘ and c4=‘c4‘ order by c3\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: index_test

type: ref

possible_keys: index_c1c2c3c4

key: index_c1c2c3c4

key_len: 60

ref: const,const

rows: 1

Extra: Using index condition; Using where

c1,c2,c3都用到了，c1,c2索引用于排序，c3索引用于排序

(3)

select * from index_test where c1=‘c1‘ and c2=‘c2‘ order by c3,c2\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: index_test

type: ref

possible_keys: index_c1c2c3c4

key: index_c1c2c3c4

key_len: 60

ref: const,const

rows: 1

Extra: Using index condition; Using where

c1,c2,c3 c3用到索引是因为c2是常量

(4)

explain select * from index_test where c1=‘c1‘ and c5=‘c5‘ order by c2,c3\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: index_test

type: ref

possible_keys: index_c1c2c3c4

key: index_c1c2c3c4

key_len: 30

ref: const

rows: 1

Extra: Using index condition; Using where

c1,c2,c3用到索引了

(5)

explain select * from index_test where c1=‘c1‘ and c2=‘c2‘ and c5=‘c5‘ order by c3,c2\G;

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: index_test

type: ref

possible_keys: index_c1c2c3c4

key: index_c1c2c3c4

key_len: 60

ref: const,const

rows: 1

Extra: Using index condition; Using where

c1,c2,c3使用了索引

MySQL 联合索引使用情况

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