MySQL多列索引怎么用
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针对此问题进行测试：
假设某个表有一个联合索引（c1,c2,c3,c4）一下___只能使用该联合索引的c1,c2,c3部分
A where c1=x and c2=x and c4>x and c3=x
B where c1=x and c2=x and c4=x order by c3
C where c1=x and c4= x group by c3,c2
D where c1=x and c5=x order by c2,c3
E where c1=x and c2=x and c5=? order by c2,c
1、创建测试表
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mysql> show create table t1 \G
*************************** 1. row ***************************
Table: t1
Create Table: CREATE TABLE `t1` (
`c1` mediumint(8) unsigned NOT NULL DEFAULT '0',
`c2` smallint(5) unsigned NOT NULL DEFAULT '0',
`c3` int(10) unsigned NOT NULL DEFAULT '0',
`c4` int(10) unsigned NOT NULL DEFAULT '0',
`c5` mediumint(8) unsigned NOT NULL,
`c6` varchar(2) DEFAULT NULL,
KEY `idx_t1_c1_c2_c3_c4` (`c1`,`c2`,`c3`,`c4`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
2、选项A执行计划
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mysql> explain select * from t1 where c1=100 and c2=2 and c4>1000 and c3=1419401948 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t1
partitions: NULL
type: range
possible_keys: idx_t1_c1_c2_c3_c4
key: idx_t1_c1_c2_c3_c4
key_len: 13
ref: NULL
rows: 1
filtered: 100.00
Extra: Using index condition
3、选项B执行计划
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mysql> explain select * from t1 where c1=100 and c2=2 and c4=1419317673 order by c3 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t1
partitions: NULL
type: ref
possible_keys: idx_t1_c1_c2_c3_c4
key: idx_t1_c1_c2_c3_c4
key_len: 5
ref: const,const
rows: 1
filtered: 10.00
Extra: Using index condition
4、选项C执行计划
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mysql> explain select * from t1 where c1=100 and c4=1419317673 group by c3,c2 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t1
partitions: NULL
type: ref
possible_keys: idx_t1_c1_c2_c3_c4
key: idx_t1_c1_c2_c3_c4
key_len: 3
ref: const
rows: 1
filtered: 10.00
Extra: Using index condition; Using temporary; Using filesort
5、选项D执行计划
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mysql> explain select * from t1 where c1=100 and c5=2 order by c2,c3 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t1
partitions: NULL
type: ref
possible_keys: idx_t1_c1_c2_c3_c4
key: idx_t1_c1_c2_c3_c4
key_len: 3
ref: const
rows: 1
filtered: 10.00
Extra: Using index condition; Using where
6、选项E执行计划
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mysql> explain select * from t1 where c1=1000 and c2=200 and c5=2 order by c2,c3 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t1
partitions: NULL
type: ref
possible_keys: idx_t1_c1_c2_c3_c4
key: idx_t1_c1_c2_c3_c4
key_len: 5
ref: const,const
rows: 1
filtered: 10.00
Extra: Using index condition; Using where
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