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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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1. mysql> show create table t1 \G

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

3.        Table: t1

4. Create Table: CREATE TABLE `t1` (

5.   `c1` mediumint(8) unsigned NOT NULL DEFAULT '0',

6.   `c2` smallint(5) unsigned NOT NULL DEFAULT '0',

7.   `c3` int(10) unsigned NOT NULL DEFAULT '0',

8.   `c4` int(10) unsigned NOT NULL DEFAULT '0',

9.   `c5` mediumint(8) unsigned NOT NULL,

10.   `c6` varchar(2) DEFAULT NULL,

11.   KEY `idx_t1_c1_c2_c3_c4` (`c1`,`c2`,`c3`,`c4`)

12. ) ENGINE=InnoDB DEFAULT CHARSET=utf8

2、选项A执行计划

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1. mysql> explain select * from t1 where c1=100 and c2=2 and c4>1000 and c3=1419401948 \G

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

3.            id: 1

4.   select_type: SIMPLE

5.         table: t1

6.    partitions: NULL

7.          type: range

8. possible_keys: idx_t1_c1_c2_c3_c4

9.           key: idx_t1_c1_c2_c3_c4

10.       key_len: 13

11.           ref: NULL

12.          rows: 1

13.      filtered: 100.00

14.         Extra: Using index condition

3、选项B执行计划

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1. mysql> explain select * from t1 where c1=100 and c2=2 and c4=1419317673 order by c3 \G

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

3.            id: 1

4.   select_type: SIMPLE

5.         table: t1

6.    partitions: NULL

7.          type: ref

8. possible_keys: idx_t1_c1_c2_c3_c4

9.           key: idx_t1_c1_c2_c3_c4

10.       key_len: 5

11.           ref: const,const

12.          rows: 1

13.      filtered: 10.00

14.         Extra: Using index condition

4、选项C执行计划

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1. mysql> explain select * from t1 where c1=100 and c4=1419317673 group by c3,c2 \G

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

3.            id: 1

4.   select_type: SIMPLE

5.         table: t1

6.    partitions: NULL

7.          type: ref

8. possible_keys: idx_t1_c1_c2_c3_c4

9.           key: idx_t1_c1_c2_c3_c4

10.       key_len: 3

11.           ref: const

12.          rows: 1

13.      filtered: 10.00

14.         Extra: Using index condition; Using temporary; Using filesort

5、选项D执行计划

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1. mysql> explain select * from t1 where c1=100 and c5=2 order by c2,c3 \G

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

3.            id: 1

4.   select_type: SIMPLE

5.         table: t1

6.    partitions: NULL

7.          type: ref

8. possible_keys: idx_t1_c1_c2_c3_c4

9.           key: idx_t1_c1_c2_c3_c4

10.       key_len: 3

11.           ref: const

12.          rows: 1

13.      filtered: 10.00

14.         Extra: Using index condition; Using where

6、选项E执行计划

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1. mysql> explain select * from t1 where c1=1000 and c2=200 and c5=2 order by c2,c3 \G

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

3.            id: 1

4.   select_type: SIMPLE

5.         table: t1

6.    partitions: NULL

7.          type: ref

8. possible_keys: idx_t1_c1_c2_c3_c4

9.           key: idx_t1_c1_c2_c3_c4

10.       key_len: 5

11.           ref: const,const

12.          rows: 1

13.      filtered: 10.00

14.         Extra: Using index condition; Using where

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