create table t2(
c1 char(1) not null default 0,
c2 char(1) not null default 0,
c3 char(1) not null default 0,
c4 char(1) not null default 0,
c5 char(1) not null default 0,
index c1234(c1,c2,c3,c4)
)engine innodb charset utf8;
insert into t2 values (1,3,5,6,7),(2,3,9,8,3),(4,3,2,7,5);假設某個表有乙個聯合索引（c1,c2,c3,c4
a where c1=1 and c2=2 and c4>3 and c3=4
b where c1=1 and c2=2 and c4=3 order by c3
c where c1=1 and c4=2 group by c3,c2
d where c1=1 and c5=2 order by c2,c3
e where c1=1 and c2=2 and c5=4 order by c2,c3
explain select * from t2 where c1=『a』 and c2=『b』 and c4>『a』 and c3=『b』\g;
mysql> explain select * from t2 where c1='a' and c2='b' and c4>'a' and c3='b'\g;
*************************** 1. row ***************************
id: 1
select_type: ******
table: t2
type: range
possible_keys: c1234
key: c1234
key_len: 12
ref: null
rows: 1
extra: using index condition
1 row in set (0.00 sec)
error:
no query specified
驗證b
explain select * from t2 where c1=『a』 and c2=『b』 and c4=『a』 order by c3\g;
mysql> explain select * from t2 where c1='a' and c2='b' and c4='a' order by c3\g;
*************************** 1. row ***************************
id: 1
select_type: ******
table: t2
type: ref
possible_keys: c1234
key: c1234
key_len: 6
ref: const,const
rows: 2
extra: using index condition; using where
1 row in set (0.00 sec)
error:
no query specified
驗證c
explain select * from t2 where c1=『a』 and c4=『b』 group by c3,c2\g;
mysql> explain select * from t2 where c1='a' and c4='b' group by c3,c2\g;
*************************** 1. row ***************************
id: 1
select_type: ******
table: t2
type: ref
possible_keys: c1234
key: c1234
key_len: 3
ref: const
rows: 2
extra: using index condition; using where; using temporary; using filesort
1 row in set (0.00 sec)
error:
no query specified
驗證d
explain select * from t2 where c1=『a』 and c5=『b』 order by c2,c3\g;
mysql> explain select * from t2 where c1='a' and c5='b' order by c2,c3\g;
*************************** 1. row ***************************
id: 1
select_type: ******
table: t2
type: ref
possible_keys: c1234
key: c1234
key_len: 3
ref: const
rows: 2
extra: using index condition; using where
1 row in set (0.00 sec)
error:
no query specified
d變種
explain select * from t2 where c1=『a』 and c5=『b』 order by c3,c2\g;
*************************** 1. row ***************************
id: 1
select_type: ******
table: t2
type: ref
possible_keys: c1234
key: c1234
key_len: 3
ref: const
rows: 2
extra: using index condition; using where; using filesort
1 row in set (0.00 sec)
error:
no query specified
驗證e
explain select * from t2 where c1=『a』 and c2=『c』 and c5=『b』 order by c2,c3\g;
mysql> explain select * from t2 where c1='a' and c2='c' and c5='b' order by c2,c3\g;
*************************** 1. row ***************************
id: 1
select_type: ******
table: t2
type: ref
possible_keys: c1234
key: c1234
key_len: 6
ref: const,const
rows: 1
extra: using index condition; using where
1 row in set (0.00 sec)
error:
no query specified
e變種
explain select * from t2 where c1=『a』 and c2=『c』 and c5=『b』 order by c3,c2\g;
*************************** 1. row ***************************
id: 1
select_type: ******
table: t2
type: ref
possible_keys: c1234
key: c1234
key_len: 6
ref: const,const
rows: 1
extra: using index condition; using where
1 row in set (0.00 sec)
error:
no query specified
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