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針對此問題進行測試：
假設某個表有一個聯合索引（c1,c2,c3,c4）一下___只能使用該聯合索引的 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、創建測試表
點擊 (此處) 折疊或打開
mysql
show
create
table
t1 \G
***************************
row
***************************
Table:
t1
Create
Table:
CREATE
TABLE
`t1`
(
`c1`
mediumint(8)
unsigned
NULL
DEFAULT
0 ,
`c2`
smallint(5)
unsigned
NULL
DEFAULT
0 ,
`c3`
int(10)
unsigned
NULL
DEFAULT
0 ,
`c4`
int(10)
unsigned
NULL
DEFAULT
0 ,
`c5`
mediumint(8)
unsigned
NULL,
`c6`
varchar(2)
DEFAULT
NULL,
KEY
`idx_t1_c1_c2_c3_c4`
(`c1`,`c2`,`c3`,`c4`)
)
ENGINE=InnoDB
DEFAULT
CHARSET=utf8
2、選項 A 執行計劃
點擊 (此處) 折疊或打開
mysql
explain
select
from
where
c1=100
c2=2
c4 1000
c3=1419401948 \G
***************************
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、選項 B 執行計劃
點擊 (此處) 折疊或打開
mysql
explain
select
from
where
c1=100
c2=2
c4=1419317673 order by c3 \G
***************************
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、選項 C 執行計劃
點擊 (此處) 折疊或打開
mysql
explain
select
from
where
c1=100
c4=1419317673 group by c3,c2 \G
***************************
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、選項 D 執行計劃
點擊 (此處) 折疊或打開
mysql
explain
select
from
where
c1=100
c5=2 order by c2,c3 \G
***************************
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、選項 E 執行計劃
點擊 (此處) 折疊或打開
mysql
explain
select
from
where
c1=1000
c2=200
c5=2 order by c2,c3 \G
***************************
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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