The mysqlor condition can be indexed to avoid full table scan _ MySQL

The mysqlor condition can be indexed to avoid full table scan of bitsCN.com.

In some cases, the or condition can avoid full table scan.

1. if the where statement contains the or condition, the myisam table can use indexes, but innodb does not.

1) myisam table:

Create table if not exists 'A '(

'Id' int (1) not null AUTO_INCREMENT,

'Uid' int (11) not null,

'Anum' char (20) default null,

Primary key ('id '),

KEY 'uid' ('uid ')

) ENGINE = MyISAM default charset = latin1 AUTO_INCREMENT = 6;

Mysql> explain select * from a where id = 1 or uid = 2;

+ ---- + ------------- + ------- + ------------- + --------------- + ------------- + --------- + ------ + --------------------------------------- +

| Id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+ ---- + ------------- + ------- + ------------- + --------------- + ------------- + --------- + ------ + --------------------------------------- +

| 1 | SIMPLE | a | index_merge | PRIMARY, uid | PRIMARY, uid | 4, 4 | NULL | 2 | Using union (PRIMARY, uid); Using where |

+ ---- + ------------- + ------- + ------------- + --------------- + ------------- + --------- + ------ + --------------------------------------- +

1 row in set (0.00 sec)

2) innodb table:

Create table if not exists 'A '(

'Id' int (1) not null AUTO_INCREMENT,

'Uid' int (11) not null,

'Anum' char (20) default null,

Primary key ('id '),

KEY 'uid' ('uid ')

) ENGINE = InnoDB default charset = latin1 AUTO_INCREMENT = 6;

Mysql> explain select * from a where id = 1 or uid = 2;

+ ---- + ------------- + ------- + ------ + --------------- + ------ + --------- + ------ + ------------- +

| Id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+ ---- + ------------- + ------- + ------ + --------------- + ------ + --------- + ------ + ------------- +

| 1 | SIMPLE | a | ALL | PRIMARY, uid | NULL | 5 | Using where |

+ ---- + ------------- + ------- + ------ + --------------- + ------ + --------- + ------ + ------------- +

1 row in set (0.00 sec)

2. all or conditions must be independent indexes:

+ ------- + Response ----------------------------------------------------------------------------------------------------------------------

| Table | Create Table

+ ------- + Response ----------------------------------------------------------------------------------------------------------------------

| A | create table 'A '(

'Id' int (1) not null AUTO_INCREMENT,

'Uid' int (11) not null,

'Anum' char (20) default null,

Primary key ('id ')

) ENGINE = MyISAM AUTO_INCREMENT = 6 default charset = latin1 |

+ ------- + Response ----------------------------------------------------------------------------------------------------------------------

1 row in set (0.00 sec)

To view the description, follow these steps:

Mysql> explain select * from a where id = 1 or uid = 2;

+ ---- + ------------- + ------- + ------ + --------------- + ------ + --------- + ------ + ------------- +

| Id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+ ---- + ------------- + ------- + ------ + --------------- + ------ + --------- + ------ + ------------- +

| 1 | SIMPLE | a | ALL | PRIMARY | NULL | 5 | Using where |

+ ---- + ------------- + ------- + ------ + --------------- + ------ + --------- + ------ + ------------- +

1 row in set (0.00 sec)

The entire table is scanned.

From the hguisu column

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