MySQL->索引的维护[20180504]

学习MySQL数据库中表的索引维护(新增和删除)

索引的好处：

提高查询的效率

可限定特定的资料（如唯一）

索引的不足：

如果过多的索引，会影响Insert，Update的速率，因为资料插入和更新同时也会维护索引。

消耗更多的存储空间

索引按列可分为，单列索引和组合索引。单列只包含表中的一列，组合索引可包含当前表中多列栏位。

创建普通索引

CREATE INDEX <index_name> ON <tab_name>(<col_name>(length));

如果列的类型是CHAR，VARCHAR可以不指定length的值。

如果列的类型是BLOB，TEXT则需指定length的值

创建表时建立索引

CREATE TABLE <tab_name>(

<col_name> TYPE,....

[INDEX_TYPE] <index_name>(<col_name>(length))

engine=InnoDB default charset=utf8;

mysql> create table index_tab03(

    ->     id int,

    ->    col01 varchar(20),

    ->     index index_01(col01)

    ->      )engine=InnoDB default charset=utf8;

Query OK, 0 rows affected (0.01 sec)

mysql> desc index_tab03;

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

| Field | Type        | Null | Key | Default | Extra |

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

| id    | int(11)     | YES  |     | NULL    |       |

| col01 | varchar(20) | YES  | MUL | NULL    |       |

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

2 rows in set (0.00 sec)

mysql> show index from index_tab03\G

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

       Table: index_tab03

  Non_unique: 1

    Key_name: index_01

Seq_in_index: 1

Column_name: col01

   Collation: A

Cardinality: 0

    Sub_part: NULL

      Packed: NULL

        Null: YES

  Index_type: BTREE

     Comment:

1 row in set (0.00 sec)

 m

ALTER语句维护索引

ALTER TABLE <tab_name> ADD PRIMARY KEY(<column_list>);#增加一个主键，唯一和非空

ALTER TABLE <tab_name> ADD UNIQUE <index_name>(<column_list>);#增加一个唯一索引，列中可以为NULL并且可以有多次

ALTER TABLE <tab_name> ADD INDEX <index_name>(<column_list>);#增加一个普通索引

ALTER TABLE <tab_name> ADD FULLTEXT <index_name>(<column_list>);增加一个全文索引，注意只有MyISAM引擎支持

mysql> alter table index_tab01 add primary key(id,col01);

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table index_tab01 add index indx_01(col01);

Query OK, 0 rows affected (0.00 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table index_tab01 add fulltext indx_02(col01,col02(300));

ERROR 1214 (HY000): The used table type doesn't support FULLTEXT indexes

mysql> alter table index_tab01 engine=MyISAM;

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table index_tab01 add fulltext index_02(col01,col02(30));

Query OK, 0 rows affected (0.00 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from index_tab01\G

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

       Table: index_tab01

  Non_unique: 0

    Key_name: PRIMARY

Seq_in_index: 1

Column_name: id

   Collation: A

Cardinality: NULL

    Sub_part: NULL

      Packed: NULL

        Null:

  Index_type: BTREE

     Comment:

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

       Table: index_tab01

  Non_unique: 0

    Key_name: PRIMARY

Seq_in_index: 2

Column_name: col01

   Collation: A

Cardinality: 0

    Sub_part: NULL

      Packed: NULL

        Null:

  Index_type: BTREE

     Comment:

*************************** 3. row ***************************

       Table: index_tab01

  Non_unique: 0

    Key_name: index_un

Seq_in_index: 1

Column_name: col01

   Collation: A

Cardinality: 0

    Sub_part: NULL

      Packed: NULL

        Null:

  Index_type: BTREE

     Comment:

*************************** 4. row ***************************

       Table: index_tab01

  Non_unique: 1

    Key_name: indx_01

Seq_in_index: 1

Column_name: col01

   Collation: A

Cardinality: NULL

    Sub_part: NULL

      Packed: NULL

        Null:

  Index_type: BTREE

     Comment:

*************************** 5. row ***************************

       Table: index_tab01

  Non_unique: 1

    Key_name: index_02

Seq_in_index: 1

Column_name: col01

   Collation: NULL

Cardinality: NULL

    Sub_part: NULL

      Packed: NULL

        Null:

  Index_type: FULLTEXT

     Comment:

*************************** 6. row ***************************

       Table: index_tab01

  Non_unique: 1

    Key_name: index_02

Seq_in_index: 2

Column_name: col02

   Collation: NULL

Cardinality: NULL

    Sub_part: NULL

      Packed: NULL

        Null: YES

  Index_type: FULLTEXT

     Comment:

6 rows in set (0.00 sec)

索引删除

DROP INDEX <index_name> ON <tab_name>;

ALTER TABLE <tab_name> DROP PRIMARY KEY;

ALTER TABLE <tab_name> DROP INDEX <index_name>;

mysql> alter table index_tab01 drop primary key;

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table index_tab01 drop index index_02;

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> drop index index_01 on index_tab01;

查看索引结构

SHOW INDEX FROM <tab_name>\G

巴特西

MySQL->索引的维护[20180504]

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