Mysql->Maintenance of Index [20180504]

Learn index maintenance (add and delete) for tables in Mysql database

The benefits of indexing:

Improve query efficiency

Can be limited to specific data (e.g. unique)

Insufficient index:

If there are too many indexes, it will affect the rate of Insert and Update, because data insertion and update will also maintain indexes at the same time.

Consume more storage space

Indexes can be divided into single-column indexes and composite indexes by column. A single column contains only one column in the table, and a composite index can contain multiple columns in the current table.

Create a normal index

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

If the type of the column is CHAR, VARCHAR can not specify the value of length.

If the type of the column is BLOB, TEXT needs to specify the value of length

Create an index when creating a table

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
  Not_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 statement maintains indexes

ALTER TABLE <tab_name> ADD PRIMARY KEY(<column_list>);#Add a primary key, unique and non-null

ALTER TABLE <tab_name> ADD UNIQUE <index_name>(<column_list>);#Add a unique index, the column can be NULL and there can be multiple times

ALTER TABLE <tab_name> ADD INDEX <index_name>(<column_list>);#Add a common index

ALTER TABLE <tab_name> ADD FULLTEXT <index_name>(<column_list>); Add a full-text index, note that only MyISAM engine supports

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)

index deletion

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;

View index structure

SHOW INDEX FROM <tab_name>\G

Mysql->Maintenance of Index [20180504]

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