Detailed explanation and example tutorial of MySQL index

MySQL Index

The establishment of MySQL index is very important for the efficient operation of MySQL. The index can greatly improve the retrieval speed of MySQL.

For example, if MySQL that is properly designed and uses indexes is a Lamborghini, then MySQL that is not designed and uses indexes is a human tricycle.

Indexes are divided into single column indexes and combined indexes. A single-column index means that an index only contains a single column. A table can have multiple single-column indexes, but this is not a combined index. Combined index, that is, one index contains multiple columns.

When creating an index, you need to ensure that the index is applied to the conditions of the SQL query statement (usually as a condition of the WHERE clause).

In fact, the index is also a table, which saves the primary key and index fields and points to the records of the entity table.

The above all talk about the benefits of using indexes, but excessive use of indexes will cause abuse. Therefore, the index will also have its shortcomings: although the index greatly improves the query speed, it will also reduce the speed of updating the table, such as INSERT, UPDATE and DELETE on the table. Because when updating the table, MySQL not only needs to save the data, but also save the index file.

Creating index files will occupy disk space.

Normal index

Create index

This is the most basic index, it has no restrictions. It has the following creation methods:

CREATE INDEX indexName ON mytable(username(length));

If it is CHAR, VARCHAR type, length can be less than the actual length of the field; if it is BLOB and TEXT type, length must be specified.

Modify the table structure

ALTER mytable ADD INDEX [indexName] ON (username(length))

Specify directly when creating the table

CREATE TABLE mytable(  
 
ID INT NOT NULL,   
 
username VARCHAR(16) NOT NULL,  
 
INDEX [indexName] (username(length))  
 
);

DeleteSyntax of index

DROP INDEX [indexName] ON mytable;

Unique index

It is similar to the previous ordinary index, except that the value of the index column must be unique, but null values ​​are allowed. In the case of a composite index, the combination of column values ​​must be unique. It has the following creation methods:

Create index

CREATE UNIQUE INDEX indexName ON mytable(username(length))

Modify the table structure

ALTER mytable ADD UNIQUE [indexName] ON (username(length))

Specify directly when creating the table

CREATE TABLE mytable(  
 
ID INT NOT NULL,   
 
username VARCHAR(16) NOT NULL,  
 
UNIQUE [indexName] (username(length))  
 
);

Use the ALTER command to add and deleting indexes

There are four ways to add indexes to the data table:

ALTER TABLE tbl_name ADD PRIMARY KEY (column_list): 该语句添加一个主键，这意味着索引值必须是唯一的，且不能为NULL。
ALTER TABLE tbl_name ADD UNIQUE index_name (column_list): 这条语句创建索引的值必须是唯一的（除了NULL外，NULL可能会出现多次）。
ALTER TABLE tbl_name ADD INDEX index_name (column_list): 添加普通索引，索引值可出现多次。
ALTER TABLE tbl_name ADD FULLTEXT index_name (column_list):该语句指定了索引为 FULLTEXT ，用于全文索引。

The following examples add indexes to the table.

mysql> ALTER TABLE testalter_tbl ADD INDEX (c);

You can also use the DROP clause in the ALTER command to drop an index. Try the following example to delete the index:

mysql> ALTER TABLE testalter_tbl DROP INDEX (c);

Use the ALTER command to add and delete primary keys

The primary key can only act on one column. When adding a primary key index, you need to ensure that the primary key is not empty by default ( NOT NULL). Examples are as follows:

mysql> ALTER TABLE testalter_tbl MODIFY i INT NOT NULL;
mysql> ALTER TABLE testalter_tbl ADD PRIMARY KEY (i);

You can also use the ALTER command to delete the primary key:

mysql> ALTER TABLE testalter_tbl DROP PRIMARY KEY;

You only need to specify PRIMARY KEY when deleting, but when deleting an index, you must know the index name.

显示索引信息

你可以使用 SHOW INDEX 命令来列出表中的相关的索引信息。可以通过添加 \G 来格式化输出信息。

尝试以下实例:

mysql> SHOW INDEX FROM table_name\G
........

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