What types of mysql indexes are there?

The main index types currently available in MySQL are: ordinary index, unique index, primary key index, combined index, and full-text index. The following article will introduce these MySQL indexes to you, I hope it will be helpful to you.

#By adding indexes to fields, you can increase the speed of data reading and improve the concurrency and stress resistance of the project. Index optimization is an optimization method in MySQL. The index is equivalent to the table of contents of a book. You can quickly find the required content based on the page numbers in the table of contents.

Let’s take a look at the main index types currently available in MySQL:

1. Ordinary index

Ordinary index is the most basic index. There are no restrictions, the value can be empty; only speeds up queries. It can be created or deleted in the following ways:

1), directly creating an index

CREATE INDEX index_name ON table(column(length))

2), adding an index by modifying the table structure

ALTER TABLE table_name ADD INDEX index_name ON (column(length))

3), Delete index

DROP INDEX index_name ON table

2. Unique index

The unique index is similar to the ordinary index, except that the value of the index column must be unique, but empty is allowed value. In the case of a composite index, the combination of column values ​​must be unique. To put it simply: a unique index speeds up queries because the column value is unique (can have null). It can be created in the following ways:

1), creating a unique index

CREATE UNIQUE INDEX indexName ON table(column(length))

2), modifying the table structure

ALTER TABLE table_name ADD UNIQUE indexName ON (column(length))

3, primary key index

The primary key index is a special unique index. A table can only have one primary key and no null values ​​are allowed. To put it simply: the primary key index speeds up queries. The column value is unique (cannot have null) and there is only one in the table.

Generally, the primary key index is created at the same time when creating the table:

CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL, PRIMARY KEY(ID) );

Of course, you can also use the ALTER command. Remember: a table can only have one primary key.

4. Composite index

Combined index refers to an index created on multiple fields. Only the first field when creating the index is used in the query conditions. The index will be used. Follow the leftmost prefix set when using combined indexes.

It can be said that the combined index is an index composed of multiple column values, specially used for combined searches, and its efficiency is greater than index merging.

ALTER TABLE `table` ADD INDEX name_city_age (name,city,age);

5. Full-text index

Full-text index is mainly used to find keywords in the text, rather than directly comparing them with values ​​in the index. The fulltext index is very different from other indexes. It is more like a search engine rather than a simple parameter matching of the where statement. The fulltext index is used with the match against operation instead of the general where statement plus like. It can be used in create table, alter table, and create index, but currently only full-text indexes can be created on char, varchar, and text columns. It is worth mentioning that when the amount of data is large, it is better to put the data into a table without a global index and then use CREATE index to create a fulltext index than to first create a fulltext for a table and then write the data. The speed is much faster.

1), it is suitable to add a full-text index when creating a table

CREATE TABLE `table` (
    `id` int(11) NOT NULL AUTO_INCREMENT ,
    `title` char(255) CHARACTER NOT NULL ,
    `content` text CHARACTER NULL ,
    `time` int(10) NULL DEFAULT NULL ,
    PRIMARY KEY (`id`),
    FULLTEXT (content)
);

2), modify the table structure to add a full-text index

ALTER TABLE article ADD FULLTEXT index_content(content)

3), create an index directly

CREATE FULLTEXT INDEX index_content ON article(content)

Simply put: full-text indexing is to segment the text content and search.

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