CREATE [UNIQUE|FULLTEXT|SPATIAL] INDEX index_name
    [index_type]
    ON tbl_name (index_col_name,...)
    [index_type]

index_col_name:
    col_name [(length)] [ASC | DESC]

index_type:
    USING {BTREE | HASH | RTREE}

CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See Section 12.1.2, “ALTER TABLE Syntax”. CREATE INDEX cannot be used to create a PRIMARY KEY; use ALTER TABLE instead. For more information about indexes, see Section 7.4.5, “How MySQL Uses Indexes”.

Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See Section 12.1.5, “CREATE TABLE Syntax”. CREATE INDEX enables you to add indexes to existing tables.

A column list of the form (col1,col2,...) creates a multiple-column index. Index values are formed by concatenating the values of the given columns.

Indexes can be created that use only the leading part of column values, using col_name(length) syntax to specify an index prefix length:

The statement shown here creates an index using the first 10 characters of the name column:

CREATE INDEX part_of_name ON customer (name(10));

If names in the column usually differ in the first 10 characters, this index should not be much slower than an index created from the entire name column. Also, using column prefixes for indexes can make the index file much smaller, which could save a lot of disk space and might also speed up INSERT operations.

Prefix lengths are storage engine-dependent (for example, a prefix can be up to 1000 bytes long for MyISAM tables, 767 bytes for InnoDB tables). Note that prefix limits are measured in bytes, whereas the prefix length in CREATE INDEX statements is interpreted as number of characters for non-binary data types (CHAR, VARCHAR, TEXT). Take this into account when specifying a prefix length for a column that uses a multi-byte character set. For example, utf8 columns require up to three index bytes per character.

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix.

MySQL Enterprise.  Lack of proper indexes can greatly reduce performance. Subscribe to the MySQL Enterprise Monitor for notification of inefficient use of indexes. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

FULLTEXT indexes are supported only for MyISAM tables and can include only CHAR, VARCHAR, and TEXT columns. Indexing always happens over the entire column; column prefix indexing is not supported and any prefix length is ignored if specified. See Section 11.8, “Full-Text Search Functions”, for details of operation.

The MyISAM, InnoDB, NDB, BDB, and ARCHIVE storage engines support spatial columns such as (POINT and GEOMETRY. (Chapter 19, Spatial Extensions, describes the spatial data types.) However, support for spatial column indexing varies among engines. Spatial and non-spatial indexes are available according to the following rules.

Spatial indexes (created using SPATIAL INDEX):

Non-spatial indexes (created with INDEX, UNIQUE, or PRIMARY KEY):

In MySQL 5.0:

An index_col_name specification can end with ASC or DESC. These keywords are allowed for future extensions for specifying ascending or descending index value storage. Currently, they are parsed but ignored; index values are always stored in ascending order.

Some storage engines allow you to specify an index type when creating an index. The allowable index type values supported by different storage engines are shown in the following table. Where multiple index types are listed, the first one is the default when no index type specifier is given.

The RTREE index type is allowable only for SPATIAL indexes.

If you specify an index type that is not legal for a given storage engine, but there is another index type available that the engine can use without affecting query results, the engine uses the available type.

Examples:

CREATE TABLE lookup (id INT) ENGINE = MEMORY;
CREATE INDEX id_index USING BTREE ON lookup (id);

TYPE type_name is recognized as a synonym for USING type_name. However, USING is the preferred form.

Before MySQL 5.0.60, the index_type option can be given only before the ON tbl_name clause. Use of the option in this position is deprecated as of 5.0.60; support for it is to be dropped in a future MySQL release. As of 5.0.60, the option should be given following the index column list. If an index_type option is given in both the earlier and later positions, the final option applies.