In MySQL 3.22 or later, CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See Section 13.2.2, “ALTER TABLE Syntax”. The CREATE INDEX statement doesn't do anything prior to MySQL 3.22.

Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See Section 13.2.5, “CREATE TABLE Syntax”. CREATE INDEX allows 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.

For CHAR and VARCHAR columns, indexes can be created that use only part of a column, using col_name(length) syntax to index a prefix consisting of the first length characters of each column value. BLOB and TEXT columns also can be indexed, but a prefix length must be given.

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));

Because most names 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 partial columns for indexes can make the index file much smaller, which could save a lot of disk space and might also speed up INSERT operations!

Prefixes can be up to 255 bytes long (or 1000 bytes for MyISAM and InnoDB tables as of MySQL 4.1.2). Note that prefix limits are measured in bytes, whereas the prefix length in CREATE INDEX statements is interpreted as number of characters. Take this into account when specifying a prefix length for a column that uses a multi-byte character set.

You can add an index on a column that can have NULL values only if you are using MySQL 3.23.2 or newer and are using the MyISAM, InnoDB, or BDB table type. You can only add an index on a BLOB or TEXT column if you are using MySQL 3.23.2 or newer and are using the MyISAM or BDB table type, or MySQL 4.0.14 or newer and the InnoDB table type.

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.

From MySQL 4.1.0 on, some storage engines allow you to specify an index type when creating an index. The syntax for the index_type specifier is USING type_name. The allowable type_name 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.

Example:

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

TYPE type_name can be used as a synonym for USING type_name to specify an index type. However, USING is the preferred form. Also, the index name that precedes the index type in the index specification syntax is not optional with TYPE. This is because, unlike USING, TYPE is not a reserved word and thus is interpreted as an index name.

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

For more information about how MySQL uses indexes, see Section 7.4.5, “How MySQL Uses Indexes”.

FULLTEXT indexes can index only CHAR, VARCHAR, and TEXT columns, and only in MyISAM tables. FULLTEXT indexes are available in MySQL 3.23.23 or later. Section 12.6, “Full-Text Search Functions”.

SPATIAL indexes can index only spatial columns, and only in MyISAM tables. SPATIAL indexes are available in MySQL 4.1 or later. Spatial column types are described in Chapter 18, Spatial Extensions in MySQL.