All MySQL column types can be indexed. Use of indexes on the relevant columns is the best way to improve the performance of SELECT operations.

The maximum number of indexes per table and the maximum index length is defined per storage engine. See Capítulo 14, Motores de almacenamiento de MySQL y tipos de tablas. All storage engines support at least 16 indexes per table and a total index length of at least 256 bytes. Most storage engines have higher limits.

With col_name(length) syntax in an index specification, you can create an index that uses only the first length characters of a CHAR or VARCHAR column. Indexing only a prefix of column values like this can make the index file much smaller.

The MyISAM and InnoDB storage engines also support indexing on BLOB and TEXT columns. When indexing a BLOB or TEXT column, you must specify a prefix length for the index. For example:

CREATE TABLE test (blob_col BLOB, INDEX(blob_col(10)));

Im MySQL 5.0, prefixes can be up to 1000 bytes long for MyISAM and InnoDB tables. Note that prefix limits are measured in bytes, whereas the prefix length in CREATE TABLE statements is interpreted as number of characters. Be sure to take this into account when specifying a prefix length for a column that uses a multi-byte character set.

You can also create FULLTEXT indexes. These are used for full-text searches. In MySQL 5.0, only the MyISAM storage engine supports FULLTEXT indexes and only for CHAR, VARCHAR, and TEXT columns. Indexing always takes place over the entire column and partial (prefix) indexing is not supported. See Sección 12.7, “Funciones de búsqueda de texto completo (Full-Text)” for details.

In MySQL 5.0, you can also create indexes on spatial column types. Spatial types are supported only by the MyISAM storage engine. Spatial indexes use R-trees.

The MEMORY (HEAP) storage engine uses hash indexes by default, but also supports B-tree indexes in MySQL 5.0.