You can provide hints to give the optimizer information about how to choose indexes during query processing. Section 12.2.8.1, “JOIN Syntax”, describes the general syntax for specifying tables in a SELECT statement. The syntax for an individual table, including that for index hints, looks like this:

tbl_name [[AS] alias] [index_hint]

index_hint:
    USE {INDEX|KEY} [FOR JOIN] (index_list)
  | IGNORE {INDEX|KEY} [FOR JOIN] (index_list)
  | FORCE {INDEX|KEY} [FOR JOIN] (index_list)

index_list:
    index_name [, index_name] ...

By specifying USE INDEX (index_list), you can tell MySQL to use only one of the named indexes to find rows in the table. The alternative syntax IGNORE INDEX (index_list) can be used to tell MySQL to not use some particular index or indexes. These hints are useful if EXPLAIN shows that MySQL is using the wrong index from the list of possible indexes.

You can also use FORCE INDEX, which acts like USE INDEX (index_list) but with the addition that a table scan is assumed to be very expensive. In other words, a table scan is used only if there is no way to use one of the given indexes to find rows in the table.

Each hint requires the names of indexes, not the names of columns. The name of a PRIMARY KEY is PRIMARY. To see the index names for a table, use SHOW INDEX.

An index_name value need not be a full index name. It can be an unambiguous prefix of an index name. If a prefix is ambiguous, an error occurs.

USE INDEX, IGNORE INDEX, and FORCE INDEX affect only which indexes are used when MySQL decides how to find rows in the table and how to do the join. They do not affect whether an index is used when resolving an ORDER BY or GROUP BY clause. As of MySQL 5.0.40, the optional FOR JOIN clause can be added to make this explicit.

Examples:

SELECT * FROM table1 USE INDEX (col1_index,col2_index)
  WHERE col1=1 AND col2=2 AND col3=3;

SELECT * FROM table1 IGNORE INDEX (col3_index)
  WHERE col1=1 AND col2=2 AND col3=3;

For FULLTEXT searches, index hints do not work before MySQL 5.0.74. As of 5.0.74, index hints work as follows:

Index hints are accepted but ignored for UPDATE statements.