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A primary index is automatically created for the primary key and ensures that the primary key is unique. You can use the primary index to retrieve and access objects from the database. The unique index is a column, or an ordered collection of columns, for which each value identifies a unique row.
A primary key index is created by default when a table is created with a primary key specified. It will match the primary key in nature, in that it will be a single-column index if the primary key is on a single column and a multi-column composite index if the primary key is a composite primary key.
When you create a table with a primary key or unique key, MySQL automatically creates a special index named PRIMARY . This index is called the clustered index. The PRIMARY index is special because the index itself is stored together with the data in the same table.
The primary key is always indexed. This is the same for MyISAM and InnoDB, and is generally true for all storage engines that at all supports indices.
According to http://dev.mysql.com/doc/refman/5.0/en/constraint-primary-key.html it would appear that this is would be implicit
Even though this was asked in 2009 figured I'd post an actual reference to the MySQL documentation on primary keys. http://dev.mysql.com/doc/refman/5.5/en/optimizing-primary-keys.html
The primary key for a table represents the column or set of columns that you use in your most vital queries. It has an associated index, for fast query performance
For MySQL 5.0 reference see: http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html
Most MySQL indexes (PRIMARY KEY, UNIQUE, INDEX, and FULLTEXT) are stored in B-trees. Exceptions are that indexes on spatial data types use R-trees, and that MEMORY tables also support hash indexes.
The primary key is implicitly indexed for both MyISAM and InnoDB. You can verify this by using EXPLAIN on a query that makes use of the primary key.
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