primary keys and an index

[davidklonski]

Hello MySQL gurus

Assuming I create the following table:

CREATE TABLE table (
ID INT UNSIGNED NOT NULL AUTO_INCREMENT,
Column1 INT UNSIGNED NOT NULL,
Column2 VARCHAR(255) NOT NULL,
PRIMARY KEY (ID)
) ENGINE=MYISAM DEFAULT CHARSET=latin1;

Now ID is the primary key of the table which means it has a unique index on it (right?)

will adding the following have any additional effect?
CREATE index_name ON table(ID);

Or is such an index automatically created for all the primary keys in a table?

Also, putting storage issues aside, what would be the disadvantages to creating an index for every column that I use in a join, (in order to speed things up)?

thanks for the help

[nemonoman]

primary key columns are indexed automatically. In earlier versions of MySQL, you could (stupidly) reindex the same column...I don't know if later versions have been made smart enough not to do this.