Why there are both index and key in some cases?

[Migrated From Jforum.net]

Hi, all.

Can some one explain to me why in the schema we have both index and key for the same column, for example, in table jforum_forums, we have:
-----------------------------------------------------------------------
CREATE TABLE jforum_forums (
forum_id INT NOT NULL auto_increment,
categories_id INT NOT NULL default '1',
forum_name varchar(150) NOT NULL default '',
forum_desc varchar(255) default NULL,
forum_order INT default '1',
forum_topics INT NOT NULL default '0',
forum_last_post_id INT NOT NULL default '0',
moderated TINYINT(1) DEFAULT '0',
PRIMARY KEY (forum_id),
KEY (categories_id),
INDEX idx_forums_cats (categories_id)
) TYPE=InnoDB;
-------------------------------------------------------------------
We have key and index for categories_id, and they actually are all for index. Can some one explain why we have both of them?

Thank you very much.


David

[originally posted on jforum.net by davidxia]

[Migrated From Jforum.net]

Probably historical inertia... e.g., the original create script from table code added it and it never got cleaned up.
[originally posted on jforum.net by monroe]