CREATE TABLE statement explanation

[misc1]

Hello,
I found an article which is describing how to develop site engine with PHP supported with MYSQL database.
Since I'm new to php and mysql I have a question on CREATE TABLE statemet that was used in that article.
What I don't quite understand is what is the purpose of UNIQUE KEY `bl_ID` (`bl_ID`) and KEY `bl_ID_2` (`bl_ID`) lines in statements below:

# Table: 'block_location'
#
CREATE TABLE `block_location` (
`bl_ID` int(11) NOT NULL auto_increment,
`block_ID` int(11) default '0',
`block_row` int(11) default '0',
`block_col` int(11) default '0',
`block_page` varchar(255) default 'all',
`site_ID` int(11) default '1',
`user_ID` int(11) default '0',
PRIMARY KEY (`bl_ID`),
UNIQUE KEY `bl_ID` (`bl_ID`),
KEY `bl_ID_2` (`bl_ID`)
) TYPE=MyISAM;


# Table: 'blocks'
#
CREATE TABLE `blocks` (
`block_ID` int(11) NOT NULL auto_increment,
`block_title` varchar(255) default 'Block Title',
`block_file` varchar(255) default '0',
`plugin_ID` int(11) default '0',
`group_ID` int(11) default '0',
`site_ID` int(11) default NULL,
`mod_ID` int(11) default NULL,
PRIMARY KEY (`block_ID`),
UNIQUE KEY `block_ID` (`block_ID`),
KEY `block_ID_2` (`block_ID`)
) TYPE=MyISAM;

Thanks
Misc

 

[TonyGroves]

You have three indexes on each primary-key field, two of which are superfluous. You should get rid of them both by (in the case of the 'blocks' table):[tt]
ALTER TABLE blocks
DROP KEY block_id,
DROP KEY block_id_2[/tt]
and the same for the 'block_location' table.

Having unnecessary indexes is harmless, but it makes updates slower.