After reading Fernando Ipar’s interesting post on partial indexes for string columns, there were two things I wanted to note:
First, this trick works quite well, but only if your like clauses only ever use the wildcard on the right hand side (or not at all). MySQL will not be able to use the index if the like contains a wildcard on the left.
Consider the following table definition:
mysql> show create table people\G
*************************** 1. row
***************************
Table: people
Create Table: CREATE TABLE `people` (
`person_id` int(15) NOT NULL default '0',
`username` varchar(255) default NULL,
`email` varchar(255) default NULL,
PRIMARY KEY (`person_id`),
KEY `people_username` (`username`(5))
) ENGINE=MyISAM DEFAULT CHARSET=latin1
1 row …