测试环境：
CPU:              Intel(R) Pentium(R) 4 3.0G （单核双线程）
内存:             1.5G
操作系统：    Window XP SP2
PHP:              5.2.6
MYSQL:         5.0.51b-community

测试过程中使用的表结构
CREATE TABLE `mysqlchar` (
`username` char(32) default NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

CREATE TABLE `mysqlchar` (
`username` varchar(32) default NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

CREATE TABLE `mysqlchar` (
`username` char(32) default NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `mysqlchar` (
`username` varchar(32) default NULL
) ENGINE= InnoDB DEFAULT CHARSET=utf8;

0;$i--){
		$result .= $str{mt_rand(0,62)};
	}
	return $result;
}
function insert(){
	$dsn = 'mysql:dbname=test;host=127.0.0.1';
	$user = 'root';
	$password = 'xxxxx';
        $dbh = new PDO($dsn, $user, $password);
	$sth = $dbh->prepare('INSERT INTO mysqlchar(`username`)
		VALUES(?)');
	echo microtime();
	echo '    ';
	for($i=0;$i<20;$i++){
		$sth->execute(array(getRandom(mt_rand(6,32))));
	}
	echo microtime();
}
function select(){
	$dsn = 'mysql:dbname=test;host=127.0.0.1';
	$user = 'root';
	$password = 'xxxxxx';
        $dbh = new PDO($dsn, $user, $password);
	$sth = $dbh->prepare('SELECT username from mysqlchar
		WHERE username = ?');
	echo microtime();
	echo '    ';
	$sth->execute(array('BOklEnL2onF'));
	echo microtime();
}
function addKey(){
	$dsn = 'mysql:dbname=test;host=127.0.0.1';
	$user = 'root';
	$password = 'xxxxx';
        $dbh = new PDO($dsn, $user, $password);
	echo microtime();
	echo '    ';
	$dbh->query('ALTER TABLE mysqlchar ADD KEY test(`username`)');
	echo microtime();
}
//insert();
//select();
//addKey();
?>

测试结果(单位：秒)