mysql index

[ragtek]

I'm teaching mysql sql now and wanted to know if i have undestood the indexpart right.

I have a query
[sql]
SELECT r_invites.*, user.username
FROM " . TABLE_PREFIX . "r_invites
LEFT JOIN " . TABLE_PREFIX . "user AS user USING(userid)
where userid = " . $vbulletin->userinfo['userid'] . " ORDER BY r_invitesid DESC
[/sql]
should now the useridcolumn have a index to be faster because its in the where part?

2. example:
[sql]
SELECT * FROM " . TABLE_PREFIX . "link where catid = ". $catid . " order by linkid DESC
[/sql]
here the catid should have one or?