求sql的优化

[soktest]

SELECT b.ads_title ads_title, b.ads_comment1 ads_comment1, b.ads_comment2, ads_comment2, b.ads_show_url ads_link, a.key_id key_id, a.key_name key_name, b.ads_id ads_id FROM key_mstr a, ads_mstr b, usr_mstr c WHERE a.key_ads_id = b.ads_id AND b.ads_userid = c.usr_userid AND c.usr_money >0 AND b.ads_state =1 and a.key_name='"+querywords+"' ORDER BY rand( ) LIMIT ";

我应该怎么建立index呢？ 有三个表ads_mstr, key_mstr,usr_mstr