MySQL查看索引是否有效

EXPLAIN
SELECT
   wtt.uuid,wtt.i,wtt.s,wtt.ua,wtt.aid,wtt.uid,wtt.`from`,wtt.gsid,rand
FROM web_target_t wtt
WHERE wtt.is_vip = 0 AND wtt.is_blue = 0 AND wtt.is_star = 0 AND invalid_state=0
   AND wtt.urank>=10
   AND wtt.urank>20
   AND wtt.rand<100000
ORDER BY `rand` limit 10;

type                          range

possible_keys          rand,idx_distribute 

rows                        254