sql优化问题，求大神啊sos

Question

需求是这样的，表有两个字段a、b，b是时间字段，数据量500w
我通过对a分组取最大时间，效率好慢，求优化，
Select a.featid,a.sj From temp a Where a.sj=(Select Max(sj) From temp Where sj=a.sj)
Select a.featid, max(a.sj) as sj From temp a Group By a.featid

Answer 1

select a.featid from temp a where 1>(select count(sj) from temp where featid=a.featid and sj>a.sj group by featid,sj);