MySQL or使索引失效

问题遇到的现象和发生背景

请问这句MySQL怎么去掉 or ，因为删掉or b.bname = "" explain显示用到了索引，查询很快，但不删掉就是all没用到索引，查询要5s，怀疑是or使索引失效
select bid from book a ,querybook b where ( a.bname in (b.bname) or b.bname = "" ) and
(a.bauthor in (b.bauthor) or b.bauthor = "" )