MySQL performance

select * 
NOT IN
!=	//index失効
<>	//index失効
is null	//index失効, 方法:NOT NULL, default=0, where xxx=0
where OR //index失効失効 
	改善前: 
	select id from t where num=10 or num=20
    改善後:
    select id from t where num=10
     union all
    select id from t where num=20

左側の計算式 OR function //index失効
	例
	select id from t where num/2=100
    改善案:
    select id from t where num=100*2

^--------
exists is better then in ???