什么时候是Now（）计算

Question

如果我有这样的SQL语句：Now（）的结果在完整语句中是否相同？或者，如果某些子查询需要几分钟，在几个地方有所不同？

select 
     *
from 
     table_x 
          join (select * from table_z where <different criteria and y = now() ) tablez on <joincontext>   
where 
     <different criteria> and x = now()
union 
select 
     * 
from 
     table_x 
where 
     <different criteria> and x = now()