explain 
The SELECT s.i d AS stuId, u.f irst_name AS firstName, u.m obile AS mobile, s.u ser_id AS studentUserId 
The FROM students s 
Inner join the users u on s.u ser_id=u.i d 
Where s.i d=1990000 XXXX 
The or u.m obile='1990000 XXXX' 
Limit 1; 

Perform special slow:
Table structure: students' id primary key table, user_id only indexes; The users table id primary key, mobile as the only index; Why students table useless to the id primary