The slow query function does not go to the index

Well, there is a pit.

 

     Early in the morning for work, other people are in a hurry. The database is severely delayed, a 600 second query. Serious delay in operation.

     Under the sql statement:

      SELECT * from user where user_id=FLOOR(rand()*10000000);

       The effect is to randomly get a user.

  

analyze

         select FLOOR(rand()*10000000);

         No problem soon.

 

          SELECT * from user where user_id = 1 , no problem,

          soon

 

          SELECT * from user where user_id=FLOOR(rand()*10000000);

           

 Finally found that it is a full table scan, this function does not take the index. . I have a headache.

The random number is placed, and the application layer is generated.