mysql 5.6 is Not referring Indexes for deterministic function DATEDIFF
Reason for indexes not being used is table column is an input part of the function.
Current SQL Query

select *
 from   users
 where  DATEDIFF(users.created_at,'2019-05-05 23:11:56.547') =  0;


Indexes will be used if the query is in below way

select *
 from   users
 where  users.created_at =  DATE_ADD('2019-05-05 23:11:56.547', INTERVAL 0 DAY);


since above refers to a point of time. To check day we can have to use range timestamp start of today to start off tomorrow,

select *
 from   users
 where  users.created_at between DATE_ADD('2019-05-05 ', INTERVAL 0 DAY)
and DATE_ADD('2019-05-05 ', INTERVAL 1 DAY);