Why we lost Uber as a user
Does anyone else think the scenario in the explanation is an unreasonable request to make of a relational database?  I think that if you've created a design that requires you to update a 50K row table 500 times a second that itself is heavily indexed and used heavily in joins, you have a software design problem more than a database problem.  I wouldn't expect any database to handle that and am surprised that mysql does.  One has to ask:  for how long will it work?  Surely the clock is running out on such a design.