Requests should be kept alive. For any system, if the requester goes away, eventually the system should stop doing work on their behalf. That seems like the root of the problem in the situation you're describing.
As for "if the requester goes away", remember that the requester might be a few hops away. E.g., the HTTP connection from the mobile client drops; the web server and its connection to the DB is still alive and well. I can forcefully shut that connection, but that's somewhat of a drag (I'd rather keep it open, since it is perfectly good).
Beyond closing the connection, and being able to issue some form of "cancel this query" request: HTTP/1 lacks it entirely, PostgreSQL requires opening a separate connection, Redis lacks it entirely, and I think both Mongo and MySQL lack it entirely.
Even support for "time this request out" is spotty.
MySQL has a kill command, which does need to be done on another connection (might also need more permissions, it's been a while). It's been a while since I used a lot of MySQL, but this was definitely a pain point when things went sideways.