Q:

How many rows can a class in Parse.com realistically handle?

I know that Parse.com has some sort of limit on the number of seconds a request/query can take. Correct me if I'm wrong.  Cloud code should typically finish within 7 seconds, for example.
If I have a Parse class with 70 million entities, and I run a query, how long would that take? Is it realistic to expect Parse to work well for that kind of data?

A:

Parse is built on MongoDB, so you should look into its limitations as they should be shared.
From experience (and yours may be different) Parse cannot handle .count or .find operations on a class whose size is above ~80 million, timeouts are basically guaranteed. Response times start to slow down noticeably around 70M, if not earlier. Most of the issues you'll experience depend on the complexity of the requests you're making. Keep it simple.
I believe one solution to these timeouts or slow responses is to .get or .fetch Objects instead of running .find or .first queries for them.
In some cases it is worth reconsidering your current schema or database structure because an "oversized data table issue" is not necessary specific to Parse. Best of luck!

