[mattxxx]

JOINs are just hard period. When you're operating at a large scale, you need to be thinking about exactly how to partition + index your data for the types of queries that you want to write with JOINs.

Streaming joins are so hard, that they're an anti pattern. If you're using external storage to make it work, then your architecture has probably gone really wrong or you're using streams for something that you shouldn't.