Yep, it currently limits you to a 10G database size. Makes sense, though - SQL techniques like LIKE filters or joins really do require all of your data to be in memory with a very high-bandwidth, low latency interconnect - ie, it all has to be in memory on a single box.

This is the reason the datastore forbids these operations - because they're extremely difficult to efficiently implement and still scale indefinitely (without making other, potentially very large sacrifices).