I never heard of a database that doesn't reside in memory if you have enough ram. Most of the time, it's the OS making this happen by maintaining a cache for disk reads, and flushing writes to disk asynchronously.
Does MemSQL just have more compact data structures that let more stuff fit in memory?
My guess is if you design your database to reside in RAM, you can use pointers in the data structures. You also can drop all layout optimizations that are meant for linear access (optimize for random access instead).
Does MemSQL just have more compact data structures that let more stuff fit in memory?