You mentioned OLTP. Erlang's Mnesia store comes to mind, but as far as I'm aware it's limited to a 4GB data set. I'm not sure if that qualifies as a main-memory

You mentioned OLTP. Erlang's Mnesia store comes to mind, but as far as I'm aware it's limited to a 4GB data set. I'm not sure if that qualifies as a main-memory db, but it might be similar.