"The only way to have such a versioned history is ..." is incorrect, and trivially so. Datomic provides one existence proof of this: Datomic's history data is

"The only way to have such a versioned history is ..." is incorrect, and trivially so. Datomic provides one existence proof of this: Datomic's history data is kept in distinct data structures, so it is "pay as you go" -- querying history is more expensive, because there is more stuff. Querying the present is cheaper. Datomic queries are datalog, and do not require writing map/reduce jobs.