To get a consistent view across multiple queries you just use the SERIALIZABLE isolation level. In PostgreSQL REPEATABLE READ also works, but the standard does not guarantee this (the standard allows for ghost reads since it assume you use locking rather than MVCC snapshots to implement REPEATABLE READ).
The ability to query any historical view of the data is indeed not there in PostgreSQL in any simple or reliable way. That is an advantage of Datomic, but I do not see why it would be impossible to implement in a "traditional database".
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To get a consistent view across multiple queries you just use the SERIALIZABLE isolation level. In PostgreSQL REPEATABLE READ also works, but the standard does not guarantee this (the standard allows for ghost reads since it assume you use locking rather than MVCC snapshots to implement REPEATABLE READ).
The ability to query any historical view of the data is indeed not there in PostgreSQL in any simple or reliable way. That is an advantage of Datomic, but I do not see why it would be impossible to implement in a "traditional database".