> When you compare a distributed database that uses immutable data against PostgreSQL, the thing that is immediately apparent to me is that garbage collection is much more difficult in the distributed setting. You can't just rewrite the network interface for PostgreSQL and get datomic, but you might be able to get single-server datomic.

PostgeSQL-XC solves this problem by adding a global transaction management server. I guess it does the same thing as the transactor for Datomic so there really is no major difference here.