In many OLTP systems, almost all work operates on multiple attributes of a single record. E.g., when logging a user in, an authentication system cares about mul

In many OLTP systems, almost all work operates on multiple attributes of a single record. E.g., when logging a user in, an authentication system cares about multiple attributes of a single user record, not facts about the aggregate pool of users. Column oriented stores are extremely efficient for aggregate queries, but they make writes and single-row reads more expensive and are thus not suitable for every workload. There's an excellent overview in Martin Kleppmann's Designing Data Intensive Applications.