The difference between pessimistic and optimistic detection is that in pessimistic detection, you detect conflicts on every load and store within any transaction and abort/stall the transaction if there is a conflict. In optimistic detection, you wait until a transaction is committing and then when it is, if there are any conflicts, you continue committing the transaction but other transactions may abort later on because they now have conflicts.

With eager versioning, it is not practical to use optimistic detection at the save time as the eager versioning will directly change data in the physical memory and conflict transactions with read will "see" the updated value, violating its atomic property.