Hybrids With On-disk Databases
The first database engine to support both in-memory and on-disk tables in a single database was released in 2003. The advantage to this approach is flexibility: the developer can strike a balance between performance (which is enhanced by sorting, storing and retrieving specified data entirely in memory, rather than going to disk); cost, because a less costly hard disk can be substituted for more memory; persistence; and form factor, because RAM chips cannot approach the density of a small hard drive.
Manufacturing efficiency is another reason a combined in-memory/on-disk database system may be chosen. Some device product lines, especially in consumer electronics, include some units with permanent storage, and others that rely on memory for storage (set-top boxes, for example). If such devices require a database system, a manufacturer can adopt a hybrid database system at lower and upper cost, and with less code customization, than using separate in-memory and on-disk databases, respectively, for its disk-less and disk-based products.
Read more about this topic: In-memory Database