A hybrid database is a database system that supports and uses both on-disk and in-memory data storage. Hybrid databases are used when the system needs high performance with the small footprint that only in-memory database systems can provide. This also provides the added benefits of durability and low cost of disk-based database systems. In short, the system makes use of hard disks for saving and retaining data, yet makes use of the memory for data that is in dynamic use to increase performance.
Techopedia explains Hybrid Database
Because hybrid databases support both in-memory and on-disk storage, the obvious benefit is flexibility. The developer can then strike a balance between performance, cost and persistence.
The benefits of a hybrid database include:
Performance: This is the in-memory part of the equation. Sorting, storing and retrieving specific in-use data entirely from memory rather than from disks makes all processes substantially quicker.
Cost: Hard disks cost less than memory, so some of the money saved can be used to add more memory to improve performance.
Persistence: Because RAM chips cannot get close to the storage density of a hard drive, disks are still used to store data needed for later use. This ensures that they are not lost in the event of a power outage.