If you've got a serious need for speed, in-memory databases deliver the fastest possible data access available today. In-memory databases are gaining popularity among organizations struggling to keep up with fast transactions and other forms of high velocity streaming data.
When SQL databases were initially designed, memory was expensive, so rotating magnetic disks in the form of mechanical disk drives became the primary storage device for databases. While disk-based storage is still used and relied upon, the steadily decreasing price of RAM has the potential to make mechanical disks obsolete as the primary data storage layer for operational systems.
However, not all in-memory databases are the same. Some in-memory RDBMS technologies provide much higher performance than others when compared to traditional disk-based products. How can that be? It really depends on how the in-memory product was designed -- was it created from the ground up to run in-memory, or was it a simple migration of a disk-based product to in-memory?