One advantage of a relational database is that users do not need to know anything about the physical structure of the database.
They can access all the data by simply using Structured Query Language (SQL). However, underneath the logical structures manipulated by SQL, the data in your Oracle database is still stored on a disk. And, as with all data on disk, the performance of data retrieval is dependent to some extent on the physical movement of the disk head over the spinning disk. If you can minimize this disk head movement, you minimize the single largest bottleneck in data retrieval. Oracle allows you to create clustered tables and indexes, whose primary purpose is to minimize the inherent delays caused by disk head movement. This module covers all you need to know about clusters, including:

1. The advantages of clustering
2. How to create and size a cluster
3. How to create a cluster key
4. The purpose of a hash cluster
5. How to create a hash cluster
6. How to drop a cluster

Although clusters are not appropriate for all types of data, in the right situation a cluster can provide a significant increase in performance.
In the next lesson you will learn when to use clustered tables.