Secondary indexes

A secondary index, commonly referred to as an index in the context of a relational database, is a structure allowing efficient lookup of records by some attribute other than their primary key. This is a widely useful capability; for instance, when developing a blog application, you would want to be able to easily retrieve all of the posts written by a particular author. Cassandra supports secondary indexes; while Cassandra's version is not as versatile as indexes in a typical relational database, it's a powerful feature in the right circumstances. A query on a secondary index can perform poorly in certain cases; hence, it should be used sparingly and only in certain cases, which we will touch upon later in Chapter 5, Establishing Relationships.