For most of the last couple decades, data modeling has centered around the relationships among various entities. A person has one account, but one or more phone numbers. That same person has one or more addresses (such as home and work). A person can belong to one or more groups, which can in turn contain many people.

We modeled these relationships using foreign keys and join tables, and we built queries by joining multiple tables together to produce the desired result. However, in recent years, we introduced another dimension to our data: time. Now we're interested in more than just how entities are connected, but how their relationships change over time. For example, while we previously were concerned only about a set of fixed locations associated with a person, we now have mobile phones with GPS radios in pockets and purses all over the world. This makes it possible to produce a timeline of a person's movements by marrying time and location.

Introducing time...