Writing your own aggregates

In this book you have most of the onboard functions provided by PostgreSQL. However, what SQL provides might not be enough for you. The good news is that it is possible to add your own aggregates to the database engine. In this section you will learn how that can be done.

Creating simple aggregates

For the purpose of this example the goal is to solve a very simple problem: if you take a taxi, you usually have to pay for getting in the taxi (for example, 2.50 EUR). Then let us assume that for each kilometer the customer has to pay 2.20 EUR. The question now is: what is the total price of a trip?

Of course, this example is simple enough to solve the problem without a custom aggregate, however, let's see how it works. First some test...