Writing your own aggregates

In this book, you will learn about most of the built-in 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 this example, the goal is to solve a very simple problem. If a customer takes a taxi, they usually have to pay for getting in the taxi—for example, €2.50. Now, let's assume that for each kilometer, the customer has to pay €2.20. The question now is, what is the total price of a trip?

Of course, this example is simple enough to solve without a custom aggregate; however, let's see how it works. First, some test data needs to be created:

test=# CREATE TABLE t_taxi (trip_id int, km numeric);  
CREATE TABLE 
test=# INSERT INTO t_taxi  
  VALUES (1, 4.0), (1, 3.2), (1, 4.5), (2, 1.9), (2, 4.5); 
INSERT 0 5 

To create aggregates...