Partial aggregation: The beautiful way

Partial aggregation: Aggregations are a fundamental feature of SQL. Functions such as min, max, count, sum, avg and so on are used in virtually every application - it is basically impossible to write a reasonably complex applications without them.

Here is a trivial aggregate: All it does is summing up numbers from 1 to 10:

So far so good. But what if we only want to sum up even numbers? We can rely on a simple trick: NULL values are ignored by aggregation functions. This makes a lot of sense because NULL really means “undefined” and you cannot average or sum up undefined values.  A simple CASE WHEN can do the job:

This works perfectly - however, it is a bit old fashioned and pretty nasty to write (especially in case of more complex operations).

The fancy way to partial aggregation

There is a more fancy way to do that:

The FILTER keywords allow users to tell the systems which rows should make it into the aggregate function. The new syntax is far more readable, as well as a lot shorter.

Find out more about partial aggregation in PostgreSQL 16 in this blog post about Parallel Aggregates by Pavlo Golub.

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