Eren Basak

SQL is a very powerful language for analyzing and reporting against data. At the core of SQL is the idea of joins and how you combine various tables together. One such type of join: outer joins are useful when we need to retain rows, even if it has no match on the other side.

And while the most common type of join, inner join, against tables A and B would bring only the tuples that have a match for both A and B, outer joins give us the ability to bring together from say all of table A even if they don’t have a corresponding match in table B. For example, let's say you keep customers in one table and purchases in another table. When you want to see all purchases of customers, you may want to see all customers in the result even if they did not do any purchases yet. Then, you need an outer join. Within this post we’ll analyze a bit on what outer joins are, and then how we support them in a distributed fashion on Citus.

PostgreSQL is known for its great extensibility and powerful plugins. One particular category of extensions is Foreign Data Wrappers or FDWs. FDWs allow us to interact from within Postgres directly with other data stores such as hdfs, columnar stores, mysql, etc. Combined with Citus’ scalability features, we can even leverage them to help us scale out those data stores where might otherwise be quite difficult.