Citus Blog

In an earlier blog post I wrote about how breaking problems down into a MapReduce style approach can give you much better performance. We've seen Citus is orders of magnitude faster than single node databases when we're able to parallelize the workload across all the cores in a cluster. And while count (*) and avg is easy to break into smaller parts I immediately got the question what about count distinct, or the top from a list, or median?

Exact distinct count is admittedly harder to tackle, in a large distributed setup, because it requires a lot of data shuffling between nodes. Count distinct is indeed supported within Citus, but at times can be slow when dealing with especially larger datasets. Median across any moderate to large size dataset can become completely prohibitive for end users. Fortunately for nearly all of these there are approximation algorithms which provide close enough answers and do so with impressive performance characteristics.

For those considering Citus, if your use case seems like a good fit, we often are willing to spend some time with you to help you get an understanding of the Citus database and what type of performance it can deliver. We commonly do this in a roughly two hour pairing session with one of our engineers. We'll talk through the schema, load up some data, and run some queries. If we have time at the end it is always fun to load up the same data and queries into single node Postgres and see how we compare. After seeing this for years, I still enjoy seeing performance speed ups of 10 and 20x over a single node database, and in cases as high as 100x.

And the best part is it didn't take heavy re-architecting of data pipelines. All it takes is just some data modeling, and parallelization with Citus.

Extensions are capable of extending, changing, and advancing the behavior of Postgres. How? By hooking into low level Postgres API hooks. The open source Citus database that scales out Postgres horizontally is itself implemented as a PostgreSQL extension, which allows Citus to stay current with Postgres releases without lagging behind like other Postgres forks. I've previously written about the various types of extensions, today though I want to take a deeper look at the most useful Postgres extension: pg_stat_statements.