For the past four months, I've been working with the incredible SQL Execution team at Cockroach Labs as a backend engineering intern to develop the first prototype of a batched, column-at-a-time execution engine. During this time, I implemented a column-at-a-time hash join operator that outperformed CockroachDB's existing row-at-a-time hash join by 40x. In this blog post, I'll be going over the philosophy, challenges, and motivation behind implementing a column-at-a-time SQL operator in general, as well as some specifics about hash join itself.