Also,

> `repo_name` LowCardinality(String),

This is not a low cardinality:

7133122498 = 7.1B

Don't use low cardinality for such columns!

This is an ad-hoc query. It does a full scan, processing slightly less than a billion rows per second on a single machine, and finishes in a reasonable time with over 7 billion events on GitHub from 2015. While it does not make sense to optimize this table for my particular query, the fact that it works well for arbitrary queries is worth noting.