tl;dr:Justin provides a deep dive into SQL's foundational aspects, highlighting the importance of a formalized approach to query behavior. Using the example of "predicate pushdown," Justin presents a SQL query that behaves differently across databases like DuckDB, SQLite, and CockroachDB. "I think it's a fun little mind bender that gives you some insight into the internals of these databases query engines without having to actually look at any code."
tl;dr:“Relational inner joins are really common in the world of databases, and one weird thing about them is that it seems like everyone has a different idea of what they are. In this post I’ve aggregated a bunch of different definitions, ways of thinking about them, and ways of implementing them that will hopefully be interesting. They’re not without redundancy, some of them are arguably the same, but I think they’re all interesting perspectives nonetheless.”
tl;dr:"The count-distinct problem is to estimate the number of distinct elements appearing in a stream. That is, given some enumeration of “objects,” which you can think of as any data type you like, we want to know approximately how many unique objects there are."