There's been a lot of fun, practical work recently on optimal join algorithms! (Full disclosure, this advisor on this paper is one of Andy and my former students, so I'm slightly biased)

RPT: https://people.iiis.tsinghua.edu.cn/~huanchen/publications/r...

Like most academic work, this one builds on some work that's been done over the last few years on ways to make the Yannakakis algorithm actually practical.