I am the author of the optimization of partial sorting and selection in Clickhouse. It uses Floyd-Rivest algorithm and we tried a lot of different things back at the time, read [1]

Overall clickhouse reads blocks of fixed sizes (64k) and finds top elements and then does top of the top until it converges.

[1] https://danlark.org/2020/11/11/miniselect-practical-and-gene...