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.
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...