Great question. Yes, eventually it does, but (at least for now) it wasn't something we could reliably force as part of the query cycle and know everything was in it's "best" state with ClickHouse. To be honest, we didn't provide the final compressed size of either database because of the need to wait.

The code that's currently used by TSBS was submitted by Altinity, a heavy supporter of ClickHouse in the U.S., but TSBS is open source and anyone is welcome to contribute and make the process/test better!

Concerning Clickhouse, yes it does - exactly the same thing e.g. as when you have 2 compressed files containing each 100 sorted rows, when you merge those 200 rows into a single file, sort them and compress them, the result will be smaller than the sum of the 2 separate files.

How much you save is again exactly the same as when dealing directly with files: it depends on the data and on the compression algo.