> If dictionary is an int i, it is functionally equivalent to zstd_decompress(data, (select dict from _zstd_dict where id = i)).

Compact saves you 8 bytes per compressed column by removing some metadata zstd uses :

> if compact is true, the output will be without magic header, without checksums, and without dictids. This will save 4 bytes when not using dictionaries and 8 bytes when using dictionaries. this also means the data will not be decodeable as a normal zstd archive with the standard tools. The same compact argument must also be passed to the decompress function.

You will need to update the dicts occasionally, but there's a helper function to do this for you, read the docs about zstd_incremental_maintenance.