Re: libpq compression

On 22.12.2020 22:03, Tom Lane wrote:
> Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> writes:
>> I don't see aby benchmark results in this thread, allowing me to make
>> that conclusion, and I find it hard to believe that 200MB/client is a
>> sensible trade-off.
>> It assumes you have that much memory, and it may allow easy DoS attack
>> (although maybe it's not worse than e.g. generating a lot of I/O or
>> running expensive function). Maybe allowing limiting the compression
>> level / decompression buffer size in postgresql.conf would be enough. Or
>> maybe allow disabling such compression algorithms altogether.
> The link Ken pointed at suggests that restricting the window size to
> 8MB is a common compromise. It's not clear to me what that does to
> the achievable compression ratio. Even 8MB could be an annoying cost
> if it's being paid per-process, on both the server and client sides.
>
> regards, tom lane
Please notice that my original intention was to not give to a user
(client) possibility to choose compression algorithm and compression
level at all.
All my previous experiments demonstrate that using compression level
larger than default only significantly decrease speed, but not compression
ratio.  Especially for compressing of protocol messages.
Moreover, on some dummy data (like generated by pgbench) zstd with
default compression level (1) shows better compression ratio
than with higher levels.

I have to add possibility to specify compression level and suggested
compression algorithms because it was requested by reviewers.
But I still think that it was wrong idea and this results just prove
prove it.
More flexibility is not always good...

Now there is a discussion concerning a way to switch compression
algorithm on the fly (particular case: toggling compression for
individual ibpq messages). IMHO it is once again excessive flexibility
which just increase complexity and gives nothing good in practice).