Re: pglz performance

Hi,

On 2019-08-02 20:40:51 +0500, Andrey Borodin wrote:
> We have some kind of "roadmap" of "extensible pglz". We plan to provide implementation on Novembers CF.

I don't understand why it's a good idea to improve the compression side
of pglz. There's plenty other people that spent a lot of time developing
better compression algorithms.


> Currently, pglz starts with empty cache map: there is no prior 4k bytes before start. We can add imaginary prefix to
anydata with common substrings: this will enhance compression ratio.
 
> It is hard to decide on training data set for this "common prefix". So we want to produce extension with aggregate
functionwhich produces some "adapted common prefix" from users's data.
 
> Then we can "reserve" few negative bytes for "decompression commands". This command can instruct database on which
commonprefix to use.
 
> But also system command can say "invoke decompression from extension".
> 
> Thus, user will be able to train database compression on his data and substitute pglz compression with custom
compressionmethod seamlessly.
 
> 
> This will make hard-choosen compression unneeded, but seems overly hacky. But there will be no need to have lz4,
zstd,brotli, lzma and others in core. Why not provide e.g. "time series compression"? Or "DNA compression"? Whatever
gunuser wants for his foot.
 

I think this is way too complicated, and will provide not particularly
much benefit for the majority users.

In fact, I'll argue that we should flat out reject any such patch until
we have at least one decent default compression algorithm in
core. You're trying to work around a poor compression algorithm with
complicated dictionary improvement, that require user interaction, and
only will work in a relatively small subset of the cases, and will very
often increase compression times.

Greetings,

Andres Freund