Re: pglz performance

Hi,

On 2019-08-04 02:41:24 +0200, Petr Jelinek wrote:
> Same here.
>
> Just so that we don't idly talk, what do you think about the attached?

Cool!

> It:
> - adds new GUC compression_algorithm with possible values of pglz (default)
> and lz4 (if lz4 is compiled in), requires SIGHUP

As Tomas remarked, I think it shouldn't be SIGHUP but USERSET. And I
think lz4 should be preferred, if available. I could see us using a
list style guc, so we could set it to lz4, pglz, and the first available
one would be used.

> - adds 1 byte header to the compressed data where we currently store the
> algorithm kind, that leaves us with 254 more to add :) (that's an extra
> overhead compared to the current state)

Hm. Why do we need an additional byte? IIRC my patch added that only
for the case we would run out of space for compression formats without
extending any sizes?

> - changes the rawsize in TOAST header to 31 bits via bit packing
> - uses the extra bit to differentiate between old and new format

Hm. Wouldn't it be easier to just use a different vartag for this?

> - I expect my changes to configure.in are not the greatest as I don't have
> pretty much zero experience with autoconf

FWIW the configure output changes are likely because you used a modified
version of autoconf. Unfortunately debian/ubuntu ship one with vendor
patches.

Greetings,

Andres Freund