Horton

Re: [HACKERS] Custom compression methods

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Alexander Korotkov <a.korotkov@postgrespro.ru>, David Steele <david@pgmasters.net>, Ildus Kurbangaliev <i.kurbangaliev@gmail.com>, Dmitry Dolgov <9erthalion6@gmail.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2020-06-23T18:27:47Z
Lists: pgsql-hackers 
On Mon, Jun 22, 2020 at 4:53 PM Andres Freund <andres@anarazel.de> wrote:
> > Or maybe we add 1 or 2 "privileged" built-in compressors that get
> > dedicated bit-patterns in the upper 2 bits of the size field, with the
> > last bit pattern being reserved for future algorithms. (e.g. 0x00 =
> > pglz, 0x01 = lz4, 0x10 = zstd, 0x11 = something else - see within for
> > details).
>
> Agreed. I favor an approach roughly like I'd implemented below
> https://postgr.es/m/20130605150144.GD28067%40alap2.anarazel.de
> I.e. leave the vartag etc as-is, but utilize the fact that pglz
> compressed datums starts with a 4 byte length header, and that due to
> the 1GB limit, the first two bits currently have to be 0. That allows to
> indicate 2 compression methods without any space overhead, and
> additional compression methods are supported by using an additional byte
> (or some variable length encoded larger amount) if both bits are 1.

I think there's essentially no difference between these two ideas,
unless the two bits we're talking about stealing are not the same in
the two cases. Am I missing something?

> One additional note: Adding additional vartag_external values does incur
> some noticable cost, distributed across lots of places.

OK.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. docs: Update TOAST storage docs for configurable compression.

  2. Further tweaking of pg_dump's handling of default_toast_compression.

  3. Fix interaction of TOAST compression with expression indexes.

  4. Tidy up more loose ends related to configurable TOAST compression.

  5. Short-circuit slice requests that are for more than the object's size.

  6. Mostly-cosmetic adjustments of TOAST-related macros.

  7. Remove useless configure probe for <lz4/lz4.h>.

  8. Error on invalid TOAST compression in CREATE or ALTER TABLE.

  9. docs: Fix omissions related to configurable TOAST compression.

  10. More code cleanup for configurable TOAST compression.

  11. Bring configure support for LZ4 up to snuff.

  12. Make compression.sql regression test independent of default.

  13. Use valid compression method in brin_form_tuple

  14. Fix up pg_dump's handling of per-attribute compression options.

  15. Allow configurable LZ4 TOAST compression.

  16. Fix inconsistencies in the code

  17. Mostly-cosmetic improvements in memory chunk header alignment coding.

  18. Allow numeric to use a more compact, 2-byte header in many cases.