Horton

Re: [HACKERS] Custom compression methods (mac+lz4.h)

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Andres Freund <andres@anarazel.de>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Alexander Korotkov <a.korotkov@postgrespro.ru>, David Steele <david@pgmasters.net>, Ildus Kurbangaliev <i.kurbangaliev@gmail.com>, Dmitry Dolgov <9erthalion6@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2021-03-22T15:47:59Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Mar 22, 2021 at 10:41 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Yeah, I thought about that too, but do we want to assume that
>> VARRAWSIZE_4B_C is the correct way to get the decompressed size
>> for all compression methods?

> I think it's OK to assume this.

OK, cool.

>> (If so, I think it would be better style to have a less opaque macro
>> name for the purpose.)

> Complaining about the name of one particular TOAST-related macro name
> seems a bit like complaining about the greenhouse gasses emitted by
> one particular car.

Maybe, but that's not a reason to make it worse.  Anyway, my understanding
of that is that the really opaque names are *only* meant to be used in
this very stretch of postgres.h, ie they are just intermediate steps on
the way to the macros below them.  As an example, the only use of
VARDATA_1B_E() is in VARDATA_EXTERNAL().

> Anyway, this particular macro name was chosen, it seems, for symmetry
> with VARDATA_4B_C, but if you want to change it to something else, I'm
> OK with that, too.

After looking at postgres.h for a bit, I'm thinking that what these
should have been symmetric with is the considerably-less-terrible
names used for the corresponding VARATT_EXTERNAL cases.  Thus,
something like

s/VARRAWSIZE_4B_C/VARDATA_COMPRESSED_GET_RAWSIZE/
s/VARCOMPRESS_4B_C/VARDATA_COMPRESSED_GET_COMPRESSION/

Possibly the former names should survive and the latter become
wrappers around them, not sure.  But we shouldn't be using the "4B"
terminology anyplace except this part of postgres.h.

			regards, tom lane

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.