Horton

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

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Robert Haas <robertmhaas@gmail.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: 2021-03-21T23:43:24Z
Lists: pgsql-hackers 
On Sun, Mar 21, 2021 at 07:11:50PM -0400, Tom Lane wrote:
> I wrote:
> > Justin Pryzby <pryzby@telsasoft.com> writes:
> >> On Sun, Mar 21, 2021 at 04:32:31PM -0400, Tom Lane wrote:
> >>> This seems somewhat repeatable (three identical failures in three
> >>> attempts).  Not sure why I did not see it yesterday; but anyway,
> >>> there is something wrong with partial detoasting for LZ4.
> 
> >> With what version of LZ4 ?
> 
> > RHEL8's, which is
> > lz4-1.8.3-2.el8.x86_64
> 
> I hate to be the bearer of bad news, but this suggests that
> LZ4_decompress_safe_partial is seriously broken in 1.9.2
> as well:
> 
> https://github.com/lz4/lz4/issues/783

Ouch

> Maybe we cannot rely on that function for a few more years yet.
> 
> Also, I don't really understand why this code:
> 
> 	/* slice decompression not supported prior to 1.8.3 */
> 	if (LZ4_versionNumber() < 10803)
> 		return lz4_decompress_datum(value);
> 
> It seems likely to me that we'd get a flat out build failure
> from library versions lacking LZ4_decompress_safe_partial,
> and thus that this run-time test is dead code and we should
> better be using a configure probe if we intend to allow old
> liblz4 versions.  Though that might be moot.

The function existed before 1.8.3, but didn't handle slicing.
https://github.com/lz4/lz4/releases/tag/v1.8.3
|Finally, an existing function, LZ4_decompress_safe_partial(), has been enhanced to make it possible to decompress only the beginning of an LZ4 block, up to a specified number of bytes. Partial decoding can be useful to save CPU time and memory, when the objective is to extract a limited portion from a larger block.

Possibly we could allow v >= 1.9.3 || (ver >= 1.8.3 && ver < 1.9.2).

Or maybe not: the second half apparently worked "by accident", and we shouldn't
need to have intimate knowledge of someone else's patchlevel releases, 

-- 
Justin

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.