Horton

Re: [HACKERS] Custom compression methods

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, 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>, pgsql-hackers@lists.postgresql.org
Date: 2021-03-01T05:36:17Z
Lists: pgsql-hackers 
On Mon, Mar 01, 2021 at 10:32:23AM +0530, Dilip Kumar wrote:
> On Sun, Feb 28, 2021 at 9:48 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
> >
> > On my PC, this new change is causing a test failure:
> >
> >  SELECT SUBSTR(f1, 2000, 50) FROM cmdata1;
> > -                       substr
> > -----------------------------------------------------
> > - 01234567890123456789012345678901234567890123456789
> > -(1 row)
> > -
> > +ERROR:  compressed lz4 data is corrupt
> 
> The older version of lz4 had this problem that while decompressing
> partial if we don't give the buffer size up to full data length it was
> failing[1] but it is solved in version 1.9.

Thanks.  It seems like that explains it.                                                                                                                                                                                           
I think if that's a problem with recent versions, then you'll have to
conditionally disable slicing.
https://packages.debian.org/liblz4-dev                                                                                                                                                                                             

Slicing isn't generally usable if it sometimes makes people's data inaccessible
and gives errors about corruption.

I guess you could make it a compile time test on these constants (I don't know
the necessary version, though)

#define LZ4_VERSION_MAJOR    1    /* for breaking interface changes  */
#define LZ4_VERSION_MINOR    7    /* for new (non-breaking) interface capabilities */
#define LZ4_VERSION_RELEASE  1    /* for tweaks, bug-fixes, or development */
#define LZ4_VERSION_NUMBER (LZ4_VERSION_MAJOR *100*100 + LZ4_VERSION_MINOR *100 + LZ4_VERSION_RELEASE)

If the version is too low, either make it #error, or disable slicing.                                                                                                                                                              
The OS usual library version infrastructure will make sure the runtime version
is at least the MAJOR+MINOR of the compile time version.

-- 
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.