Horton

Re: [HACKERS] Custom compression methods

Tomas Vondra <tomas.vondra@2ndquadrant.com>

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Robert Haas <robertmhaas@gmail.com>, Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru>
Cc: Oleg Bartunov <obartunov@gmail.com>, Craig Ringer <craig@2ndquadrant.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Andres Freund <andres@anarazel.de>
Date: 2017-11-19T23:23:23Z
Lists: pgsql-hackers 

On 11/15/2017 02:13 PM, Robert Haas wrote:
> On Wed, Nov 15, 2017 at 4:09 AM, Ildus Kurbangaliev
> <i.kurbangaliev@postgrespro.ru> wrote:
>> So in the next version of the patch I can just unlink the options from
>> compression methods and dropping compression method will not affect
>> already compressed tuples. They still could be decompressed.
> 
> I guess I don't understand how that can work.  I mean, if somebody
> removes a compression method - i.e. uninstalls the library - and you
> don't have a way to make sure there are no tuples that can only be
> uncompressed by that library - then you've broken the database.
> Ideally, there should be a way to add a new compression method via an
> extension ... and then get rid of it and all dependencies thereupon.
> 

I share your confusion. Once you do DROP COMPRESSION METHOD, there must
be no remaining data compressed with it. But that's what the patch is
doing already - it enforces this using dependencies, as usual.

Ildus, can you explain what you meant? How could the data still be
decompressed after DROP COMPRESSION METHOD, and possibly after removing
the .so library?

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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.