Horton

Re: [HACKERS] Custom compression methods

Dilip Kumar <dilipbalaut@gmail.com>

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From: Dilip Kumar <dilipbalaut@gmail.com>
To: Justin Pryzby <pryzby@telsasoft.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>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2021-03-11T10:51:38Z
Lists: pgsql-hackers

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On Thu, Mar 11, 2021 at 10:07 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
>  On Thu, Mar 11, 2021 at 8:50 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
> For INSERT SELECT we were already doing in the older version so we can
> include that code here, we will also have to include the patches for
> decompressing data before forming the composite types because without
> that we can not ensure that lz4 does not exist anywhere in the table.
> Said that with that also we can not ensure that it doesn't exist anywhere
> in the system because it might exist in the WAL and if you do the crash
> recovery then might get those lz4 compressed data back.

In updated patches, now INSERT INTO SELECT/VACUUM FULL/ CLUSTER will
re compress the data as per the latest compression method.

create table t(a text compression lz4);
insert into t values(repeat('a', 9000));
postgres[129360]=# select pg_column_compression(a) from t;
 pg_column_compression
-----------------------
 lz4
(1 row)

postgres[129360]=# alter TABLE t ALTER COLUMN a SET COMPRESSION pglz;
ALTER TABLE
postgres[129360]=# select pg_column_compression(a) from t;
 pg_column_compression
-----------------------
 lz4
(1 row)

postgres[129360]=# VACUUM FULL t;
VACUUM
postgres[129360]=# select pg_column_compression(a) from t;
 pg_column_compression
-----------------------
 pglz
(1 row)

IMHO, now we have a way for user to rewrite table using VACUUM
FULL/CLUSTER so I don't think we should force rewrite in ALTER SET
COMPRESSION.

In attached patch I am re compressing in INSERT INTO SELECT as well,
but honestly I think maybe we don't need to force that also on user
and we can only do this in VACUUM FULL or CLUSTER, thoughts?

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

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.