Horton

Re: [HACKERS] Custom compression methods

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

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru>
Cc: pgsql-hackers@postgresql.org, Ildar Musin <i.musin@postgrespro.ru>
Date: 2017-11-23T20:54:32Z
Lists: pgsql-hackers 
Hi,

On 11/23/2017 10:38 AM, Ildus Kurbangaliev wrote:
> On Tue, 21 Nov 2017 18:47:49 +0100
> Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> 
>>>   
>>
>> Hmmm, it still doesn't work for me. See this:
>>
>>     test=# create extension pg_lz4 ;
>>     CREATE EXTENSION
>>     test=# create table t_lz4 (v text compressed lz4);
>>     CREATE TABLE
>>     test=# create table t_pglz (v text);
>>     CREATE TABLE
>>     test=# insert into t_lz4 select repeat(md5(1::text),300);
>>     INSERT 0 1
>>     test=# insert into t_pglz select * from t_lz4;
>>     INSERT 0 1
>>     test=# drop extension pg_lz4 cascade;
>>     NOTICE:  drop cascades to 2 other objects
>>     DETAIL:  drop cascades to compression options for lz4
>>     drop cascades to table t_lz4 column v
>>     DROP EXTENSION
>>     test=# \c test
>>     You are now connected to database "test" as user "user".
>>     test=# insert into t_lz4 select repeat(md5(1::text),300);^C
>>     test=# select * from t_pglz ;
>>     ERROR:  cache lookup failed for compression options 16419
>>
>> That suggests no recompression happened.
> 
> Should be fixed in the attached patch. I've changed your extension a
> little bit according changes in the new patch (also in attachments).
> 

Hmm, this seems to have fixed it, but only in one direction. Consider this:

    create table t_pglz (v text);
    create table t_lz4 (v text compressed lz4);

    insert into t_pglz select repeat(md5(i::text),300)
    from generate_series(1,100000) s(i);

    insert into t_lz4 select repeat(md5(i::text),300)
    from generate_series(1,100000) s(i);

    \d+

     Schema |  Name  | Type  | Owner | Size  | Description
    --------+--------+-------+-------+-------+-------------
     public | t_lz4  | table | user  | 12 MB |
     public | t_pglz | table | user  | 18 MB |
    (2 rows)

    truncate t_pglz;
    insert into t_pglz select * from t_lz4;

    \d+

     Schema |  Name  | Type  | Owner | Size  | Description
    --------+--------+-------+-------+-------+-------------
     public | t_lz4  | table | user  | 12 MB |
     public | t_pglz | table | user  | 18 MB |
    (2 rows)

which is fine. But in the other direction, this happens

    truncate t_lz4;
    insert into t_lz4 select * from t_pglz;

     \d+
                       List of relations
     Schema |  Name  | Type  | Owner | Size  | Description
    --------+--------+-------+-------+-------+-------------
     public | t_lz4  | table | user  | 18 MB |
     public | t_pglz | table | user  | 18 MB |
    (2 rows)

which means the data is still pglz-compressed. That's rather strange, I
guess, and it should compress the data using the compression method set
for the target table instead.

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.