Horton

Re: [HACKERS] Custom compression methods

Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru>

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From: Ildus K <i.kurbangaliev@postgrespro.ru>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org
Date: 2017-11-21T20:28:55Z
Lists: pgsql-hackers 
On Tue, 21 Nov 2017 18:47:49 +0100
Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:

 
> 
> I propose to use either
> 
>    CompressionMethodOptions (and CompressionMethodRoutine)
> 
> or
> 
>    CompressionOptions (and CompressionRoutine)

Sounds good, thanks.

> 
> OK. But then I don't understand why tsvector.c does things like
> 
>     VARSIZE(data) - VARHDRSZ_CUSTOM_COMPRESSED - arrsize
>     VARRAWSIZE_4B_C(data) - arrsize
> 
> instead of
> 
>     VARSIZE_ANY_EXHDR(data) - arrsize
>     VARSIZE_ANY(data) - arrsize
> 
> Seems somewhat confusing.
> 

VARRAWSIZE_4B_C returns original size of data, before compression (from
va_rawsize in current postgres, and from va_info in my patch), not size
of the already compressed data, so you can't use VARSIZE_ANY here.

VARSIZE_ANY_EXHDR in current postgres returns VARSIZE-VARHDRSZ, despite
the varlena is compressed or not, so I just kept this behavior for
custom compressed varlenas too. If you look into tuptoaster.c you will
also see lines like 'VARSIZE(attr) - TOAST_COMPRESS_HDRSZ'. So I think
if VARSIZE_ANY_EXHDR will subtract different header sizes then it
should subtract them for usual compressed varlenas too.

> >   
> 
> 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.

I will check that. Is your extension published somewhere?

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.