Horton

Re: [HACKERS] Custom compression methods

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Alexander Korotkov <a.korotkov@postgrespro.ru>
Cc: Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, Евгений Шишкин <itparanoia@gmail.com>, Andres Freund <andres@anarazel.de>, Oleg Bartunov <obartunov@gmail.com>, Craig Ringer <craig@2ndquadrant.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2017-12-11T17:46:20Z
Lists: pgsql-hackers 
On Mon, Dec 11, 2017 at 12:41 PM, Alexander Korotkov
<a.korotkov@postgrespro.ru> wrote:
> Thus, in your example if user would like to further change awesome
> compression for evenbetter compression, she should write.
>
> SET COMPRESSION evenbetter PRESERVE pglz, awesome; -- full list of previous
> compression methods

Right.

> I wonder what should we do if user specifies only part of previous
> compression methods?  For instance, pglz is specified but awesome is
> missing.
>
> SET COMPRESSION evenbetter PRESERVE pglz; -- awesome is missing
>
> I think we should trigger an error in this case.  Because query is specified
> in form that is assuming to work without table rewrite, but we're unable to
> do this without table rewrite.

I think that should just rewrite the table in that case.  PRESERVE
should specify the things that are allowed to be preserved -- its mere
presence should not be read to preclude a rewrite.  And it's
completely reasonable for someone to want to do this, if they are
thinking about de-installing awesome.

> I also think that we need some way to change compression method for multiple
> columns in a single table rewrite.  Because it would be way more efficient
> than rewriting table for each of columns.  So as an alternative of
>
> ALTER TABLE tbl ALTER COLUMN c1 SET COMPRESSION awesome; -- first table
> rewrite
> ALTER TABLE tbl ALTER COLUMN c2 SET COMPRESSION awesome; -- second table
> rewrite
>
> we could also provide
>
> ALTER TABLE tbl ALTER COLUMN c1 SET COMPRESSION awesome PRESERVE pglz; -- no
> rewrite
> ALTER TABLE tbl ALTER COLUMN c2 SET COMPRESSION awesome PRESERVE pglz; -- no
> rewrite
> VACUUM FULL tbl RESET COMPRESSION PRESERVE c1, c2; -- rewrite with
> recompression of c1 and c2 and removing depedencies
>
> ?

Hmm.  ALTER TABLE allows multi comma-separated subcommands, so I don't
think we need to drag VACUUM into this.  The user can just say:

ALTER TABLE tbl ALTER COLUMN c1 SET COMPRESSION awesome, ALTER COLUMN
c2 SET COMPRESSION awesome;

If this is properly integrated into tablecmds.c, that should cause a
single rewrite affecting both columns.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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.