Horton

Re: [HACKERS] Custom compression methods

Alexander Korotkov <a.korotkov@postgrespro.ru>

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From: Alexander Korotkov <a.korotkov@postgrespro.ru>
To: Robert Haas <robertmhaas@gmail.com>
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: 2017-12-11T17:41:05Z
Lists: pgsql-hackers 
On Mon, Dec 11, 2017 at 8:25 PM, Robert Haas <robertmhaas@gmail.com> wrote:

> On Mon, Dec 11, 2017 at 7:55 AM, Ildus Kurbangaliev
> <i.kurbangaliev@postgrespro.ru> wrote:
> > On Fri, 8 Dec 2017 15:12:42 -0500
> > Robert Haas <robertmhaas@gmail.com> wrote:
> >> Maybe a better idea is ALTER COLUMN SET COMPRESSION x1, x2, x3 ...
> >> meaning that x1 is the default for new tuples but x2, x3, etc. are
> >> still allowed if present.  If you issue a command that only adds
> >> things to the list, no table rewrite happens, but if you remove
> >> anything, then it does.
> >
> > I like this idea, but maybe it should be something like ALTER COLUMN
> > SET COMPRESSION x1 [ PRESERVE x2, x3 ]? 'PRESERVE' is already used in
> > syntax and this syntax will show better which one is current and which
> > ones should be kept.
>
> Sure, that works.  And I think pglz should exist in the catalog as a
> predefined, undroppable compression algorithm.  So the default for
> each column initially is:
>
> SET COMPRESSION pglz
>
> And if you want to rewrite the table with your awesome custom thing, you
> can do
>
> SET COMPRESSION awesome
>
> But if you want to just use the awesome custom thing for new rows, you can
> do
>
> SET COMPRESSION awesome PRESERVE pglz
>
> Then we can get all the dependencies right, pg_upgrade works, users
> have total control of rewrite behavior, and everything is great.  :-)
>

Looks good.

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

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 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

?

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres 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.