Horton

Re: [HACKERS] Custom compression methods

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Dilip Kumar <dilipbalaut@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>, pgsql-hackers@lists.postgresql.org
Date: 2021-03-11T00:40:54Z
Lists: pgsql-hackers 
On Wed, Mar 10, 2021 at 03:50:48PM -0500, Robert Haas wrote:
> On Wed, Mar 10, 2021 at 6:52 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > The pending comment is providing a way to rewrite a table and
> > re-compress the data with the current compression method.
> 
> I spent some time poking at this yesterday and ran couldn't figure out
> what was going on here. There are two places where we rewrite tables.
> One is the stuff in cluter.c, which handles VACUUM FULL and CLUSTER.
> That eventually calls reform_and_rewrite_tuple(), which deforms the
> old tuple and creates a new one, but it doesn't seem like there's
> anything in there that would expand toasted values, whether external
> or inline compressed. But I think that can't be right, because it
> seems like then you'd end up with toast pointers into the old TOAST
> relation, not the new one, which would cause failures later. So I must
> be missing something here.

I did this the empirical way.

postgres=# CREATE TABLE t (a text compression lz4);
postgres=# INSERT INTO t SELECT repeat('a',99999);
postgres=# ALTER TABLE t ALTER a SET COMPRESSION pglz;
postgres=# VACUUM FULL t;
postgres=# SELECT pg_column_compression(a) FROM t;
pg_column_compression | lz4

Actually, a --without-lz4 build can *also* VACUUM FULL the lz4 table.

It looks like VACUUM FULL t processes t but not its toast table (which is
strange to me, since VACUUM processes both, but then autovacuum also processes
them independently).

-- 
Justin

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.