Horton

Re: [HACKERS] Custom compression methods

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andrew Dunstan <andrew@dunslane.net>, Justin Pryzby <pryzby@telsasoft.com>, Dilip Kumar <dilipbalaut@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Andres Freund <andres@anarazel.de>, 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>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2021-03-24T17:23:23Z
Lists: pgsql-hackers 
On Wed, Mar 24, 2021 at 12:45 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I wouldn't be proposing this if the xversion failures were the only
> reason; making them go away is just a nice side-effect.  The core
> point is that the charter of pg_dump is to reproduce the source
> database's state, and as things stand we're failing to ensure we
> do that.

Well, that state is just a mental construct, right? In reality, there
is no such state stored anywhere in the old database. You're choosing
to attribute to it an implicit state that matches what would need to
be configured in the newer version to get the same behavior, which is
a reasonable thing to do, but it is an interpretive choice rather than
a bare fact.

I don't care very much if you want to change this, but to me it seems
slightly worse than the status quo. It's hard to imagine that someone
is going to create a new cluster, set the default to lz4, run
pg_upgrade, and then complain that the new columns ended up with lz4
as the default. It seems much more likely that they're going to
complain if the new columns *don't* end up with lz4 as the default.
And I also can't see any other scenario where imagining that the TOAST
compression property of the old database simply does not exist, rather
than being pglz implicitly, is worse.

But I could be wrong, and even if I'm right it's not a hill upon which
I wish to die.

--
Robert Haas
EDB: http://www.enterprisedb.com

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.