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: 2021-03-24T16:24:38Z
Lists: pgsql-hackers 
On Wed, Mar 24, 2021 at 11:42 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> On reflection, though, I wonder if we've made pg_dump do the right
> thing anyway.  There is a strong case to be made for the idea that
> when dumping from a pre-14 server, it should emit
>         SET default_toast_compression = 'pglz';
> rather than omitting any mention of the variable, which is what
> I made it do in aa25d1089.  If we changed that, I think all these
> diffs would go away.  Am I right in thinking that what's being
> compared here is new pg_dump's dump from old server versus new
> pg_dump's dump from new server?
>
> The "strong case" goes like this: initdb a v14 cluster, change
> default_toast_compression to lz4 in its postgresql.conf, then
> try to pg_upgrade from an old server.  If the dump script doesn't
> set default_toast_compression = 'pglz' then the upgrade will
> do the wrong thing because all the tables will be recreated with
> a different behavior than they had before.  IIUC, this wouldn't
> result in broken data, but it still seems to me to be undesirable.
> dump/restore ought to do its best to preserve the old DB state,
> unless you explicitly tell it --no-toast-compression or the like.

This feels a bit like letting the tail wag the dog, because one might
reasonably guess that the user's intention in such a case was to
switch to using LZ4, and we've subverted that intention by deciding
that we know better. I wouldn't blame someone for thinking that using
--no-toast-compression with a pre-v14 server ought to have no effect,
but with your proposal here, it would. Furthermore, IIUC, the user has
no way of passing --no-toast-compression through to pg_upgrade, so
they're just going to have to do the upgrade and then fix everything
manually afterward to the state that they intended to have all along.
Now, on the other hand, if they wanted to make practically any other
kind of change while upgrading, they'd have to do something like that
anyway, so I guess this is no worse.

But also ... aren't we just doing this to work around a test case that
isn't especially good in the first place? Counting the number of lines
in the diff between A and B is an extremely crude proxy for "they're
similar enough that we probably haven't broken anything."

-- 
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.