Horton

Re: Custom compression methods

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Oleg Bartunov <obartunov@gmail.com>, Craig Ringer <craig@2ndquadrant.com>, Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Andres Freund <andres@anarazel.de>
Date: 2017-11-06T03:32:08Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> A basic problem here is that, as proposed, DROP COMPRESSION METHOD may
> break your database irretrievably.  If there's no data compressed
> using the compression method you dropped, everything is cool -
> otherwise everything is broken and there's no way to recover.  The
> only obvious alternative is to disallow DROP altogether (or make it
> not really DROP).

> Both of those alternatives sound fairly unpleasant to me, but I'm not
> exactly sure what to recommend in terms of how to make it better.
> Ideally anything we expose as an SQL command should have a DROP
> command that undoes whatever CREATE did and leaves the database in an
> intact state, but that seems hard to achieve in this case.

If the use of a compression method is tied to specific data types and/or
columns, then each of those could have a dependency on the compression
method, forcing a type or column drop if you did DROP COMPRESSION METHOD.
That would leave no reachable data using the removed compression method.
So that part doesn't seem unworkable on its face.

IIRC, the bigger concerns in the last discussion had to do with
replication, ie, can downstream servers make sense of the data.
Maybe that's not any worse than the issues you get with non-core
index AMs, but I'm not sure.

			regards, tom lane

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.