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: Tom Lane <tgl@sss.pgh.pa.us>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Dilip Kumar <dilipbalaut@gmail.com>, Andres Freund <andres@anarazel.de>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, David Steele <david@pgmasters.net>, Ildus Kurbangaliev <i.kurbangaliev@gmail.com>, Dmitry Dolgov <9erthalion6@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2021-03-22T17:58:08Z
Lists: pgsql-hackers 
On Mon, Mar 22, 2021 at 01:38:36PM -0400, Robert Haas wrote:
> On Mon, Mar 22, 2021 at 12:16 PM Robert Haas <robertmhaas@gmail.com> wrote:
> > But, what about giving the default_toast_compression_method GUC an
> > assign hook that sets a global variable of type "char" to the
> > appropriate value? Then GetDefaultToastCompression() goes away
> > entirely. That might be worth exploring.
> 
> Actually, we can do even better. We should just make the values
> actually assigned to the GUC be TOAST_PGLZ_COMPRESSION etc. rather
> than TOAST_PGLZ_COMPRESSION_ID etc. Then a whole lot of complexity
> just goes away. I added some comments explaining why using
> TOAST_PGLZ_COMPRESSION is the wrong thing anyway. Then I got hacking
> and rearranged a few other things. So the attached patch does these
> thing:
> 
> - Changes default_toast_compression to an enum, as in your patch, but
> now with values that are the same as what ultimately gets stored in
> attcompression.
> - Adds a comment warning against incautious use of
> TOAST_PGLZ_COMPRESSION_ID, etc.
> - Moves default_toast_compression_options to guc.c.
> - After doing the above two things, we can remove the #include of
> utils/guc.h into access/toast_compression.h, so the patch does that.
> - Moves NO_LZ4_SUPPORT, GetCompressionMethodName, and
> CompressionNameToMethod to guc.c. Making these inline functions
> doesn't save anything meaningful; it's more important not to export a
> bunch of random identifiers.
> - Removes an unnecessary cast to bool from the definition of
> CompressionMethodIsValid.
> 
> I think this is significantly cleaner than what we have now, and I
> also prefer it to your proposal.

+1

guc.c should not longer define this as extern:
default_toast_compression_options

I think you should comment that default_toast_compression is an int as far as
guc.c is concerned, but storing one of the char value of TOAST_*_COMPRESSION

Shouldn't varlena.c pg_column_compression() call GetCompressionMethodName () ?
I guess it should already have done that.

Maybe pg_dump.c can't use those constants, though (?)

-- 
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.