Horton

Re: [HACKERS] Custom compression methods

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Justin Pryzby <pryzby@telsasoft.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>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2021-02-05T16:23:46Z
Lists: pgsql-hackers 
On Fri, Feb 5, 2021 at 11:07 AM Robert Haas <robertmhaas@gmail.com> wrote:
> Personally, my preference is to just update the test outputs. It's not
> important whether many people look closely to verify the differences;
> we just need to look them over on a one-time basis to see if they seem
> OK. After that it's 0 effort, vs. having to maintain HIDE_COMPRESSAM
> forever.

Oh, I guess you're thinking about the case where someone wants to run
the tests with a different default. That might be a good reason to
have this. But then those changes should go in 0002.

Regarding 0002, I'm not feeling very excited about having every call
to TupleDescInitEntry() do an extra syscache lookup. It's going to be
the same lookup every time forever to get the same value every time
forever. Now maybe that function can never get hot enough for it to
matter, but can't we find a way to be smarter about this? Like,
suppose we cache the OID in a global variable the first time we look
it up, and then use CacheRegisterSyscacheCallback() to have it zeroed
out if pg_am is updated?

Taking that idea a bit further, suppose you get rid of all the places
where you do get_compression_am_oid(default_toast_compression, false)
and change them to get_default_compression_am_oid(), which is defined
thus:

static Oid
get_default_compression_am_oid(void)
{
    if (unlikely(!OidIsValid(cached_default_compression_oid))
      // figure it out;
    return cached_default_compression_oid;
}

Also, how about removing the debugging leftovers from syscache.c?

-- 
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.