Horton

Re: [HACKERS] Custom compression methods

Tomas Vondra <tomas.vondra@enterprisedb.com>

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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Robert Haas <robertmhaas@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Justin Pryzby <pryzby@telsasoft.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: 2021-03-20T09:35:26Z
Lists: pgsql-hackers 
Hi,

I think this bit in brin_tuple.c is wrong:

    ...
    Form_pg_attribute att = TupleDescAttr(brdesc->bd_tupdesc,
                                          keyno);
    Datum       cvalue = toast_compress_datum(value,
                                              att->attcompression);

The problem is that this is looking at the index descriptor (i.e. what
types are indexed) instead of the stored type. For BRIN those may be
only loosely related, which is why the code does this a couple lines above:

    /* We must look at the stored type, not at the index descriptor. */
    TypeCacheEntry *atttype
        = brdesc->bd_info[keyno]->oi_typcache[datumno];

For the built-in BRIN opclasses this happens to work, because e.g.
minmax stores two values of the original type. But it may not work for
other out-of-core opclasses, and it certainly doesn't work for the new
BRIN opclasses (bloom and minmax-multi).

Unfortunately, the only thing we have here is the type OID, so I guess
the only option is using GetDefaultToastCompression(). Perhaps we might
include that into BrinOpcInfo too, in the future.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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.