Horton

Re: [HACKERS] Custom compression methods

Dilip Kumar <dilipbalaut@gmail.com>

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From: Dilip Kumar <dilipbalaut@gmail.com>
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: Robert Haas <robertmhaas@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: 2021-03-01T05:02:23Z
Lists: pgsql-hackers 
On Sun, Feb 28, 2021 at 9:48 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
>
> On my PC, this new change is causing a test failure:
>
>  SELECT SUBSTR(f1, 2000, 50) FROM cmdata1;
> -                       substr
> -----------------------------------------------------
> - 01234567890123456789012345678901234567890123456789
> -(1 row)
> -
> +ERROR:  compressed lz4 data is corrupt

The older version of lz4 had this problem that while decompressing
partial if we don't give the buffer size up to full data length it was
failing[1] but it is solved in version 1.9.

> @@ -119,15 +119,15 @@ lz4_cmdecompress_slice(const struct varlena *value, int32 slicelength)
>         int32           rawsize;
>         struct varlena *result;
>
> -       /* allocate memory for holding the uncompressed data */
> -       result = (struct varlena *) palloc(VARRAWSIZE_4B_C(value) + VARHDRSZ);
> +       /* allocate memory for the uncompressed data */
> +       result = (struct varlena *) palloc(slicelength + VARHDRSZ);
>
> -       /* decompress partial data using lz4 routine */
> +       /* decompress the data */
>         rawsize = LZ4_decompress_safe_partial((char *) value + VARHDRSZ_COMPRESS,
>                                                                                   VARDATA(result),
>                                                                                   VARSIZE(value) - VARHDRSZ_COMPRESS,
>                                                                                   slicelength,
> -                                                                                 VARRAWSIZE_4B_C(value));
> +                                                                                 slicelength);

This is done for the latest version as now we don't need to allocate
the buffer of full size, it is enough the allocate just equal to the
slicelength.

> Also, in the tests, you have this at both the top and bottom of the file:
>
> src/test/regress/sql/compression.sql:\set HIDE_COMPRESSAM false
> src/test/regress/sql/compression.sql:\set HIDE_COMPRESSAM false
>
> Whereas the patch I sent had at the end:
>
> +\set HIDE_COMPRESSAM on
>
> ("on" is the default when run under pg_regress)

I will fix this.

[1] https://docs.unrealengine.com/en-US/API/Runtime/Core/Compression/LZ4_decompress_safe_partial/index.html

-- 
Regards,
Dilip Kumar
EnterpriseDB: 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.