Horton

Re: [HACKERS] Custom compression methods

Dilip Kumar <dilipbalaut@gmail.com>

View thread View on postgr.es
From: Dilip Kumar <dilipbalaut@gmail.com>
To: Robert Haas <robertmhaas@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: 2021-03-18T11:10:41Z
Lists: pgsql-hackers

Attachments

On Thu, Mar 18, 2021 at 1:32 AM Robert Haas <robertmhaas@gmail.com> wrote:
>
> ).On Mon, Mar 15, 2021 at 6:58 PM Andres Freund <andres@anarazel.de> wrote:
> > - Adding all these indirect function calls via toast_compression[] just
> >   for all of two builtin methods isn't fun either.
>
> Yeah, it feels like this has too many layers of indirection now. Like,
> toast_decompress_datum() first gets TOAST_COMPRESS_METHOD(attr). Then
> it calls CompressionIdToMethod to convert one constant (like
> TOAST_PGLZ_COMPRESSION_ID) to another constant with a slightly
> different name (like TOAST_PGLZ_COMPRESSION). Then it calls
> GetCompressionRoutines() to get hold of the function pointers. Then it
> does an indirect functional call. That seemed like a pretty reasonable
> idea when we were trying to support arbitrary compression AMs without
> overly privileging the stuff that was built into core, but if we're
> just doing stuff that's built into core, then we could just switch
> (TOAST_COMPRESS_METHOD(attr)) and call the correct function. In fact,
> we could even move the stuff from toast_compression.c into detoast.c,
> which would allow the compiler to optimize better (e.g. by inlining,
> if it wants).
>
> The same applies to toast_decompress_datum_slice().

Changed this, but I have still kept the functions in
toast_compression.c.  I think keeping compression related
functionality in a separate file looks much cleaner.  Please have a
look and let me know that whether you still feel we should move it ti
detoast.c.  If the reason is that we can inline, then I feel we are
already paying cost of compression/decompression and compare to that
in lining a function will not make much difference.

> There's a similar issue in toast_get_compression_method() and the only
> caller, pg_column_compression(). Here the multiple mapping layers and
> the indirect function call are split across those two functions rather
> than all in the same one, but here again one could presumably find a
> place to just switch on TOAST_COMPRESS_METHOD(attr) or
> VARATT_EXTERNAL_GET_COMPRESSION(attr) and return "pglz" or "lz4"
> directly.

I have simplified that, only one level of function call from
pg_column_compression,  I have kept a toast_get_compression_id
function because in later patch 0005, we will be using that for
getting the compression id from the compressed data.

> In toast_compress_datum(), I think we could have a switch that invokes
> the appropriate compressor based on cmethod and sets a variable to the
> value to be passed as the final argument of
> TOAST_COMPRESS_SET_SIZE_AND_METHOD().

Done

> Likewise, I suppose CompressionNameToMethod could at least be
> simplified to use constant strings rather than stuff like
> toast_compression[TOAST_PGLZ_COMPRESSION_ID].cmname.

Done


Other changes:
- As suggested by Andres, remove compression method comparision from
eualTupleDesc, because it is not required now.
- I found one problem in existing patch, the problem was in
detoast_attr_slice, if externally stored data is compressed then we
compute max possible compressed size to fetch based on the slice
length, for that we were using pglz_maximum_compressed_size, which is
not correct for lz4.  For lz4, I think we need to fetch the complete
compressed data.  We might think that for lz4 we might compute lie
Min(LZ4_compressBound(slicelength, total_compressed_size);  But IMHO,
we can not do that and the reason is same that why we should not use
PGLZ_MAX_OUTPUT for pglz (explained in the comment atop
pglz_maximum_compressed_size).


-- 
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.