Re: [HACKERS] Custom compression methods - Mailing list pgsql-hackers

On Tue, Nov 24, 2020 at 7:11 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> About (4), one option is that we directly call the correct handler
> function for the built-in type directly from
> toast_(de)compress(_slice) functions but in that case, we are
> duplicating the code, another option is that we call the
> GetCompressionRoutine() a common function and in that, for the
> built-in type, we can directly call the corresponding handler function
> and get the routine.  The only thing is to avoid duplicating in
> decompression routine we need to convert CompressionId to Oid before
> calling GetCompressionRoutine(), but now we can avoid sys cache lookup
> for the built-in type.

Suppose that we have a variable lz4_methods (like heapam_methods) that
is always defined, whether or not lz4 support is present. It's defined
like this:

const CompressionAmRoutine lz4_compress_methods = {
    .datum_compress = lz4_datum_compress,
    .datum_decompress = lz4_datum_decompress,
    .datum_decompress_slice = lz4_datum_decompress_slice
};

(It would be good, I think, to actually name things something like
this - in particular why would we have TableAmRoutine and
IndexAmRoutine but not include "Am" in the one for compression? In
general I think tableam is a good pattern to adhere to and we should
try to make this patch hew closely to it.)

Then those functions are contingent on #ifdef HAVE_LIBLZ4: they either
do their thing, or complain that lz4 compression is not supported.
Then in this function you can just say, well, if we have the 01 bit
pattern, handler = &lz4_compress_methods and proceed from there.

BTW, I think the "not supported" message should probably use the 'by
this build' language we use in some places i.e.

[rhaas pgsql]$ git grep errmsg.*'this build' | grep -vF .po:
contrib/pg_prewarm/pg_prewarm.c: errmsg("prefetch is not supported by
this build")));
src/backend/libpq/be-secure-openssl.c: (errmsg("\"%s\" setting \"%s\"
not supported by this build",
src/backend/libpq/be-secure-openssl.c: (errmsg("\"%s\" setting \"%s\"
not supported by this build",
src/backend/libpq/hba.c: errmsg("local connections are not supported
by this build"),
src/backend/libpq/hba.c: errmsg("hostssl record cannot match because
SSL is not supported by this build"),
src/backend/libpq/hba.c: errmsg("hostgssenc record cannot match
because GSSAPI is not supported by this build"),
src/backend/libpq/hba.c: errmsg("invalid authentication method \"%s\":
not supported by this build",
src/backend/utils/adt/pg_locale.c: errmsg("ICU is not supported in
this build"), \
src/backend/utils/misc/guc.c: GUC_check_errmsg("Bonjour is not
supported by this build");
src/backend/utils/misc/guc.c: GUC_check_errmsg("SSL is not supported
by this build");

-- 
Robert Haas
EDB: http://www.enterprisedb.com