Re: [HACKERS] Custom compression methods

).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().

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.

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().

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

> - why is HIDE_TOAST_COMPRESSION useful? Doesn't quite seem to be
>   comparable to HIDE_TABLEAM?

Andres, what do you mean by this exactly? It's exactly the same issue:
without this, if you change the default compression method, every test
that uses \d+ breaks. If you want to be able to run the whole test
suite with either compression method and get the same results, you
need this. Now, maybe you don't, because perhaps that doesn't seem so
important with compression methods as with table AMs. I think that's a
defensible position. But, it is at the underlying level, the same
thing.

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