Re: [HACKERS] Custom compression methods

On Mon, Mar 22, 2021 at 11:13 AM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
> The first iteration was pretty rough, and there's still some question in my
> mind about where default_toast_compression_options[] should be defined. If
> it's in the header file, then I could use lengthof() - but then it probably
> gets multiply defined.

What do you want to use lengthof() for?

> In the latest patch, there's multiple "externs". Maybe
> guc.c doesn't need the extern, since it includes toast_compression.h. But then
> it's the only "struct config_enum_entry" which has an "extern" outside of
> guc.c.

Oh, yeah, we certainly shouldn't have an extern in guc.c itself, if
we've already got it in the header file.

As to the more general question of where to put stuff, I don't think
there's any conceptual problem with putting it in a header file rather
than in guc.c. It's not very scalable to just keeping inventing new
GUCs and sticking all their accoutrement into guc.c. That might have
kind of made sense when guc.c was invented, since there were probably
fewer settings there and guc.c itself was new, but at this point it's
a well-established part of the infrastructure and having other
subsystems cater to what it needs rather than the other way around
seems logical. However, it's not great to have "utils/guc.h" included
in "access/toast_compression.h", because then anything that includes
"access/toast_compression.h" or "access/toast_internals.h" sucks in
"utils/guc.h" even though it's not really topically related to what
they intended to include. We can't avoid that just by choosing to put
this enum in guc.c, because GetDefaultToastCompression() also uses it.
But, what about giving the default_toast_compression_method GUC an
assign hook that sets a global variable of type "char" to the
appropriate value? Then GetDefaultToastCompression() goes away
entirely. That might be worth exploring.

> Also, it looks like you added default_toast_compression out of order, so maybe
> you'd fix that at the same time.

You know, I looked at where you had it and said to myself, "surely
this is a silly place to put this, it would make much more sense to
move this up a bit." Now I feel dumb.

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