contrib/showguc (was Re: [HACKERS] revised sample SRF C function; proposed SRF API)

Tom Lane wrote:
> Well, we're not doing that; and I see no good reason to make the thing
> be a builtin function at all. Since it's just an example, it can very
> well be a contrib item with a creation script. Probably *should* be,
> in fact, because dynamically created functions are what other people are
> going to be building; an example of how to do it as a builtin function
> isn't as helpful.

Here is a patch for contrib/showguc. It can serve as a reference
implementation for a C function which returns setof composite. It
required some small changes in guc.c and guc.h so that the number of GUC
variables, and their values, could be accessed. Example usage as shown
below:

test=# select * from show_all_vars() where varname = 'wal_sync_method';
varname | varval
-----------------+-----------
wal_sync_method | fdatasync
(1 row)

test=# select show_var('wal_sync_method');
show_var
-----------
fdatasync
(1 row)

show_var() is neither composite nor set returning, but it seemed like a
worthwhile addition. Please apply if there are no objections.

Thanks,

Joe