Re: CustomScan in a larger structure (RE: CustomScan support on readfuncs.c)

> On Wed, Jan 27, 2016 at 9:36 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> > On Mon, Jan 25, 2016 at 8:06 PM, Kouhei Kaigai <kaigai@ak.jp.nec.com> wrote:
> >> Sorry for my late response. I've been unavailable to have enough
> >> time to touch code for the last 1.5 month.
> >>
> >> The attached patch is a revised one to handle private data of
> >> foregn/custom scan node more gracefully.
> >>
> >> The overall consensus upthread were:
> >> - A new ExtensibleNodeMethods structure defines a unique name
> >>   and a set of callbacks to handle node copy, serialization,
> >>   deserialization and equality checks.
> >> - (Foreign|Custom)(Path|Scan|ScanState) are first host of the
> >>   ExtensibleNodeMethods, to allow extension to define larger
> >>   structure to store its private fields.
> >> - ExtensibleNodeMethods does not support variable length
> >>   structure (like a structure with an array on the tail, use
> >>   separately allocated array).
> >> - ExtensibleNodeMethods shall be registered on _PG_init() of
> >>   extensions.
> >>
> >> The 'pgsql-v9.6-custom-private.v3.patch' is the main part of
> >> this feature. As I pointed out before, it uses dynhash instead
> >> of the self invented hash table.
> >
> > On a first read-through, I see nothing in this patch to which I would
> > want to object except for the fact that the comments and documentation
> > need some work from a native speaker of English.  It looks like what
> > we discussed, and I think it's an improvement over what we have now.
> 
> Well, looking at this a bit more, it seems like the documentation
> you've written here is really misplaced.  The patch is introducing a
> new facility that applies to both CustomScan and ForeignScan, but the
> documentation is only to do with CustomScan.  I think we need a whole
> new chapter on extensible nodes, or something.  I'm actually not
> really keen on the fact that we keep adding SGML documentation for
> this stuff; it seems like it belongs in a README in the source tree.
> We don't explain nodes in general, but now we're going to have to try
> to explain extensible nodes.  How's that going to work?
>
The detail of these callbacks are not for end-users, administrators and
so on except for core/extension developers. So, I loves idea not to have
such a detailed description in SGML.
How about an idea to have more detailed source code comments close to
the definition of ExtensibleNodeMethods?
I haven't seen the src/backend/nodes/README yet, and it has only 6 updates
history from Postgres95 era. I guess people may forget to update README
file if description is separately located from the implementation.

> I think you should avoid the call to GetExtensibleNodeMethods() in the
> case where extnodename is NULL.  On the other hand, I think that if
> extnodename is non-NULL, all four methods should be required, so that
> you don't have to check if (methods && methods->nodeRead) but just if
> (extnodename) { methods = GetExtensibleNodeMethods(extnodename);
> methods->nodeRead( ... ); }.  That seems like it would be a bit
> tidier.
>
OK, I'll fix up. No need to have 'missing_ok' argument here.

Thanks,
--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai@ak.jp.nec.com>