Re: Table AM Interface Enhancements

Hi, Jeff!

On Tue, Apr 2, 2024 at 8:19 AM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> On Sat, 2024-03-30 at 23:33 +0200, Alexander Korotkov wrote:
> > I've pushed 0001, 0002 and 0006.
>
> Sorry to jump in to this discussion so late. I had worked on something
> like the custom reloptions (0002) in the past, and there were some
> complications that don't seem to be addressed in commit c95c25f9af.
>
> * At minimum I think it needs some direction (comments, docs, tests)
> that show how it's supposed to be used.
>
> * The bytea returned by the reloptions() method is not in a trivial
> format. It's a StdRelOptions struct with string values stored after the
> end of the struct. To build the bytea internally, there's some
> infrastructure like allocateRelOptStruct() and fillRelOptions(), and
> it's not very easy to extend those to support a few custom options.
>
> * If we ever decide to add a string option to StdRdOptions, I think the
> design breaks, because the code that looks for those string values
> wouldn't know how to skip over the custom options. Perhaps we can just
> promise to never do that, but we should make it explicit somehow.
>
> * Most existing heap reloptions (other than fillfactor) are used by
> other parts of the system (like autovacuum) so should be considered
> valid for any AM. Most AMs will just want to add a handful of their own
> options on top, so it would be good to demonstrate how this should be
> done.
>
> * There are still places that are inappropriately calling
> heap_reloptions directly. For instance, in ProcessUtilitySlow(), it
> seems to assume that a toast table is a heap?

Thank you for the detailed explanation. This piece definitely needs
more work. I've just reverted the c95c25f9af.

I don't like the idea that every custom table AM reltoptions should
begin with StdRdOptions. I would rather introduce the new data
structure with table options, which need to be accessed outside of
table AM. Then reloptions will be a backbox only directly used in
table AM, while table AM has a freedom on what to store in reloptions
and how to calculate externally-visible options. What do you think?

------
Regards,
Alexander Korotkov