Re: Small improvements to pg_list.h's linitial(), lsecond(), lthird() etc macros

On Tue, 29 Sep 2020 at 11:37, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> I wrote:
> > list_second_cell() does have uses, although I observe that they
> > are almost exclusively in locutions such as
> >       for_each_cell(lc, rollups, list_second_cell(rollups))
> > to iterate over all-but-the-first elements of a list.  I wonder if
> > we ought to come up with a better notation for that.  I'm imagining
> > something like
> >       for_each_from(lc, rollups, 1)
> > to start from list index 1.  It looks like this would be marginally
> > more efficient, and perhaps more readable.
>
> Concretely, I'm thinking about the attached.

I had a look over this and I like it.  It seems good as it allows
consumers to choose N programmatically rather than be fixed into using
list_second_cell() or list_fortysecond_cell().

> I'm somewhat inclined to back-patch this into v13.  The missing
> const decoration seems arguably a bug, which we've missed noticing
> only because of our generally lamentable under-usage of const.
> And I think it'll be helpful for future back-patching if
> for_each_from is available in all versions with the new List API.

It does seem fairly low risk and having personally experienced
backpatching pain, I understand your motivation to backpatch.  I
certainly wouldn't object to backpacking but will defer to your better
judgement on whether you choose to or not.

David