On Thu, Mar 2, 2017 at 5:57 PM, David Steele <da...@pgmasters.net> wrote:
> Hi Alexander,
>
> On 2/16/17 11:20 AM, Robert Haas wrote:
> > On Thu, Feb 16, 2017 at 10:59 AM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> >> Robert Haas <robertmh...@gmail.com> writes:
> >>> On Thu, Feb 16, 2017 at 8:05 AM, Alexander Korotkov
> >>> <a.korot...@postgrespro.ru> wrote:
> >>>> My idea is that we need more general redesign of specifying ordering
> which
> >>>> index can produce. Ideally, we should replace amcanorder,
> amcanbackward and
> >>>> amcanorderbyop with single callback. Such callback should take a
> list of
> >>>> pathkeys and return number of leading pathkeys index could satisfy
> (with
> >>>> corresponding information for index scan). I'm not sure that other
> hackers
> >>>> would agree with such design, but I'm very convinced that we need
> something
> >>>> of this level of extendability. Otherwise we would have to hack our
> planner
> >>>> <-> index_access_method interface each time we decide to cover
> another index
> >>>> produced ordering.
> >>
> >>> Yeah. I'm not sure if that's exactly the right idea. But it seems
> >>> like we need something.
> >>
> >> That's definitely not exactly the right idea, because using it would
> >> require the core planner to play twenty-questions trying to guess which
> >> pathkeys the index can satisfy. ("Can you satisfy some prefix of this
> >> pathkey list? How about that one?") It could be sensible to have a
> >> callback that's called once per index and hands back a list of pathkey
> >> lists that represent interesting orders the index could produce, which
> >> could be informed by looking aside at the PlannerInfo contents to see
> >> what is likely to be relevant to the query.
> >>
> >> But even so, I'm not convinced that that is a better design or more
> >> maintainable than the current approach. I fear that it will lead to
> >> duplicating substantial amounts of code and knowledge into each index
> AM,
> >> which is not an improvement; and if anything, that increases the risk of
> >> breaking every index AM anytime you want to introduce some fundamentally
> >> new capability in the area. Now that it's actually practical to have
> >> out-of-core index AMs, that's a bigger concern than it might once have
> >> been.
> >
> > Yeah, that's all true. But I think Alexander is right that just
> > adding amcandoblah flags ad infinitum doesn't feel good either. The
> > interface isn't really arm's-length if every new thing somebody wants
> > to do something new requires another flag.
> >
> >> Also see the discussion that led up to commit ed0097e4f. Users objected
> >> the last time we tried to make index capabilities opaque at the SQL
> level,
> >> so they're not going to like a design that tries to hide that
> information
> >> even from the core C code.
> >
> > Discoverability is definitely important, but first we have to figure
> > out how we're going to make it work, and then we can work out how to
> > let users see how it works.
>
> Reading through this thread I'm concerned that this appears to be a big
> change making its first appearance in the last CF. There is also the
> need for a new patch and a general consensus of how to proceed.
>
Yes, refactoring of amcanorder/amcanorderbyop should be very thoughtful.
I recommend moving this patch to 2017-07 or marking it RWF.
>
I agree. Done.
------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
