Re: [HACKERS] [PATCH] kNN for btree

On Thu, Feb 16, 2017 at 8:05 AM, Alexander Korotkov
<a.korotkov@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.

>> info->amcanorderbyop = (void (*)()) amroutine->amcanorderbyop;
>
> It's not necessary to use cast here.  For instance, we don't use cast for
> amcostestimate.

In fact, it's bad to use the cast here, because if in future the
signature of one of amroutine->amcanorderbyop is changed and
info->amcanorderbyop is not changed to match, then the cast will
prevent a compiler warning, but at runtime you may crash.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company