Re: get_actual_variable_range vs idx_scan/idx_tup_fetch

On Sat, Oct 18, 2014 at 02:20:45PM -0400, Bruce Momjian wrote:
> On Sat, Oct 18, 2014 at 06:15:03PM +0200, Marko Tiikkaja wrote:
> > On 10/18/14, 5:46 PM, Tom Lane wrote:
> > >Marko Tiikkaja <marko@joh.to> writes:
> > >>Yes, exactly; if I had had the option to disable the index from the
> > >>optimizer's point of view, I'd have seen that it's not used for looking
> > >>up any data by any queries, and thus I would have known that I can
> > >>safely drop it without slowing down queries.  Which was the only thing I
> > >>cared about, and where the stats we provide failed me.
> > >
> > >This argument is *utterly* wrongheaded, because it assumes that the
> > >planner's use of the index provided no benefit to your queries.  If the
> > >planner was touching the index at all then it was planning queries in
> > >which knowledge of the extremal value was relevant to accurate selectivity
> > >estimation.  So it's quite likely that without the index you'd have gotten
> > >different and inferior plans, whether or not those plans actually chose to
> > >use the index.
> >
> > Maybe.  But at the same time that's a big problem: there's no way of
> > knowing whether the index is actually useful or not when it's used
> > only by the query planner.
>
> That is a good point.  Without an index, the executor is going to do a
> sequential scan, while a missing index to the optimizer just means worse
> statistics.

I have applied the attached patch to document that the optimizer can
increase the index usage statistics.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +