[postgis-devel] estimating quality of spatial indexes

[Sandro Santilli]

On Mon, Mar 02, 2015 at 10:57:29AM -0800, Paul Ramsey wrote:
> Yes, I think the n-d index is still ang-tan.
> Oleg and Teodor's site had some code for visualizing r-tree boxes that
> could maybe be used to get a feel? Doing higher dimensions is going to
> be difficut, I would imagine.

I got Oleg and Teodor's "Gevel" in place, great tool!
Now the problem is that we hide so much our index types under the carpet
that converting index keys to geometries is not easy.

Not only BOX2DF type lacks a cast to GEOMETRY but does not even implement
text output, so there's basically no way to get it printed.
Hadn't tried yet, but I guess the same holds for GIDX.

Do you guys have an problem with me implementing the casts ?

--strk;

> On Mon, Mar 2, 2015 at 10:54 AM, Sandro Santilli <strk at keybit.net> wrote:
> > I'm running performance tests with n-dimensional data and
> > found cases in which fetching 5 points out of ~35 million
> > can take 20K reads _or_ 2K reads depending on whether the
> > Z/M selectivity is higher or the 2d selectivity is.
> >
> > In other words, if the query box is large but "low" it takes
> > 10 times more seeking than if it is narrow but tall.
> >
> > I guess this has to do with the way splits were choosen and of
> > course to the distribution of the data. To know more about the
> > case I'd like to be able to "see" the index volumes in some way.
> >
> > Do you have ideas about how to visualize contents of the index ?
> > I remember someone did it a long time ago.
> >
> > Another though is that the 2d index was changed 2 years ago
> > to use Korotkov ( double-sorting ) approach for splitting, whereas the
> > nd seems to be still using Ang-Tan one, is that correct ?
> >
> > REF: http://en.wikipedia.org/wiki/R-tree
> >      http://trac.osgeo.org/postgis/ticket/1895
> >
> > --strk;