[postgis-devel] estimating quality of spatial indexes

[Sandro Santilli]

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;