On Sat, Feb 26, 2011 at 06:44:52PM +0000, Greg Stark wrote:
> 2011/2/26 PostgreSQL - Hans-Jürgen Schönig <postgres@cybertec.at>:
> > what we are trying to do is to explicitly store column correlations. so, a histogram for (a, b) correlation and so
on.
>
> The problem is that we haven't figured out how to usefully store a
> histogram for <a,b>. Consider the oft-quoted example of a
> <city,postal-code> -- or <city,zip code> for Americans. A histogram
> of the tuple is just the same as a histogram on the city.
But there are cases where it can work. Frankly the example you mention
is odd because for we can't even build useful 1D histograms for <city>
and <zip code>, so the fact that 2D is hard is not surprising.
The histograms we do build work fine from > and <, just equality. The
2D will handle the same.
Have a nice day,
--
Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/
> Patriotism is when love of your own people comes first; nationalism,
> when hate for people other than your own comes first.
> - Charles de Gaulle