From: Calum Grant (calum_at_[hidden])
Date: 2005-10-21 12:41:18
Arkadiy Vertleyb wrote:
> "Calum Grant" <calum_at_[hidden]> wrote
> > I frankly have no idea how I would implement that problem
> in SQL, but
> > then this was a problem you chose to be very difficult to
> solve using
> > a SQL notation.
>
> I am not an SQL expert, but should be something like this (if
> I remember
> correctly):
>
> SELECT number, name, city, COUNT(*) AS cnt
> FROM mytable a, mytable b
> WHERE [the long condition based on coordinates]
> GROUP BY a.number
> ORDER BY cnt DESC;
>
> Then you hope that the optimizer figures out how to not
> compare every possible pair ;-)
Needless to say, implementing a query analyser to do all that purely in
TMP would be rather difficult! But I don't believe there are any hidden
costs in RML - what you see is what you get.
> This exact statement I modeled in RTL, but RTL also specifies
> HOW to do things in addition to WHAT to do, so I was able to
> do a smartrer
> (range-based) query.
Ah, but you can specify "how" using C++.
> I am thrilled with your performance results, though (although
> I consider them the results of a by-hand solution). Did you
> completely avoid calculating the distance? If so, I think
> any such solution would provide only approximate result
> (although it might happen to be a pretty good approximation).
I compare on Euclidean distance squared. It does not actually matter
whether Euclidean distance, Euclidean distance squared, or surface
distance is used, the ordering will be preserved.
Best regards, Calum