Hi Tom,

> What needs more discussion is that it seems to me to make sense to extend
the standard
> opclasses to handle the four Y-direction operators comparable to the
X-direction
> operators that are already there, that is "above", "below", "overabove",
and
> "overbelow".

As part of PostGIS (http://postgis.refractions.net), I submitted a patch a
while back to add additional Y-direction operators to the code which is a
slightly modified version of rtree_gist (and yes, the PostGIS implementation
will suffer from the same issues you've found with the existing R-tree
implementations).

The operators I went for were as follows:

A &<| B - true if A's bounding box overlaps or is below B's bounding
box
A |&> B - true if B's bounding box overlaps or is above B's bounding
box
A <<| B - true if A's bounding box is strictly below B's bounding
box
A |>> B - true if A's bounding box is strictly above B's bounding
box

Since the rtree_gist implementation and operators were 2D, my thoughts were
to use another op-class only if the indexing were upgraded to 3D. So with
this in mind, I created the following new GiST strategies:

#define RTOverBelowStrategyNumber 9
#define RTBelowStrategyNumber 10
#define RTAboveStrategyNumber 11
#define RTOverAboveStrategyNumber 12

This is basically what you are suggesting but with a | instead of a ^ in the
operator name (my original choice was to try and use } to indicate the
positive sense of the Y-axis but this was not accepted as a valid operator
character which is why I changed to |).

It would be harder for us to change these operators since they already
exist, but then again it would be useful from a maintenance point of view to
keep the strategy numbers and operators the same across both
implementations. Of course strategy numbers are just used internally so
these aren't such a big issue - it's more the choice of operators that would
be useful to agree on.


Kind regards,

Mark.

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"Mark Cave-Ayland" <m.cave-ayland@webbased.co.uk> writes:
> The operators I went for were as follows:

> A &<| B - true if A's bounding box overlaps or is below B's bounding
> box
> A |&> B - true if B's bounding box overlaps or is above B's bounding
> box
> A <<| B - true if A's bounding box is strictly below B's bounding
> box
> A |>> B - true if A's bounding box is strictly above B's bounding
> box

Well, I was proposing more or less that but with ^ because of the
precedent of the two existing box_above/box_below operator names.
However, I'm quite happy to adopt your names, since that's probably
a more widely used precedent. Sold, unless there are objections.

(BTW, it does look a bit odd that the "|" moves around in your names.
But I don't dislike it enough to not follow the precedent.)

> It would be harder for us to change these operators since they already
> exist, but then again it would be useful from a maintenance point of view to
> keep the strategy numbers and operators the same across both
> implementations.

Agreed, I'll use your strategy number assignments too.

regards, tom lane

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> -----Original Message-----
> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Sent: 24 June 2005 14:27
> To: Mark Cave-Ayland (External)
> Cc: bwhite@frognet.net; teodor@sigaev.ru; oleg@sai.msu.su;
> pgsql-hackers@postgresql.org; 'PostGIS Development Discussion'
> Subject: Re: Fixing r-tree semantics

(cut)

> Well, I was proposing more or less that but with ^ because of
> the precedent of the two existing box_above/box_below
> operator names. However, I'm quite happy to adopt your names,
> since that's probably a more widely used precedent. Sold,
> unless there are objections.
>
> (BTW, it does look a bit odd that the "|" moves around in
> your names. But I don't dislike it enough to not follow the
> precedent.)

The thinking behind it was that the | represents the x-axis if you tilt you
head right 90 degrees. In effect, it would allow you to 'read' the operator
without having to go and look up what it does.

> > It would be harder for us to change these operators since they already
> > exist, but then again it would be useful from a maintenance point of
> > view to keep the strategy numbers and operators the same across both
> > implementations.
>
> Agreed, I'll use your strategy number assignments too.

Alright no problems.


Many thanks,

Mark.

------------------------
WebBased Ltd
17 Research Way
Tamar Science Park
Plymouth
PL6 8BT

T: +44 (0)1752 797131
F: +44 (0)1752 791023
W: http://www.webbased.co.uk



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