[postgis-devel] Multi-D Indexing and Serialized Form

[David William Bitner]

If we're opening up the whole ball of wax and looking at the potential
things that could go into a liblwgeom2 serialized form, would there be any
possibilities to have a T dimension that was an actual timestamp (as opposed
to the typical use of an offset from a time)???

On Mon, Jul 13, 2009 at 4:04 PM, Paul Ramsey <pramsey at opengeo.org> wrote:

> For my "geography" work, I am trying to ensure that what I do either
> uses existing liblwgeom hooks, or creates new things that liblwgeom2
> could use. I'll be doing indexing as a 3-space index, and that works
> OK for geography, things are static:
>
> bbox dims = 3
> coordinate dims = 2
>
> However, for geometry, we were thinking about adding multidimensional
> indexing. Right now we have a situation like this for geometry:
>
> bbox dims = 2
> coordinate dims = 2/3/4
>
> No problem, because we only calculate a 2d box, no matter what our
> dimensionality. But when we add the possibility of multi-dimensional
> indexes, we get in a bind. How many dimensions of bbox should we
> calculate? We need enough to support the index, but we don't know what
> the dimensionality of the index using the bbox will be. That leaves us
> forced to calculate the full multi-dimensional bbox even if we only
> want a 2-d index. We also could end up with some interesting problems,
> for example, returning results from a 4d index given a 2d query bbox.
>
> The straightest line solution, IMO, is to just create a bbox with
> enough dimensionality to handle the coordinates. So we'll end up with
>
> coordinate dims = 2 => bbox dims = 2
> coordinate dims = 3 => bbox dims = 3
> coordinate dims = 4 => bbox dims = 4
>
> For 2-vertex 3d points that you only want a 2d index on (the worst
> case, since we don't put boxes on points), that implies a storage
> overhead of about 10%.
>
> P.
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-- 
************************************
David William Bitner
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