[postgis-devel] 3D Index / 2D Index concerns

[Paul Ramsey]

On Fri, Mar 18, 2011 at 8:12 AM, Paragon Corporation <lr at pcorp.us> wrote:
> Paul,
>
>>> I guess that means in order to implement the && (2d interacts)  vs.
>>> &&& (3d
>>> interacts) --- you had have them share separate set of index creation
>>> and operators?
>
>> They are actually two totally different indexes on the same serialization.
> The 2D index builds based on a fixed-length key and the ND index builds
> based on a variable-length key.
> Not quite sure what the above means, but maybe I'll have a better idea if
> you answer these questions:
>
> a) Do you have to explicitly specify you want a 2D/3D index during the build
> process or does it go by the dimensionality of geometry?

To build a 2D index (the default)

create index geoindex on mytable using gist (geom);

To build an ND index (not the default)

create index geoindex on mytable using gist (geom gist_geometry_ops_nd);

(don't try the above, not currently implemented)


> b) If it goes by the dimensionality of the geometry, what happens if you
> have a table where the column is just geometry with no constraints etc.  Is
> it a mix of both types?  Is that what you mean by they share the same
> serialization?

In the ND case, the index is a mix and can handle multiple dimensionality.

> c) does 3D have an && and if so is it just an alias for &&&?

One operator can only have one definition for a particular type (it
seems) so && will be ever and always only a 2D op for geometry.

> d) Is it possible to do a intersect check between a 2D geometry and 3D
> geometry or because the indexes are built differently this is invalid?

The 2D index will continue to operate as currently, so basically 2.5D
behavior (third/fourth dimensions exist but are ignored for indexing
purposes)

The ND index behavior is not fleshed out yet, I have to walk the line
between what I need the ops to do for good index building and what I'd
like them to do for "rational" user facing behavior.

P.