[postgis-devel] RFC: Remove mmin and mmax from GBOX

[Mark Cave-Ayland]

On 06/07/11 17:38, Paul Ramsey wrote:

> Yeah, by removing the fourth dimension you're foreclosing on 4d
> indexing to some extent, since gboxes are used to pass around full
> extents of 4d objects.

But that's the misunderstanding here - M isn't a dimension. It's an 
arbitrary numerical attribute attached to the geometry.

Thinking about this some more, I was wondering if we could change 
GSERIALIZED a bit. Currently we have this:

typedef struct
{
	uint32 size; /* For PgSQL use, use VAR* macros to manipulate. */
	uchar srid[3]; /* 21 bits of SRID (and 3 spare bits) */
	uchar flags; /* HasZ, HasM, HasBBox, IsGeodetic */
	uchar data[1]; /* See gserialized.txt */
} GSERIALIZED;

I'm thinking that instead of HasZ we should simply reserve 2 bits for 
the number of dimensions, and then if a measure is present the HasM bit 
can be set. This will give us up to 4 dimensions for people to play 
with, and will also tidy up the HasZ/HasM handling. Do people think 4 
dimensions is currently enough? I'm happy to go ahead and do the work to 
make the relevant changes.


ATB,

Mark.

-- 
Mark Cave-Ayland - Senior Technical Architect
PostgreSQL - PostGIS
Sirius Corporation plc - control through freedom
http://www.siriusit.co.uk
t: +44 870 608 0063

Sirius Labs: http://www.siriusit.co.uk/labs