[postgis-devel] Oracle SDO_GEOMETRY vs PostGIS WKT

[Martin Davis]

Here's my take:

Yes, reference [2] is the best reference for the true SFS Geometry 
standard.  It's an abstract standard, however.  Each implementation has 
its own idiosyncracies, which can only be understood by reading the 
implementation manuals (and more than likely working with the 
implementation directly).

It's necessary to distinguish between the naming conventions used by 
vendors, and their underlying implementations.  The naming conventions 
reflect an attempt to (more or less) fulfull the requirements of the SFS 
and ISO standards.  But each vendor has a different implementation, 
which seeks to meet the varying marketing goals of the vendor.

Oracle's SDO_GEOMETRY type is more full-featured than is required by 
SFS, which is one reason for its complexity.  Also, it uses existing 
features of the Oracle Object-Relational model, which also means that 
complexity is perhaps more exposed than is desirable.

ESRI recently released their own ST_GEOMETRY type for Oracle, presumably 
in an attempt to both better match the requirements of their other 
software products, as well as having the pleasant side effect of 
"plugging the leak" of spatial data from their walled garden to Oracle's 
more accessible native spatial format.

AFAICS, the Oracle ST_GEOMETRY types are provided in order to increase 
compliance with the SQL/MM standard.

It's nice to hear that you find the PostGIS ST_GEOMETRY implementation 
easier to work with and more standards compliant.  I think this is the 
result of a phenomenon which occurs in many open-source projects. Having 
no commercial agenda and limited resources means that easiest path is to 
simply follow a standard as closely as possible, and to remove as many 
rough edges as possible.

Martin

Jorge Arévalo wrote:
> Hello,
>
> I'm studying Oracle SDO_GEOMETRY type [1], and I find it really
> tangled. But, AFAIK, the SFS standard [2] ST_Geometry is implemented
> too, provided by ESRI [3]. And even a third ST_Geometry object,
> provided by Oracle and basically equal to SDO_GEOMETRY type, is
> available [4].
>
> On the other hand, and please correct me if I'm wrong, PostGIS simply
> implements the standard ST_Geometry type. I'm reading the SFS
> documents [2], and the PostGIS documentation [5], and I find it
> simpler to specify if the geometry is compound or not, if it has
> holes... this is: the geometry's properties.
>
> So, my questions, as a beginner, are:
> - Why does Oracle use a tangled format like SDO_GEOMETRY, if they
> could simply implement the easier standard? Any obvious reason I'm
> missing? Apart from "we are a private company and define our own
> formats".
> - If I want to really understand spatial formats used in Oracle
> Spatial, PostGIS... are the [2] standards the best source? I think
> they are THE source... Am I right?
>
> Thanks in advance, and best regards
>
>   

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022