[Mapserver-users] Joining geospatial data to attribute data as basis for layer defi nition

[Sean Gillies]

On Tuesday, October 7, 2003, at 08:19  AM, Pryor, Clayton J wrote:

>
> Hi all,
>
> I am building a prototype application in MapServer to evaluate whether 
> we
> should migrate from ArcIMS or not.  One of the requirements of this
> application is to render colors of geospatial data based on attributes 
> in a
> table outside of the geospatial data.  In ArcIMS I defined these types 
> of
> layers as a SPATIALQUERY in the layer definition (as shown below).
>
...
> In my review of the MapServer documentation and discussions, I found a 
> post
> in 2001 that addresses the subject.  In part, it reads as follows:
>
> "No joins are supported for classification purposes.  You can only use
> directly attached attributes.  This was done for performance reasons."
>
> I have a few questions.
>
> 1)  Is this statement still true?
>
> 2)  What exactly are "directly attached attributes?"  I did a search 
> for
> this term and could find nothing.  I am assuming this means that the
> attributes must be part of the geospatial data table.
>
> 3)  How would you go about defining the equivalent to the above ArcIMS 
> layer
> definition in a MapServer application?  In this question, I am 
> concerned
> about the join.  I have seen examples of how to reference a singe 
> geospatial
> table and how to set up color classifications based on attribute 
> values.
>
> 4)  One thing I haven't explored is the possibility of handling the 
> join at
> the database level and then referencing the resulting join in the map
> definition.  Will that work?
>
> Thanks in advance for any help.
>
> Sincerely,
>
> Clay
>

Clay,

Go in the direction that you mention in your #4.  Even if MapServer was
enhanced to allow joins for classification, it's highly doubtful that 
the
implementation would ever match the performance of a join in your RDBMS.

cheers,
Sean

--
Sean Gillies
sgillies at frii dot com
http://www.frii.com/~sgillies