MapServer, PostGIS, Subquery with JOIN, WMS GetFeatureInfo / Followup Question

[Paul Ramsey]

Nick,

The answer to this takes either Assefa's input, or a browse of the  
source code to find out what the GetFeatureInfo is going.  It is  
probably doing a 'query' rather than an 'nquery', so having a multi- 
item template may make no difference at all.  The WMS spec is silent  
on what the actual behavior of the GetFeatureInfo should be, so it is  
very much an implementation question.

Paul

On 24-Feb-06, at 12:59 PM, Nick Floersch wrote:

> I thought of a way to rephrase/reask the question I am now stuck with.
>
> I was reading through my MapServer book ('Beginning MapServer') on  
> Queries and Joins, to see if mr. Kropla had any suggestions.
>
> He wrote that, in doing regular MapServer JOINs, if you want to  
> have a JOIN produce one-to-many results, you need to specify a  
> template for the JOIN to format each record beyond the first one  
> that is returned into the final HTML result. Otherwise, only the  
> first record will be returned.
>
> How does this principal apply to the GML generated by  
> GetFeatureInfo requests? Obviously we don't need to define a valid  
> HTML template to output the data... but how do GetFeatureInfo  
> requests deal with one-to-many situations, and does the output  
> format make any difference on how it is handled? If I have my  
> GetFeatureInfo request return HTML rather than GML, can I use  
> templates and have it handle a one-to-many arrangement succesfully?
>
> Thoughts, ideas?
>
> Thanks!
> Nick Floersch
>
> From: Nick Floersch
>
> Hello Paul, Steve, Jeff, and other MapServer users,
>
> Thanks for the replies.
>
> By adding an appropriate 'using unique' clause to my DATA entry in  
> the mapfile, I got my layer based on a view to draw. I am glad that  
> views can be used.
>
>
> There is a trick that had to be realized. At first, I put in a  
> 'using unique the_geom' clause which of course assumed that my  
> geometry field was unique. But, because the source of my layer is a  
> view (or a subquery in its previous life) which has a left outer  
> join in it, the geometry column is far from unique. This is what I  
> thought I wanted - to generate a layer which had multiple points at  
> the same locations with different attributes... a series of  
> attributes for a given point location. In my case, the idea is that  
> the feature point can have images associated with it from a table  
> of images. So, a left outer join on that table gives me a layer  
> with duplicate points that have different values for the image name  
> attribute.
>
> Anyway, initially, after I realized that 'the_geom' is not a unique  
> field for me, I switched to using a field that is unique in my  
> view, and things came to life. My GetFeatureInfo requests suddenly  
> started returning attributes, and life looked good.
>
> But no. Not perfect. What I had in mind has not worked quite right  
> - my GetFeatureInfo tool clicks on a point feature, and I get a  
> list of attributes, by layer, for each feature under the pointer.  
> Except, not all the duplicate point features are returned. The best  
> I can think to describe it is this: I have a stack of points all  
> defined in the same layer, and I click on the stack, and only the  
> top point in that stack is returned. I was hoping it would return  
> data for each of those points in the stack.
>
> So, is there some way I can make this work better? Am I totally  
> barking up the wrong tree? I have one GetFeatureInfo request that  
> needs to return multiple values for the same field of a given  
> feature, based on a join...
>
>
> Thanks for any thoughts and help!
>
> Nick Floersch
>
> -------
>
> If you have the option, please don't use oid, use a primary key  
> (like the 'gid' created by shp2pgsql) as your unique key. Primary  
> keys already have indexes, oids do not. Primary keys show up  
> automatically in a "select *" query, oids do not. oids are
>
> deprecated in pgsql and not available by default in pgsql 8.1.
>
> Basically oid is now a deadend, and we need to start erasing all  
> uses of them.
>
> P
>
> -------
>
> Nick,
>
> I seem to remember a post from one of the postGIS guys awile back  
> the you needed to add an entry in the geometry_columns table for  
> the view.
>
> -Steve W.
>
> --------
>
> This is on my "figure out some day myself" list, too. I'm doing the  
> view thing right now, but I'd like to not have to create views for  
> everything.
>
> > The only thing I can think of is... does the PostGIS connector  
> require
>
> > the table to have OIDs? It looks that way.
>
> Yes. It needs some unique field in order to randomly access an  
> individual rows, it just so happens that OID is a convenient way to  
> get that in most cases. You can also specify your own unique column  
> name with "using unique <column name>" if your view doesn't have an  
> OID column but you have some other key you can use. I just pull in  
> the OID from the main geometry-containing table when I define the  
> view.
>
> --
>
> Jeff Hoffmann