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

Sun Feb 26 10:02:41 PST 2006

Hi!

Thanks so much for looking into this. I was trying to dig through the
code as well. I noticed the "FEATURE_COUNT" parameter, and also a
different mode setting ... i.e. mode = MS_SINGLE vs MS_MULTIPLE, but I
couldn't really grasp the details enough to know if that had anything to
do with my situation. It sounds like you are saying it does.

Are you also suggesting that, in theory, if I use text/html as my output
format, and have a one-to-many type relationship, as a JOIN might, the
HTML generated from the templates will have one body entry per record
(as a one-to-many JOIN might)?

Assefa, are you the query maintainer? Or query guru? What is your role
in all this? Who would I e-mail to discuss/propose changes to the GML
output so that the records returned via GML are the same as returned via
HTML or Text?

Thank you again! I will let you know how this turns out.

Nick Floersch

-----Original Message-----
From: Yewondwossen Assefa [mailto:assefa at dmsolutions.ca] 
Sent: Sunday, February 26, 2006 12:56 PM
To: Nick Floersch
Cc: Paul Ramsey; MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] MapServer, PostGIS, Subquery with
JOIN, WMS GetFeatureInfo / Followup Question

Hi There,

  From what I can see in the code, here is what you should do :

    * when doing the GetFeatureInfo use the request parametetr
FEATURE_COUNT and set it to be above 1. The default is 1. This would
trigger the nquery.
    * define on your server FEATURE_INFO_MIME_TYPE metadata to something
like text/html
    * do your wms request with info_format=test/html (same mime type
that the one defined in the map)

   This conditions should normally trigger all the logic used when doing
regular query with the mapserver cgi (with the template processing).

  Give it a try and let me know. If It does not work, I can dig more
into it.

Later,

Paul Ramsey wrote:
> 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

-- 
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Assefa Yewondwossen
Software Analyst

Email: assefa at dmsolutions.ca
http://www.dmsolutions.ca/

Phone: (613) 565-5056 (ext 14)
Fax:   (613) 565-0925
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