GeoServer SLD Functions

Question

I am in need to use a postgres query inside an sld file. Based on the documentation, before testing the now() function, I decided to make a test with the max function. I want to style only the max value in the qtd_deteccoes property, ref to CODE 1. I retrieved the max value into the ogc:Literal tag, in other words (ogc:Literal has the value returned by the max function) and after that, I compared the value using the filter ogc:PropertyIsEqualTo. While validating the sld, GeoServer does not show any error, but I am not sure if the sld makes any sense. Could anyone give an insight to the code bellow?

<ogc:Filter>
    <ogc:PropertyIsEqualTo>    
         <ogc:PropertyName>qtd_deteccoes</ogc:PropertyName>
             <ogc:Literal>
                  <ogc:Function name="max">    
                   <ogc:PropertyName>qtd_deteccoes</ogc:PropertyName>    
             </ogc:Function>
            </ogc:Literal>
     </ogc:PropertyIsEqualTo>
</ogc:Filter>
      

Answer 1

You are currently trying to compare the literal string:

         <ogc:Function name="max">    
               <ogc:PropertyName>qtd_deteccoes</ogc:PropertyName>    
         </ogc:Function>

with the value of the attribute qtd_deteccoes which will never be true, unless you have some very strange data.

If you remove the <ogc:Literal>s from your query:

<ogc:Filter>
    <ogc:PropertyIsEqualTo>    
         <ogc:PropertyName>qtd_deteccoes</ogc:PropertyName>
                  <ogc:Function name="max">    
                   <ogc:PropertyName>qtd_deteccoes</ogc:PropertyName>    
             </ogc:Function>
     </ogc:PropertyIsEqualTo>
</ogc:Filter> 

You are comparing the value of qtd_deteccoes with the result of the max function applied to the same value - which will always be true.

An SLD function only has access to the feature being rendered so you can't calculate the maximum of all the values in a FeatureCollection that way.