Re: [Geotools-devel] [Geoapi-devel] On handling multiplicity, why AttributeDescriptor exists

[Andrea A. <and...@gm...>]

Rob Atkinson ha scritto:
> On the Xpath front...
>
> fully implementing xpath seems an unecessary burden, but partial 
> implementation has its issues.
>
> The OGC allows xpath in the Filter specification, stating
>
> "In order to handle property references in a consistent manner, a 
> filter expression processor must use the subset of XPath [10] 
> expressions defined in this document for referencing simple properties 
> and the properties and sub-properties of objects with complex or 
> aggregate non-geometric properties or properties encoded as XML 
> attributes."
Interesting, I'll have a look at the filter spec and see what the subset is.
>
> It seems reasonable IMHO to implement this subset.
>
> The other thing is whether there are optimisations possible to 
> minimise the repeated run-time dereferencing - if you invoke an xpath 
> expression once, its likely you are going to do if for every feature.  
> If you knew the xpath before feature instantiation (as per responding 
> to a filter request) you could possibly promote the accessor rather 
> than traverse the model each time. Maybe traversing the xpath isnt so 
> expensive - but if we're worried about the difference between getting 
> a descriptor and getting a value we might need to explore this.
Oh you bet it we'll have to explore some way to minimize the access cost 
or complex
features will fire back at us when users trying to exploit them.
Just look at that 300% slowdown the new property accessors caused, 
granted that
example is extreme and was easily fixed, but it's a sign of how fragile 
high performance
is, that's why I'm worried.

Cheers
Andrea