[Qgis-developer] Potentially serious performance regression in new geometry - should 2.10 be delayed?

[Marco Hugentobler]

Hi Nyall

 >This potentially has huge impacts on the performance of common tasks 
such as selecting all features which intersect a geometry.

Are you really sure the impact is huge? If you select features on the 
map, you get a new one from provider each time. So also with the old 
geometry class, it was necessary to convert to geos for each intersect.

But yes, if several geos operations are done in sequence on really the 
same geometry instance, there is an overhead now. However, I would 
assume the geos conversion takes far less time than the geos operation 
itself. Did you observe cases where the geos conversion really takes 
very long time?

Lets assume the geos conversion indeed takes a long time. Couldn't we 
just store the pointer to QgsGeos (or better QgsGeometryEngine class) 
inside QgsGeometry? This would be straightforward to do. Furthermore, it 
is possible to prepare QgsGeos. A prepared geos geometry can do _much_ 
faster intersects/touches/ etc. operations if called repeatedly. We can 
cache a prepared QgsGeos now with the new engine, but we couldn't do 
that with the old QgsGeometry class. So for repeated predicate 
calculations, the new geometry is much better, also performance wise.


Regards,
Marco

On 23.06.2015 04:09, Nyall Dawson wrote:
>
> Hi all,
>
> Unfortunately, we've become aware of a serious performance regression 
> caused by the new geometry engine. Basically, the situation is that 
> for all geometry operations which rely on geos (think buffers, splits, 
> spatial relation operations such as intersects and within,... ) the 
> geometry now needs to be converted into a geos representation with 
> *every* operation. In the old geometry engine this conversion was done 
> once, and the result stored so that follow up operations would not 
> need to recalculate it. This potentially has huge impacts on the 
> performance of common tasks such as selecting all features which 
> intersect a geometry.
>
> I've had a look, and unfortunately it's not trivial to fix this. I 
> think the correct solution to this is to:
>
> - make QgsGeometryEngine accept and return QgsGeometry containers, not 
> QgsAbstractGeometryV2
> - store the generated geos representation of geometries within 
> QgsGeometryPrivate inside the QgsGeometry container. This way it will 
> be reusable between different geometry operations, and shared when 
> QgsGeometry objects are copied. This will also have the benefit that 
> if a geometry is prepared using geos then subsequent geos operations 
> performed on that QgsGeometry and its shared copies will be much faster.
> - make QgsGeometry a friend class of QgsGeo, so that it can access 
> QgsGeometryPrivate to retrieve or set the geos representation of the 
> geometry as required
>
> An alternative (short term) solution would be to just cache the geos 
> representation when geometry operations are called through the older 
> QgsGeometry modification/relationship operations. This would be 
> easier, but means that the API of QgsGeometryEngine will be stuck with 
> the current design, and we won't be able to properly fix this until 
> breaking the api for 3.0.
>
> Either way, I doubt this can be addressed within the remaining 3 days 
> we have until release. Should we delay to address this? Release with 
> the regression? Or am I missing something and there's an easier 
> solution we could implement? Or even possibly this additional cost of 
> recalculating the geos representation is trivial and can be ignored 
> (maybe someone could test this with a little repeated intersection 
> script)?
>
> Thoughts?
>
> Nyall
>
>
>
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-- 
Dr. Marco Hugentobler
Sourcepole -  Linux & Open Source Solutions
Weberstrasse 5, CH-8004 Zürich, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee

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