[Qgis-developer] why Difference takes much more time then Clip?

[G. Allegri]

Just two graphics two show a brutal benchmark (measuring time in every
cycle step).

Clip: http://cdn.img42.com/31e0205c1273a1d1c705e5109da48db1.png
Difference: http://cdn.img42.com/eeba68e62ea7d4db50662c2745a6d912.png


2014-07-01 20:05 GMT+02:00 G. Allegri <giohappy at gmail.com>:

> To be correct, the two postgis queries are:
>
> select min(l.osm_id),st_difference(l.geom,st_union(e.geom)) as geom from
> linee_selezione_s l join edifici_buffer_s e
> on ST_Intersects(l.geom, e.geom) group by l.geom
>
> select min(l.osm_id),st_intersection(l.geom,st_union(e.geom)) as geom from
> linee_selezione_s l join edifici_buffer_s e
> on ST_Intersects(l.geom, e.geom) group by l.geom
>
>
> 2014-07-01 19:37 GMT+02:00 G. Allegri <giohappy at gmail.com>:
>
> I'm doing some tests on fTools/Processing spatial operators, with a cover
>> multilinestring layer and a base polygon layer.
>>
>> Running the following query on PostGIS I get similar timings with
>> st_intersection and st_difference:
>>
>> select l.osm_id,ST_intersection(l.geom,e.geom) as geom from
>> linee_selezione as l,edifici_buffer as e where ST_Intersects(l.geom, e.geom)
>>
>> while if I run the Difference tool QGIS chokes and it takes magnitudes of
>> more time then Clip.
>> If made both tools as similar as possible, just changing this [1] line of
>> code, but the difference in execution time remains big.
>>
>> I can't find out where is the difference, because both PostGIS and QGIS
>> base the operations on GEOS.
>> Any hints?
>>
>> giovanni
>>
>> [1]
>> https://github.com/qgis/QGIS/blob/master/python/plugins/processing/algs/qgis/ftools/Clip.py#L107
>>
>> --
>> Giovanni Allegri
>> http://about.me/giovanniallegri
>> Twitter: https://twitter.com/_giohappy_
>> blog: http://blog.spaziogis.it
>> GEO+ geomatica in Italia http://bit.ly/GEOplus
>>
>
>
>
> --
> Giovanni Allegri
> http://about.me/giovanniallegri
> Twitter: https://twitter.com/_giohappy_
> blog: http://blog.spaziogis.it
> GEO+ geomatica in Italia http://bit.ly/GEOplus
>



-- 
Giovanni Allegri
http://about.me/giovanniallegri
Twitter: https://twitter.com/_giohappy_
blog: http://blog.spaziogis.it
GEO+ geomatica in Italia http://bit.ly/GEOplus
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