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

[G. Allegri]

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
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