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 <gioha...@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 <gioha...@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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