On 4 March 2017 at 18:03, Tobias Wendorff <tobias.wendo...@tu-dortmund.de> wrote: > Am Sa, 4.03.2017, 08:34 schrieb Alessandro Pasotti: >> >> Yes, that would be a strategy, but you might run into problem with >> sorting and filtering if the data provider does not support for it. > > All database-like formats, even Shapefile's DBF, support sorting > on database level. I've never understood, why some applications > download the whole content first and
Because it depends on whether the front-ends filtering and sorting functions match up with the backend's. In the case of QGIS our expression engine supports a lot of functions which just can't be translated to Postgres/PostGIS functions. In this case you have no choice but to fetch everything and do the filtering/sorting on the client side. QGIS does have robust support for "compiling expressions", which is basically translating them from QGIS' expression engine to functions supported by the backend. This works great for simple expressions, and depending on the provider even some complex ones (eg postgres provider has large coverage of mapping qgis functions->provider functions. OGR does not and can only handle simple field=value compilation). Some providers also have support for compiling the sort order to perform on the database side - unfortunately postgres does not (see https://github.com/qgis/QGIS/blob/master/src/providers/postgres/qgspostgresfeatureiterator.cpp#L130). I'm sure Matthias would love to re-enable that code and give postgres a performance boost if someone wants to step up and sponsor it! Nyall _______________________________________________ Qgis-user mailing list Qgis-user@lists.osgeo.org List info: https://lists.osgeo.org/mailman/listinfo/qgis-user Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
