[QGIS-Developer] QGIS virtual layer incredibly slow

[Totò Fiandaca]

Hello everyone,
I use virtual layers a lot and confirm the slowness and the continuous
updating that makes the tool almost unusable.

it would be very useful to speed things up.

thank you

Il giorno gio 2 mag 2019 alle ore 11:02 Andreas Neumann <a.neumann at carto.net>
ha scritto:

> Hi,
>
> One of my colleagues created a QGIS project where they loaded a parcel
> layer with approx. 24000 polygons (data source Postgis) and another layer
> with 3 polygons representing project perimeters of contruction sites. That
> second layer is in a Geopackage.
>
> Now they want to select all parcels that are intersecting the project
> perimeters.
>
> The query is straightforward:
>
> SELECT lie.nummer_grundstueck, lie.gemeinde, tbap.bemerkungen, lie.geometry
> FROM lie_liegenschaft lie, tba_projektperimeter tbap
> WHERE ST_Intersects(lie.geometry, tbap.geometry)
>
> Now the problem is: this query (24000 parcels, 3 project perimeters) is
> just incredibly slow: about 40 seconds. In a Postgis view this would be way
> less then a second! The worst thing: that virtual layer is updated with
> each and every zoom and pan. Is there no way to cache a virtual layer?
>
> Can I do something to speed up our virtual layer? The parcel layer only
> chanes once a month and the perimeters only occasionally, so it would be
> best to cache the result within a QGIS session, because there are no
> changes in the geometries of both layers expected.
>
> I understand that virtual layers somehow have to transfer all data formats
> (through ogr) into SpatiaLite or SQLite to do the query, but this is really
> way too slow for real world usage in my opinion (unless I do something
> wrong in our query).
>
> If you have any ideas how to speed up virtual layers in QGIS it would be
> much appreciated.
>
> Thanks,
> Andreas
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