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I have a layer coming from a WFS that itself gets its data from a PostGIS database via GeoServer.

As of the capatbilities of the service I can perform a Crosses operation in oder to select features that cross a specific geometry. I assume such a request is directly mapped to the appropriate geometry-function within PostGIS, i.e. ST_Crosses.

I wonder if performing a spatial query in QGIS using "Select by Location" for instance will actually route to a spatial query on the database, or if QGIS just uses its own client-side implementation. Or in other words: Does QGIS use the spatial capabilities of the underlying datastore?

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When we bring a WFS into QGIS Canvas. QGIS send request to get all features and create a local layer (how? that is a different question). The operations you perform now, like "select by location" will be performed on local copy as it is any other layer in QGIS.

This can be confirmed by changing the data on the server; the QGIS will not update the local layer once it has created the local copy. To get new data you have to remove that layer and bring the WFS layer again into the canvas.

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