I am trying to make a Virtual layer that is based on a map layer that only returns the features in the current map canvas extent. The purpose is to setup a Data Graph that shows statistical data of only the features in the current map extent.

I cannot find any examples of this being done in QGIS or using SQL. Is it possible?

1 Answer 1


The trick is to create a function that access QGIS graphical interface, and that is piped to the query of the virtual layer.

  1. Open a function editor (from anywhere, including from field calculator) and create a new function that reads the canvas extent and returns it as a geometry.
from qgis.core import *
from qgis.gui import *
from qgis.utils import iface

@qgsfunction(args='auto', group='Custom')
def currentExtent(feature, parent):
    return QgsGeometry.fromRect(iface.mapCanvas().extent())
  1. Create a new virtual layer and use the new function, selecting features that intersects the displayed area. To get features entirely within the displayed area, use ST_Within() instead.
FROM mylayer
WHERE st_intersects(mylayer.geometry, currentExtent());

Note that if you have the attribute table open, you would need to click the refresh icon after panning the map.

  • How will this handle features that are partially outside the map canvas?
    – csk
    Commented Apr 26, 2019 at 18:16
  • 2
    @csk Since it uses st_intersects, features that are partially displayed will be returned. One can change the query and use ST_Within instead (or any other spatial predicate)
    – JGH
    Commented Apr 26, 2019 at 18:19
  • 1
    @csk and since it return *, the original (complete) geometry will be returned
    – JGH
    Commented Apr 26, 2019 at 18:21
  • Thank you for the explanation. That's what I suspected, but wanted to be sure. A large feature that only slightly extends into the map canvas could have an unexpected impact on the graph.
    – csk
    Commented Apr 26, 2019 at 18:24
  • I didn't know custom expressions could be used in virtual layers ... that's cool ... i suppose they can have a big impact on queries execution times ... ?
    – Snaileater
    Commented Apr 26, 2019 at 18:50

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