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<p>Hi list,</p>
<p>using spatial predicates on virtual layers is slow by default as
spatial indexes are not used.</p>
<p><a class="moz-txt-link-freetext" href="https://docs.qgis.org/testing/en/docs/user_manual/managing_data_source/create_layers.html#use-of-indexes">https://docs.qgis.org/testing/en/docs/user_manual/managing_data_source/create_layers.html#use-of-indexes</a>
says that one can use the _search_frame_ column to "trigger
the use of a spatial index". The page has an example where this is
compared to a MBR/BBOX:</p>
<p>SELECT * FROM vtab WHERE
_search_frame_=BuildMbr(-2.10,49.38,-1.3,49.99,4326)<br>
</p>
<p>What does it actually mean and what does it really do? What is
the _search_frame_ really and why can we compare it for *equality*
against a simple bounding box? Is the "=" operator used for a
different logic?<br>
</p>
<p>Does it magically "trigger" the use of the sources' spatial
indexes in the background? That would surprise me.</p>
<p>The original docs of virtual layers at
<a class="moz-txt-link-freetext" href="https://github.com/mhugo/qgis_vlayers#index-support">https://github.com/mhugo/qgis_vlayers#index-support</a> _search_frame_
say</p>
<p>> For spatial indexes, a 'hidden' field named '<em>search_frame</em>'
is created for each virtual table (i.e. each referenced layer of
the virtual layer). The bounding box of the given geometry will be
used
to restrain the query to a particular region of space. This
bounding box is passed to the underlyng provider by a
QgsFeatureRequest with a "filterRect".</p>
<p>and show this example:</p>
<p>> SELECT * FROM pt, poly WHERE pt._search_frame_ =
poly.geometry AND Intersects(pt.geometry, poly.geometry)</p>
<p>I have some trouble parsing that in my brain. Does it mean that
there are N QgsFeatureRequests and each uses the _search_frame_
BBOX of its feature geometry as filterRect?</p>
<p><br>
</p>
<p>It would be great to demystify this and make people more aware of
how to make their virtual layers not slow.<br>
</p>
<p>Thanks!</p>
<p>Hannes<br>
<br>
PS: QgsFeatureRequest.<span class="sig-name descname"><span
class="pre">setFilterRect</span></span><span class="sig-paren">
could use a note about the spatial predicate being used to
determine which features to return. Intersection probably but it
is not clear. People might expect Within. Might file/fix this
later but feel welcome to do it before me. :)<br>
</span></p>
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Johannes Kröger / GIS-Entwickler/-Berater
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