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How does the virtual layer engine work in QGIS?
With default parameters set, the virtual layer engine will try its best to detect the type of the different columns of the query, including the type of the geometry column if one is present. This is done by introspecting the query when possible or by fetching the first row of the query (LIMIT 1) at last resort.
What do you call a virtual layer in SQL?
These layers are called virtual layers: they do not carry data by themselves and can be seen as views to other layers. Open the virtual layer creation dialog by clicking on Add Virtual Layer in the Layer menu or from the corresponding toolbar. The dialog allows you to specify an SQL query.
Can a virtual layer ignore the geometry field?
No geometry: this option forces the virtual layer to ignore any geometry field. The resulting layer is an attribute-only layer. Geometry Column: this option allows to specify the name of the column that is to be used as the geometry of the layer.
How to create a virtual layer in SQLite?
Functions from SQLite and spatial functions from Spatialite can also be used in a virtual layer query. For instance, creating a point layer out of a attribute-only layer can be done with a query similar to: SELECT id, MakePoint (x, y, 4326) as geometry FROM coordinates
When to use the keyword geometry in QGIS?
To refer the geometry column of a layer, use the name geometry. Contrary to a pure SQL query, all the fields of a virtual layer query must be named. Don’t forget to use the as keyword to name your columns if they are the result of a computation or function call.
How are joins and complex queries created in QGIS?
Joins and complex queries can also be created simply by directly using the names of the layers that are to be joined. The underlying engine uses SQLite and Spatialite to operate. It means you can use all of the SQL your local installation of SQLite understands.
How to speed up the rendering in QGIS?
How to Speed up QGIS (Rendering) 1 Enable multi-threaded rendering: Modern CPUs are multi-core processors. QGIS uses by default only one CPU core for… 2 Large vector data sets: Go to Layer Properties -> General -> Create Spatial Index When working with vector files… 3 Large raster data sets: More