Views, PostGIS 2, and QGIS

If you wanted to create a view with spatial capabilities with previous versions of PostGIS all you needed to do was manually register the view in the geometry_columns table.  This is no longer possible since from PostGIS 2.0 forward geometry_columns is a view, not a table.  However, if you use typemods to define the geometry column your view the geometry_columns view will pick it up.  For example, I wanted to take some of the data I loaded from my .fit files and convert the metric data to imperial units, and reproject the data from WGS84 to UTM.   Here’s the view that does that for me:

SELECT id, no
, utility.units_from_to('meter', 'feet', altitude) AS edge_elev
, 32+(temperature*(1.8)) AS temp_f, speed*2.2369 AS mph, cadence
, ride_date, ride_time, elev_dem
, ST_Transform(the_geom, 26916)::geometry(Point, 26916) As the_geom FROM rides.ride;

The units_from_to function is something I picked up from PostGIS in Action.  The geometry column uses ST_Transform to reproject the data, and the typemod after defines the column as having point geometry with my UTM projection.

With the geometry column defined in that manner the geometry_columns view is properly populated.  When you want to use this view in QGIS it is visible as a loadable layer.  However, before you can use it you must tell QGIS which field is the unique row identifier.  To the left of the layer’s SRID number there will be a drop box on the PostGIS layer load screen.  This dropbox will give you a list of possible fields to use as your ID column.  Just select the column that contains unique values and you’re good to go.

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