[gdal-dev] Selecting just the geometry column with OGR SQL

[Even Rouault]

Jukka,

> I believe that OGR SQL dialect adds the geometry column into SQL selection
> by default. Am I right with this? I volunteer to edit the documentation
> https://gdal.org/user/ogr_sql_dialect.html if this is the case. But what I
> can't understand is another side of the story, how to select just the
> geometry field with OGR SQL dialect from some data sources.
> 
> This command returns both time column and geometry from GML
> ogrinfo -sql "select time from timetest" timetest.gml
> I can select just the geometry from geometry column named as
> "geometryProperty" by ogrinfo -sql "select geometryProperty from timetest"
> timetest.gml
> 
> The behavior is the same with SpatiaLite when I use "-dialect OGRSQL".
> Geometry is selected automatically but it can be selected also by name. But
> I cannot discover any way to select just the geometry field from shapefile.
> Is it because geometry column in shapefile does not have any name as this
> Python test seems to prove?
> >>> from osgeo import ogr
> >>> shp_ds = ogr.Open('timetest.shp')
> >>> shp_lyr = shp_ds.GetLayer(0)
> >>> shp_lyr.GetGeometryColumn()
> 
> ''
> 
> OpenJUMP JML driver does not report a name for geometry column either. Is it
> rather an exception that drivers give names for the geometry columns? I
> know I can select only the geometry column with the SQLite dialect, but I
> am curious.

Indeed, in OGR SQL, if the geometry column has a non-empty name (typically for 
database drivers), you can use it to select it. When the geometry column name 
is empty, such as in the shapefile or OpenJUMP driver, you can use 
_ogr_geometry_ (with leading and trainling underscore) to select it. This is 
likely undocumented.

As far as I can see, this was introduced per
https://github.com/OSGeo/gdal/commit/1519dcb00bd7802d2f701114f0e1cb9b524a547f
to fix an issue with comparison with empty string literals (this was before 
OGR enforced the difference between single-quoted string literals and double-
quoted identifiers)

Even

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