[gdal-dev] RFC 29: OGR Set Desired Fields

[Frank Warmerdam]

Martin Dobias wrote:
> Hi,
> 
> based on the discussion about optimizing the performance of reading
> features in OGR few days ago, I've prepared an RFC that proposes API
> for limiting which fields will be fetched when reading features:
> 
> http://trac.osgeo.org/gdal/wiki/rfc29_desired_fields
> 
> Any comments or suggestions are highly welcome.

Martin,

Some thoughts:

1) Should there be a TestCapability() value indicating whether
the field selection option works for a particular driver?  It
doesn't seem critical to me.

2) Should we try to overload this function to also allow
selection or deselection of the "special" fields geometry
and style?  We try to treat them a bit like fields in
things like http://trac.osgeo.org/gdal/wiki/rfc6_sqlgeom.

3) I dislike the passing of an unnecessary nNumFields
parameter and unintuitive use of it as a special flag.

4) I think you need to make it clear whether where the
method is implemented.  I get the impression the OGRLayer
class will implement the method, and the OGRLayer class
will keep around a desired fields array.  Should driver
specific classes be able to override this method to detect
when the desired list changes?   I think so, even though
most drivers using the desired fields functionality will
not need to do so.

5) I'd like the RFC to be clear that some (many) drivers
may not support this functionality in which case all
fields will be returned instead of some being null.

As an alternative, if we would like to support the "special
fields" for geometry and style perhaps we should offer
a method like:

virtual OGRErr OGRLayer::SetIgnoredFields( const char **papszFields );

The argument is a list of fields to be ignored, by name, and
the special field names "OGR_GEOMETRY" and "OGR_STYLE" will
be interpreted to refer to the geometry and style values of
a feature.

Passing NULL for papszFields will clear the ignored list.

The method will return OGRERR_NONE as long as all the field
names are able to be resolved, even if the method does not
support selection of fields.

I chose passing by field name so that we could handle
OGR_GEOMETRY, OGR_STYLE and possibly some other special
fields in the future.  I changed the sense to "ignored"
so that we wouldn't accidentally drop things like
geometry and style just because they weren't explicitly
listed in a desired list.

Note, I'm willing to implement this functionality for a
bunch of drivers I think might benefit, and I also think
the RFC should indicate we will connect this to the
-select option of ogr2ogr.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent