[gdal-dev] reading features without loading geometries

[Milton Jonathan]

Hello Frank

Thanks a lot for your quick answer! I guess that approach will do in 
practice, since large data will usually be in a RDBMS. Actually, I also 
thought about this, but I didn't have a spatial database at hand (I did 
test it on a shapefile, but as you said in that case the driver loads 
the geometry anyway..)

Well, another similar thing that I was trying to do was to just load the 
Envelope of each object without loading the whole geometry. Do you think 
that would be possible with OGR? The reason for doing this is to quickly 
scan all the objects' bounding boxes in order to build a spatial tree 
for caching purposes. It would be great if we could do that!

Thanks again
Milton


Frank Warmerdam wrote:
> Milton Jonathan wrote:
>> Dear all,
>>
>> I would like to know if there would be a standard mechanism in OGR for 
>> reading features without loading their geometries (e.g., for 
>> efficiently building a list of features from which to choose from, 
>> even for layers with a large number of features with significantly 
>> sized geometries)
>>
>> I suppose that some drivers may not able to do it (e.g., drivers for 
>> simple file formats), but for database drivers that should be simple.
> 
> Milton,
> 
> One approach to this is to execute a custom query using ExecuteSQL().
> 
> 
> eg.
> 
> OGRLayer *poResult = datasource->ExecuteSQL(
>       "SELECT FID from ROADS where class in (\"M1\",\"M2\")" );
> 
> This would give back features with only the attribute FID matching the 
> query.
> When the backend is a real RDBMS this would generally be done efficiently
> without touching the geometries.  For flat formats like Shapefiles the
> machinery still reads the whole features to execute the query, so it would
> not be particularly fast.
> 
> 
> Best regards,