[gdal-dev] GetFID incorrect after sorting layer

[Even Rouault]

Jon,

this is a bug on the result SQL layer implementation of GetFeature(). 
Can you file a ticket in https://github.com/OSGeo/gdal/issues/new with 
the below elements? Thanks

Even

Le 22/06/2022 à 15:37, Jon Morris a écrit :
>
> Hello all,
>
> I’m having trouble getting features in a layer after sorting with SQL 
> query. It seems that the FIDs change when the layer is sorted. I’m ok 
> with that, but if you take the id returned by GetFID and call 
> GetFeature on the layer, you get a different feature. I would have 
> thought GetFID/GetFeature should remain in sync even after sorting. 
> The docs for GetFeature say “If this method returns a non-NULL 
> feature, it is guaranteed that its feature id (OGRFeature::GetFID()) 
> will be the same as nFID.“
>
> https://gdal.org/doxygen/classOGRLayer.html#ace902dfb44335fb1a877aa035ecff209 
>
>
> I’m aware I could just iterate through the layer, but I want to 
> add/delete features so prefer to iterate through a list of ids 
> instead. Should I just ignore the FIDs and iterate through 
> range(layer.GetFeatureCount()) instead? Is that guaranteed to give me 
> every feature in size order?
>
> I’m using GDAL 3.4.1 on Python 3.8.12
>
> Repro case:
>
> https://gist.github.com/jontwo/01d8cbf1e047e36d2fb2f7786ad60e73
>
> Creates a layer with 4 features, areas 1, 64, 4, 25
>
> Then sorts them with ds.ExecuteSQL(f"SELECT * FROM lname ORDER BY 
> OGR_GEOM_AREA ASC")
>
> Gets a list of FIDs in the layer using GetFID
>
> Calls GetFeature for each of these FIDs.
>
> Output:
>
> Areas before: [(0, 1.0), (1, 64.0), (2, 4.0), (3, 25.0)]
>
> Areas after: [(0, 1.0), (2, 4.0), (3, 25.0), (1, 64.0)]
>
> Requested FID 0 (area 1.0) got 0 (area 1.0)
>
> Requested FID 2 (area 4.0) got 3 (area 25.0)
>
> Requested FID 3 (area 25.0) got 1 (area 64.0)
>
> Requested FID 1 (area 64.0) got 2 (area 4.0)
>
> Thanks,
>
> Jon
>
>
>
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