[QGIS-Developer] GPKG and FID -- can we fix this mess?

[Matthias Kuhn]

If there is a managed database where there has been a conscious decision to
have an integer id column (postgres case) and workflows are built around
this: filling the id on insert, building foreign keys against it etc, that
works just fine.

But if GeoPackage is used as a generic dataformat (typical Shapefile
replacement), requirements are different and workflows less managed. E.g.
The "simple" case of merging two datasets already poses the question what
to do with the two colliding fid attributes. Delete the original fid
columns and regenerate one. Or preserve both and generate a new one
(leading to fid, fid_1 and fid_2).

If we look at the fid attribute as part of the data (where we have foreign
keys pointing two etc) we want it to be preserved.
If we look at it as a purely internal technical id (for RTree management
etc) we don't care for it and can just regenerate and forget about it.

It looks as if we need to keep it as a technical id.
I like the idea to transparently generate it in addFeature.
I would also prefer to hide it away from the user and only leave it
available through $id. This would make it clear to users that it's for
internal use and no one would expect it to be preserved after a merge
operation. For backwards compatibility there could be a provider flag to
re-enable it.
I hope that makes sense.

Matthias
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