[Qgis-developer] Expression based labeling

[Martin Dobias]

On Fri, Oct 28, 2011 at 4:15 AM, Sandro Santilli <strk at keybit.net> wrote:
> On Thu, Oct 27, 2011 at 09:46:11AM -0300, Martin Dobias wrote:
>> That is because not all data providers use ids in interval [0,N-1].
>> For example, postgres provider uses the column with primary key as ID
>> - so the values may be arbitrary. It is better to do a select()
>> followed by just one nextFeature().
>
> Would it make sense to abstract provider-specific IDs so that the
> [0, N-1] range holds for everyone ?

This is actually done within attribute table - we store feature IDs
for the range [0,N-1] so that we can later access the feature data by
row number. But that comes at an additional price: the layer has to be
scanned first in order to obtain the mapping and it has to be updated
whenever some changes to the layer are done.

What would be the requirements of such provider-specific IDs? What to
do when a feature is deleted? If we recreate the mapping, some IDs
stored by users would change, leading to problems if the user stores
them as references to features. If we keep the original mapping, then
it may contain holes and therefore the convenient [0, N-1] range would
not be valid.

Martin