[Qgis-user] Qgis : Union process with aggregation/unique feature by overlay

[celati Laurent]

Good afternoon,
I have a very big polygonal layer (over 1 million objects). This layer
contains a 'gid' column whose values uniquely identify each object. There
is also a 'hierarchy_level_id' column:
this string column allowing to characterize the importance/priority level
of each object: 1, 2A, 2B, 3A, 3B.

My goal for this layer would be:

   - To perform a union process in the QGIS meaning:
   https://docs.qgis.org/3.40/en/docs/user_manual/processing_algs/qgis/vectoroverlay.html#union
   - In the result/output file, I want to create two new columns. The first
   column list the 'hierarchy_level_id' for each feature. And a second column
   would retrieve the maximum value of the 'hierarchy_level_id' for each
   feature.


In other words, for areas where there is one or more overlaps, I don't want
that my union result contains as many identical features as there are
features participating in this overlap. I want a single unique feature to
be created per overlap/overlapping area.

How can I perform this process with QGIS? Should I use the Qgis Union
process? Use the aggregate function? Or something else?

In advance, thanks so much.
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