I have data about the basic municipality areas of Mexico in a shapefile. I also have data about which metro areas they correspond to (csv). I would like to dissolve them -or so I understand from this article and others.
This picture gives an idea of the number of polygons. But the white edges are NOT the ones I want to dissolve (they're there to give an idea).
I managed to get the .csv table into the shapefile via other procedures. And I keep the remaining attributes in this table.
Overall there are about 3000 municipalities, but most of them are linked to no metro areas, and have NULL values. Only 300 are linked to metro areas, of which there are about 80. The metro areas with the larges number of municipalities has 75 (that is Mexico City, as it is very big).
I run Vector -> Geoprocessing tools -> Dissolve, and choose the
zonas_metr = metro areas field as dissolver.
The problem that I have is that it goes very slowly, and it freezes after more than an hour taking up all RAM. So I wonder if there's something I can do better or maybe leave it working more time. I also hope that the NULLed entries are not being merged. Are they?