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).
Mexico municipalities

I managed to get the .csv table into the shapefile via other procedures. And I keep the remaining attributes in this table.

enter image description here

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.

enter image description here

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?

  • I would say shapefiles are the issue, not QGIS. You should look into loading your data into SQLite and best case using PostGIS to do the processing... Dec 3, 2016 at 1:50
  • Which QGIS version? This should work much better in 2.18
    – ndawson
    Dec 3, 2016 at 6:46
  • Have you tried using dissolve from the processing toolbox? I find those tools to work better
    – Liam G
    Dec 3, 2016 at 11:59
  • Thanks for the suggestions. I hadn't considered any of the three: DBformats, QGIS version, and toolboxes. Perhaps toolboxes I had, from other posts that mentioned GRASS, but I couldn't find how to load them. So I'll start with that. The version is an easy fix. And SQLite, PostGIS seem more ellaborate, so I'll leave that for the end =P.
    – Diego-MX
    Dec 5, 2016 at 0:35


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