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I have a multipart polygon layer that resides on a PostGIS server. The layer is a parcel map, with each parcel having a single unique PIN. Most of the features are single parts, but some are two or more parts for various historical reasons. There are close to 40,000 features. If I check geometries for multipart geometries with only a single part, it returns all of my single part features. For small numbers of parcels I can use this tool to show these errors in the attribute table, invert the selection, and I have the parcels I am looking for. However, QGIS chokes when I try doing this with the full layer, and I really want to avoid having to do this in little bites when I'm sure there is a better way.

I want to select all the parcels that have more than one part. If I look at the Identify Results panel, the (derived) group shows the number of parts in that feature. This is the attribute I am looking for. How is this calculated, and how can I use this derived value as my criteria for a selection, or is there another way?

celeryfarmer
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  • ST_Dump on the PostGIS side is another route. https://gis.stackexchange.com/questions/307959/turning-a-single-multipolygon-into-many-small-polygons – Mapperz Oct 01 '20 at 16:04

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Go to the select by expression tool and use

num_geometries( $geometry) > 1

Then click on Select Features (lower right) and all the multi part geometries will be selected.

enter image description here

Ian Turton
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  • I did not think this would work as num_geometries() takes a collection of geometries as the argument. So, a feature with multiple parts is a collection of geometries? Not a single geometry object with? Your simple solution is correct, of course, and did exactly what I needed. Thank you. – celeryfarmer Oct 01 '20 at 17:44