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?