Solr: Sort documents by number of fields

Question

Is there a way sort documents in Solr by the number of fields in each document?

The solr core in question has about 200 different fields, while not every field must be present in every doucment. To circle out datasets that contain not enough fields to be correct, I'd like to work through a *:* query sorted from the lowest number of fields per documents upwards.

I didn't find anything on this specific use case. Most results I found were about the relevance of individual fields, however this might not help here given the large field spectrum of the core.

Answer 1

It might be possible by sorting on a function query. That function would return a value that would be higher the more fields the doc has. But I am afraid that function would be huge (and slow), as it would need to enumerate all fields in the function.

By far the easiest thing would be to, at index time, add a 'nbFields' field containing the number of fields. Then you can sort easily on that one.