How difficult is adding a new aggregate function to Json Facet?
it has been awhile since I'm using Solr, but I have never see the code. Now
I'm in the need to add some capability to Solr.
What I need to add is another aggregate function to Json Facet API, for
example "sum(field1)-sum(field2)". How hard do you think it would be? Also,
it would be great if you give me some advice on where to start.