Assume that I have a field called price for the documents in Solr and I have that field faceted. I want to get the facets as ranges of values (eg: 0-100, 100-500, 500-1000, etc). How to do it?
I can specify the ranges beforehand, but I also want to know whether it is possible to calculate the ranges (say for 5 values) automatically based on the values in the documents?
There may well be a better Solr-specific answer, but I work with straight Lucene, and since you're not getting much traction I'll take a stab. There, I'd create a populate a Filter with a FilteredQuery wrapping the original Query. Then I'd get a FieldCache for the field of interest. Enumerate the hits in the filter's bitset, and for each hit, you get the value of the field from the field cache, and add it to a SortedSet. When you've got all of the hits, divide the size of the set into the number of ranges you want (five to seven is a good number according the user interface guys), and rather than a single-valued constraint, your facets will be a range query with the lower and upper bounds of each of those subsets.
I'd recommend using some special-case logic for a small number of values; obviously, if you only have four distinct values, it doesn't make sense to try and make 5 range refinements out of them. Below a certain threshold (say 3*your ideal number of ranges), you just show the facets normally rather than ranges.
To answer your first question, you can get facet ranges by using the the generic facet query support. Here's an example:
http://localhost:8983/solr/select?q=video&rows=0&facet=true&facet.query=price:[*+TO+500]&facet.query=price:[500+TO+*]
As for your second question (automatically suggesting facet ranges), that's not yet implemented. Some argue that this kind of querying would be best implemented on your application rather that letting Solr "guess" the best facet ranges.
Here are some discussions on the topic: