How to get the list of unique values for a specific attribute

Question

I'm trying to get a list of all the unique values for a specific field in my solr index. I know that the number of unique values is relatively small (<1000), so I shouldn't have any issues parsing them all out, but my current approach is giving me out of memory exceptions when the index has a large number of documents.

Here is what I'm currently trying:

public IList<string> GetAllShopNumbers()
        {
            ISolrQuery q= GetSomeSimpleSolrQuery();
            var r = solr.Query(q, new QueryOptions
            {
                Facet = new FacetParameters
                {
                    Queries = new[] {new SolrFacetFieldQuery("shop_number")}
                }
            });

            IList<string> shopNumbers = new List<string>();
            foreach (var facet in r.FacetFields["shop_number"])
            {
                shopNumbers.Add(facet.Key);
            }
            return shopNumbers;
        }

As mentioned, this throws an out of memory exception on a large index. How can we efficiently grab the list of unique values for a specific field in a Solr index?