private string MakeQueryCacheKey(string storeName, long commitTime, SparqlQuery query, string[] defaultGraphUris, ISerializationFormat targetFormat)
{
var graphHashCode = defaultGraphUris == null ? 0 : String.Join(",", defaultGraphUris.OrderBy(s=>s)).GetHashCode();
return storeName + "_" + commitTime + "_" + query.GetHashCode() + "_" + graphHashCode + "_" + targetFormat;
}