: Now suppose I have a lot of docs with same signature and signature : is a very long string. It seems to me indexing the signature will save me : hard disk space.
that's true, and if you were using Lucene directly you could do this and then use the StringIndex FieldCache to lookup the value for each doc, but Solr doesn't have any special optimization like that at the moment. If you don't store it, none of hte standard request handlers will retrieve it when generating results, but you could write a custom request handler to do that if you wished (it could even be done fairly programaticly: look for any fields with type "string" which are indexed but not stored and return them) -Hoss
