>From an theoretical IR standpoint, there is no reason to index null values, or even empty strings for that matter. However in practice there are plenty of cases that I've encountered where it is necessary to obtain a list of documents where a particular field is null (i.e. hasn't been specified at index time) or an empty string.
For example, you may need to generate a list of products contained in your index that do not have a part number. A dirty, ugly hack work-around to this problem that we've used in the past is to replace null or unset values at index time with a special token value like "__null__" that (hopefully) won't appear in normal indexed data. This then allows you to perform a query something like part_number:"__null__" to obtain all documents without a part number. This approach has worked in the past for string fields, not sure how effective it would be for numerical field types though. Ultimately, this leads to the situation where you are using Lucene (and Solr) as a RDBMS, which it clearly is not. While I'd love to have support for querying null / empty string fields, I don't think it's going to happen in the near future. PIete
