Fun with DocValues in Solr 4.2 - Lucidworks
When Solr/Lucene returns a set of document ids from a query, it will then use the row-oriented (aka, stored fields) view of the documents to retrieve the actual field values. This requires a very few number of seeks since all of the field data will be stored close together in the fields data file.
However, for faceting/sorting/grouping Lucene needs to iterate over every document to collect the field values. Traditionally, this is achieved by uninverting the term index. This performs very well actually, since the field values are already grouped (by nature of the index), but it is relatively slow to load and is maintained in memory. DocValues aim to alleviate both of these problems while keeping performance comparable.
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