Great question. We rank in two stages for a number of different reasons. First, it would be too expensive from a performance perspective to rank all of the messages in your corpus. Second, some of the features that we use to rank are much more easily accessed at the application layer. It would require more of an engineering effort to make these signals accessible in SOLR. The first pass which is done in SOLR is a high recall, low precision pass. The second pass through our custom ranker is a high precision pass. It is possible that we would lose some messages that might end up being important in the first pass but it's a tradeoff between performance and accuracy. Hope this helps answer your question.