Or better said when Sphinx can help MySQL on queries that at first look they don’t involve any fulltext searching.
Sphinx was build in mind to help the DB on fulltext queries. But it can also help on where there is no text search. That is everyday used queries with combined filtering,grouping and sorting used for various analytics, reporting of simply general usage.
In Sphinx, the fulltext query is executed first, creating a result set that is passed to the remaining operations ( filters, groups, sorts). By reducing the size of the set that is interogated, the whole query will not be only faster, but it will consume less resources.
Because of design for speed, Sphinx can group and sort a lot faster and can do easy segmentations or getting top-N best group matches in a single query.
The result will be offloading heavy work done by database nodes to even a single Sphinx server.
Slides were presented at PerconaLive London 2013