I really like using ElasticSearch to index the couchdb database[1]. It allows you to scale out the data store and the search index separately, since they actually have separate concerns.

You can also do a lot of really complex queries with the elasticsearch query-dsl[2].

[1] http://daemon.co.za/2012/05/elasticsearch-5-minutes/ [2] http://www.elasticsearch.org/guide/reference/query-dsl