This module provides a backend for the Search API that uses a normal database to index data. It is therefore a cheap and simple alternative to backends like Solr, but can also be a great option for larger sites if you know what you're doing.
The prime example of a large site using this module is this site itself – all issue queue listings are generated by the Search API using this module as the backend.
Note for non-MySQL users
For backends other than MySQL, there might currently be problems with umlauts, accented characters and the like. Seefor details, and report there if you are experiencing problems.