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.
Note about Drupal 8
The Drupal 8 version of this module was moved back into the main Search API project. It is not necessary anymore to download a separate project for it.