In the MySQL QA teams in Oracle we have been using a tool called the Random Query Generator (or "RQG" for short) for some time now. The main RQG testing effort has been on new server development, including regression testing. The recent MySQL 5.6 GA is a result of a couple of years of hard work, including a lot of RQG testing and bugfixing as a result of that.
I can easily say that the RQG has helped making MySQL 5.6 a better release than it would otherwise be. It is of course not our only testing tool, but there are plenty of bugs this tool has uncovered that would likely not have been seen in our other testing. Such issues include:
- crashes and asserts
- memory management bugs (valgrind errors)