We just discussed cases when conflicts between parallel threads or transactions created performance issues or even aborted queries. You saw how locks set by SQL statements or the storage engines affect parallel threads. Such locks are visible to users and easy to track, although they are not always easy to debug. You need to account for the possibility of parallel, competing threads when an application is capable of creating multiple MySQL connections. This can be as common as a web server that opens parallel connections to a MySQL server because multiple users have opened a web page provided by that web server.

We also discussed wrong results or dramatic slowdowns that are hard to miss if your application handles errors. When I collected examples for this book, I put all such issues in Wrong Results from a SELECT. I distinguish those from performance problems because you can see their results immediately, whereas performance problems are generally hidden at first, and you notice them only after examining the slow query log or getting complaints from users about slow applications.