I've got several useful habits over the years of work in MySQL
Support. One of them is to start working on every problem with
search for known MySQL bugs related to the problem at hand.
I'd like to share one recent case where this habit helped me to
get a solution for customer almost instantly.
It was one of rare cases when customer opened a support request with a very clear question and even a test case. The problem was described very precisely, more or less as follows (with table and column names, and data changed for this blog post, surely).
Let's assume we have two tables created like these: