One of the things that sets MySQL apart (in, IMHO, a very bad way) from other Open Source database projects/products such as PostgreSQL (license) and Firebird (license) is that the MySQL documentation is NOT Open Source. The MySQL documentation is and always has been copyright MySQL AB, and “… use of this documentation, in whole or in part, in another publication, requires the prior written consent from an authorized representative of MySQL AB”. This presents a major impediment to forking the server: who wants to re-write many hundreds of pages of documentation on things that haven’t even changed? Even if you’re OK with all of your new code being GPL (and anyone forking ought to be), and never being able to dual license or re-license your MySQL fork (oh well), you will have to start with no documentation, or publish an errata against the official MySQL documentation (which will only go so far).
Does this mean that MySQL is not really Open Source? I would say not exactly, although I could probably be convinced either way. Others may say yes. But it does go a long way to making the point that some things may not be quite as “open” as they initially appear. What do you think? Can a piece of software really be Open Source while its primary/only documentation is not? Did you know that the MySQL documentation was not Open Source?