div's comment summed it up: one of the most useful MySQL patch sets floating around was released by Google to help address scalability (http://code.google.com/p/google-mysql-tools/) and this appears to have continued until the present: https://groups.google.com/d/msg/google-mysql-tools/Bc7P5SIRC...

I also know that they use at least one big financial reporting packages but I'm not sure whether the use of Oracle Hyperion is evidence for or against wisdom.

Google has contributed quite a few large patches to MySQL. I doubt they would be spending engineering time on improving MySQL if they did not use it internally for at least _some_ projects, and probably many.

Do a Google search for mysql google contributions and plenty of stuff pops up.

This probably counts as an argument for both sides: http://research.google.com/pubs/pub37200.html