It’s been nine months since partition pruning code has been pushed into MySQL 5.1. Another available optimization, partition selection, has been in the main tree for even longer. There haven’t been any bugs reported for some time, so the code should be reasonably stable now.
And if you’re interested in what’s under the hood, a rather detailed description of partitioning optimizations is now available here:
It is a part of the internals manual, but I tried to write it so it doesn’t
require any knowledge of MySQL source code. (and don’t be scared away by my English – the text in the manual has passed the scrutiny of the documentation team 🙂