[dspillett]

> I wonder why the caching layer can't just pass any query it is unable to process on to the underlying database?

A caching layer can do that, and I'd be surprised if serious ones don't. It might not be perfect when moving between DB versions though: the new version might perhaps introduce syntax that is mistaken for something else which the layer _thinks_ it understands. While this can be tested for to a large extent by the devs having access to alph/beta/rc versions, but I can still see things slipping through and you are relying on people keeping their versions of the caching layer as up-to-date as everything else which is not a given.