Is cross-table indexing possible?

Question

Consider a structure where you have a many-to-one (or one-to-many) relationship with a condition (where, order by, etc.) on both tables. For example:

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Answer 1

Cross-table indexing is not possible in MySQL except via the now-defunct Akiban(?) Engine.

I have a rule: "Do not normalize 'continuous' values such as INTs, FLOATs, DATETIMEs, etc." The cost of the JOIN when you need to sort or range-test on the continuous value will kill performance.

DATETIME takes 5 bytes; INT takes 4. So any 'space' argument toward normalizing a datetime is rather poor. It is rare that you would need to 'normalize' a datetime in the off chance that all uses of a particular value were to change.