I was under the impression that the query optimiser rather than scan the query to know how to eliminate the unrequired modifiers, it simply reorgansied the query internally so it was more efficient in how it opened and closed tables and read from keys and such like.

After all without running the query multiple times it wouldn't know if the modifiers made a difference or not to the results would it?

i don't really know what it does and doesn't do ... just that it uses clevers to recognise queries that can be better performed differently, without actually trying the different versions.

I was under the impression that it did things like notice that one of the items in a group by is a unique index, so would ignore the rest?

No idea how i got this idea though, so its most probably wrong.

SIGSIZESWITCHER