INDEX on DATE field and MONTH() YEAR() functions

[Tristan Crombez]

Hi,

I was wondering if it is possible to create a table index on the MONTH(datefield) or YEAR(datefield) of a DATE type field.
Or does creating an index on a datefield also create an index on (year,month,day) of that datefield?
Or do I have to create 3 separate year, month, day fields and create an index on those instead of 1 date type field?

Thanks