myisamchk --sort-index

[Ron Last]

I have a MyISAM table with 100 million records which I join to other tables with a unique key. Will using the sort index option (see below) improve the join performance by much?

myisamchk --sort-index --sort-records=1 myTable

I also regularly use a WHERE statement on another indexed field that only has 20 possible values so I am wondering if I should sort on this fields.

Below is a typical query
SELECT .... FROM X JOIN Y USING(ID) WHERE X.theYear=2012
Should my table be sorted on ID or theYear for best performance.
Maybe the answer is obvious, or maybe it is data dependent, would appreciate comments.
thanks