Help with query. (*)

I'm having difficulty writing a query which I really can't live without...

I need to get a list of records from table A for which there are
corresponding records in table B.  I've tried to use the intersect clause,
but it doesn't seem to work, or it runs far too long.  For example:

select * from A
    where 1=1
    intersect select * from A where
    B.x=A.x
    and A.y=B.y
    and A.z=B.z
    limit 100

I need the most efficient method possible; my A tables have upward of 5
Million records.  The B table, btw, only has about 100 records.


Any help will be most appreciated.