Automatic use of keys

Our customer have Item table with 70000 records. In the table I created a new boolean field for signing special items.
I created a new menu item too, which runs the form Item Card with RunFormView=WHERE(BooleanField=CONST(Yes)), and a new key for Item table (on BooleanField).
Everything works fine, but.. Navision doesn't use my key, but primary key, so showing Item Card is very slow.

How can I force Navision to select the proper key?
(am I clear? huhh, my english is wrong...)