Given a working memory containing a large number of rather static facts which are from several subclasses (Ape, Bear, Crocodile,...) of some base class (Animal) and a single fact of class Hunter with a field target of type Animal, the shooting of an animal might be written with a rule like this:

rule shoot-1
    when
        ?h : Hunter( ?target : target )
        ?a : Animal()
        eval(?target == ?a)
    then
       ?h.shoot( ?a );
       retract( ?a );
end

Avoiding eval (which is said to be inefficient), it could also be written as

rule shoot-2
    when
        ?a: Animal()
        ?h : Hunter( target == ?a )
    then
       ?h.shoot( ?a );
       retract( ?a );
end

This, however, places the pattern with many instances up front, which is (AFAIK) not so good in a Rete implementation.

Which one is to be preferred in Drools?

Ideally, one might want to write

rule shoot-3
    when
        ?h : Hunter( ?target : target )
        ?a == ?target : Animal()                   ### not valid DRL
    then
       ?h.shoot( ?a );
       retract( ?a );
end

This avoids eval, has the single instance fact up front, but it isn't available.

-W