Why does the order of how we specify the variables in a '==' comparison matter?

Question

What i noticed is that there is a big performance difference by just changing the order of the variables that are compared with the \'==\' operator. For example $variable ==

Answer 1

I think what happens is that

Class1(... , $var : var)
Class2(var == $var, ...)

produces a network where all Class1 facts are taken, and then the Cartesian product with all Class2 facts with identical var field is created.

In contrast,

Class1(... , $var : var)
Class2($var == var, ...)

"rewritten" by the compiler as

Class1(... , $var : var)
$c2: Class2(...)
eval( $var == $c2.var )

creates the Cartesian product of all Class1 facts and all (!) Class2 facts and only thereafter filters all where the eval is false.

The traditional syntax (Drools 5 and earlier) forced you to have the field name on the left-hand side; only later on (late 5.x, 6.x), any logical expression was permitted.

After speaking to s.o. from the Drools team, a more accurate description might be this:- It is likely that where an attribute is compared to something else an optimization is triggered. Someone from the Drools team will take a look and possibly improve it by checking also the reversed expression.