Refining the algorithm

We now introduce the following refinements to the algorithm:

• If the parent expression is a relation, then do not perform substitutions on the children themselves. Proper subexpressions of the children are still considered for substitution.
• Consider attributes as well as the children of an expression for substitution.
• Use a different weighting scheme for deciding when to substitute for attributes by introducing a third constant, *attribute-complexity-threshold*. This constant is used to scale *proportional-complexity-threshold* when considering attributes.