public boolean foldable() {
        return false;
    }
Returns true when an expression is a candidate for static evaluation before the query is
executed.

The following conditions are used to determine suitability for constant folding:
- A [[Coalesce]] is foldable if all of its children are foldable
- A [[BinaryExpression]] is foldable if its both left and right child are foldable
- A [[Not]], [[IsNull]], or [[IsNotNull]] is foldable if its child is foldable
- A [[Literal]] is foldable
- A [[Cast]] or [[UnaryMinus]] is foldable if its child is foldable
indexr-query-opt/src/main/java/io/indexr/query/expr/Expression.java