An unconstrained join is one for which a WHERE clause is not specified for the tables that are joined. See Cross Join.
The result of an unconstrained join is a Cartesian product. A Cartesian product is the product of the number of rows in each table that is joined. An unconstrained join can return an undesired result that puts a large performance burden on the system.
If a SELECT statement specifies correlation and real names (for example, correlation names in a WHERE clause and real names in the select list), the Optimizer may specify an unconstrained join, depending on the conditions of the query and their specification.