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, which is rarely the desired result. A Cartesian product is the product of the number of rows in each table that is joined. An unconstrained join can produce a great many rows, returning a result that not only is not desired, but one that places 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 what the conditions of the query are and how they are specified.