A method, computer program, and database system are disclosed for executing database queries. The database includes a first table (T1) having a primary key (PK) column and a first correlated value column (CV1) and a second table (T2) having a foreign key (FK) column related to the primary key column of the first table and a second correlated value column (CV2). One implementation of the method includes preparing a database query for execution based at least in part on application of a derived constraint rule (DCR) having the form, (PK=FK)→CV2+C1≦CV1≦CV2+C2, where C1 and C2 are constants and “→” means “implies,” to produce an execution plan. A frequency of errors due to changes in DCRs is taken into account when preparing the execution plan. The plan is then executed.