CONSTRAINT: nested iteration

This option appears in the SHOWPLAN output for a query that includes a constraint. The CONSTRAINT: nested iteration method is the technique used to join tables when the purpose of the join is to enforce a constraint. The following example demonstrates the SHOWPLAN output for a query that updates a row in the employee table, which has a FOREIGN KEY constraint that references the jobs table. The updated row from the employee table is joined with the jobs table. If no matching row is found in the jobs table, then the update is aborted.

Query:    UPDATE employee 
          SET job_id = 14
          WHERE emp_id = 'KFJ64308F'
  
SHOWPLAN: STEP 1
          The type of query is UPDATE
          The update mode is direct
          FROM TABLE
          employee
          Nested iteration
          Table Scan
          FROM TABLE
          jobs
          CONSTRAINT : nested iteration
          Table Scan
          TO TABLE
          employee