The Nested iteration method is the default technique used to join tables and/or to return rows from a table. It simply indicates that the optimizer is using one or more sets of loops to go through a table and retrieve a row, qualify the row based on the search criteria given in the WHERE clause, return the row to the front-end, and loop again to get the next row. The method in which it gets the rows (such as using an available index) is discussed later.
The following example shows the optimizer doing nested iterations through each of the tables in the join:
SELECT title_id, title FROM titles, publishers WHERE titles.pub_id = publishers.pub_id AND publishers.pub_id = '1389'
STEP 1 The type of query is SELECT FROM TABLE publishers Nested iteration Table Scan FROM TABLE titles Nested iteration Table Scan