To determine the cost of search strategies where the value of a column is compared with the hit rows of a subquery, the Optimizer must know the number of hit rows. However, the number of hit rows is not known until the subquery has been processed.
For a SELECT statement with subqueries, the EXPLAIN statement determines the possible search strategies, but does not execute these subqueries. Therefore, the search strategy NO STRATEGY NOW (ONLY AT EXECUTION TIME) is usually displayed for the outer SQL statement.