To determine the cost of search strategies where the value of a column is compared with the hit rows of a subquery, you must know the number of hit rows. You can therefore only determine the cost of the search strategy after the subquery has been processed.
For a SELECT statement with subqueries, the EXPLAIN statement determines the possible search strategies, but does not execute the contained subqueries. Therefore, the search strategy NO STRATEGY NOW (ONLY AT EXECUTION TIME) for the outer SQL statement is usually displayed.
SQL statements that can only be processed on an index are an exception to this rule.