EXPLAIN Statement for SELECT Statements with Subqueries
To determine the cost of
search strategies with which, the value of a column is compared with the hit lines of a subquery, you must know the number of hit lines. 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) is usually displayed for the outer SQL statement.SQL statements that can only be processed on an index are an exception to this rule.