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Analysis: Constant table optimization of the outer query finds that the right side of the equality is a constant that can be used for an eq_ref access to fetch one row from t1, and substitute t1 with a constant. Thus constant optimization triggers evaluation of the subquery during the optimize phase of the outer query. The innermost subquery requires a plan with a temporary table because with InnoDB tables the exact count of rows is not known, and the empty tables cannot be optimzied way. JOIN::exec for the innermost subquery substitutes the subquery tables with a temporary table. When EXPLAIN gets to print the tables in the innermost subquery, EXPLAIN needs to print the name of each table through the corresponding TABLE_LIST object. However, the temporary table created during execution doesn't have a corresponding TABLE_LIST, so we get a null pointer exception. Solution: The solution is to forbid using expensive constant expressions for eq_ref access for contant table optimization. Notice that eq_ref with a subquery providing the value is still possible during regular execution.