基于案例理解MySQL执行计划

原始SQL语句：
SELECT c.customerNumber, c.customerName, e.lastname
FROM customers c
     INNER JOIN employees e ON c.salesRepEmployeeNumber = e.employeeNumber
WHERE e.lastname LIKE 'T%'
      AND EXISTS
             (SELECT orderNumber, SUM(priceEach * quantityOrdered)
              FROM orderdetails INNER JOIN orders USING (orderNumber)
              WHERE customerNumber = c.customerNumber
              GROUP BY orderNumber
              HAVING SUM(priceEach * quantityOrdered) > 50000)
UNION
SELECT customerNumber, customerName, 'NULL'
FROM customers
WHERE customerName LIKE 'T%'