例如,以下查询可能会执行全表扫描(忽略dept_id的选择性)：

SELECT * FROM employees 
WHERE dept_id = 'SALE'
START WITH manager_id is null
CONNECT BY PRIOR employee_id = manager_id

我试图以两种方式提高性能：

查询A：

SELECT * FROM employees 
START WITH manager_id is null AND dept_id = 'SALE'
CONNECT BY PRIOR employee_id = manager_id

查询B：

SELECT * FROM (
               SELECT * FROM employees 
                WHERE dept_id = 'SALE'
              )
START WITH manager_id is null
CONNECT BY PRIOR employee_id = manager_id

虽然这两个查询都比原来好得多,但在Oracle 10g第2版上,查询B的执行效果要好于A.

您是否有类似的性能优化来处理CONNECT BY和WHERE子句？你如何解释查询B比查询A好多了？