Oracle Parallel Execution of SQL Statements

oracle 当查询比较慢的时候，能够考虑并发查询，能够很快出结果，预计比以前不加快2-4倍。

Parallel Query Intra- and Inter-Operation Example

As an example of parallel query with intra- and inter-operation parallelism, consider a more complex query:

SELECT /*+ PARALLEL(employees 4) PARALLEL(departments 4) USE_HASH(employees) 
ORDERED */
       MAX(salary), AVG(salary) 
FROM employees, departments
WHERE employees.department_id = departments.department_id 
GROUP BY employees.department_id;

http://docs.oracle.com/cd/B10500_01/server.920/a96524/c20paral.htm