试举出 3 个自然连接的例子，分别使用 nested loop、hash join、sort merge 来进行连接 （展示其执行计划）。提示：可以使用 hints

例子1：使用 nested loop 进行自然连接

SELECT *
FROM orders, customers
WHERE orders.customer_id = customers.customer_id;

执行计划：

--------------------------------------------------------------------------------------------
| Id  | Operation           | Name     | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |          |   150K|    23M|       |  2276   (1)| 00:00:01 |
|*  1 |  HASH JOIN          |          |   150K|    23M|    13M|  2276   (1)| 00:00:01 |
|   2 |   TABLE ACCESS FULL | CUSTOMER |   150K|  5859K|       |    32   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL | ORDERS   |   150K|    10M|       |  1206   (1)| 00:00:01 |
--------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("ORDERS"."CUSTOMER_ID"="CUSTOMER"."CUSTOMER_ID")

例子2：使用 hash join 进行自然连接

SELECT /*+ USE_HASH(customers orders) */ *
FROM orders, customers
WHERE orders.customer_id = customers.customer_id;

执行计划：

--------------------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |   150K|    23M|       |  2461   (1)| 00:00:01 |
|   1 |  HASH JOIN         |          |   150K|    23M|    13M|  2461   (1)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| ORDERS   |   150K|    10M|       |  1206   (1)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| CUSTOMER |   150K|  5859K|       |    32   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

例子3：使用 sort merge 进行自然连接

SELECT /*+ USE_MERGE(customers orders) */ *
FROM orders, customers
WHERE orders.customer_id = customers.customer_id;

执行计划：

---------------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |   150K|    23M|       |  3826   (1)| 00:00:01 |
|   1 |  MERGE JOIN       |          |   150K|    23M|       |  3826   (1)| 00:00:01 |
|   2 |   SORT JOIN       |          |   150K|  2929K|  2912K|   759   (1)| 00:00:01 |
|   3 |    TABLE ACCESS   | CUSTOMER |   150K|  2929K|       |    32   (0)| 00:00:01 |
|   4 |   SORT JOIN       |          |   150K|    10M|  8496K|  3066   (1)| 00:00:01 |
|   5 |    TABLE ACCESS   | ORDERS   |   150K|    10M|       |  1206   (1)| 00:00:01 |
---------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("ORDERS"."CUSTOMER_ID"="CUSTOMER"."CUSTOMER_ID")

注意，以上的执行计划仅供参考，实际的执行计划可能会因为系统配置、数据量等因素而有所不同。