通过分析执行计划，Oracle执行了WITH子查询一次，并将结果放到了临时表中，在随后对子查询的多次访问中，都从临时表中直接读取了数据，这应该也是那1个物理读的由来。
通过上面的例子可以看到，将子查询放到WITH语句中不仅可以简化查询语句的结构，对于子查询需要多次执行的情况，还有可能提示查询的性能。
可惜的是，WITH语句只能用在SELECT语句中，UPDATE和DELETE语句不支持WITH语法:
SQL> SET AUTOT OFF
SQL> SET TIMING OFF
SQL> WITH SUBQ AS (SELECT 1 FROM DUAL)
2 SELECT ID, NAME FROM T_WITH WHERE ID IN (SELECT * FROM SUBQ);
ID NAME
---------- ------------------------------
1 STANDARD
SQL> WITH SUBQ AS (SELECT 1 FROM DUAL)
2 UPDATE T_WITH SET ID = 1 WHERE ID IN (SELECT * FROM SUBQ);
UPDATE T_WITH SET ID = 1 WHERE ID IN (SELECT * FROM SUBQ)
*第 2 行出现错误:
ORA-00928: 缺失 SELECT 关键字
SQL> WITH SUBQ AS (SELECT 1 FROM DUAL)
2 DELETE T_WITH WHERE ID IN (SELECT * FROM SUBQ);
DELETE T_WITH WHERE ID IN (SELECT * FROM SUBQ)
*第 2 行出现错误:
ORA-00928: 缺失 SELECT 关键字 |
【责编:yunyun 纠错】
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2006/10/18
