filter下挂一个节点：挂一个节点时，下面的例子可以看到，对表扫描一次，就是对返回的结果集进行过滤，不过有趣的是当条件为假时
filter这个词总让人很费解，它下一级可以挂 一个子节点，二个子节点，三个子节点...。
挂一个子节点意思过滤，，如对全表进行扫描后，按照条件过滤，丢弃不满足条件的数据。
挂二个子节点类似是nest loop。
挂三个子节点类似1和2做nest loop，结果集再与3做nest loop（这个是我推测的）。
为了证明当时的观点，我们下面来做个试验：
filter下挂一个节点：
挂一个节点时，下面的例子可以看到，对表扫描一次，就是对返回的结果集进行过滤，不过有趣的是当条件为假时，不会对表进行扫描。
SQL> select * from emp e where e.hiredate > sysdate - 10000 and e.hiredate
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- -------------- ---------- ---------- ----------
7788 SCOTT ANALYST 7566 19-4月 -87 3000 20
7876 ADAMS CLERK 7788 23-5月 -87 1100 20
SQL> alter session set statistics_level=all;
会话已更改。
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------
SQL_ID 982y5g2z9sn6f, child number 0
-------------------------------------
select * from emp e where e.hiredate > sysdate - 10000 and e.hiredate sysdate
Plan hash value: 3896240783
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
|* 1 | FILTER | | 1 | | 2 |00:00:00.01 | 8 |
|* 2 | TABLE ACCESS FULL| EMP | 1 | 2 | 2 |00:00:00.01 | 8 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(SYSDATE@!-100002 - filter(("E"."HIREDATE">SYSDATE@!-10000 AND "E"."HIREDATE"
SQL> select * from emp e where e.hiredate > sysdate - 10000 and e.hiredate
未选定行
SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID 0pywmdg63gf0m, child number 0
-------------------------------------
select * from emp e where e.hiredate > sysdate - 10000 and e.hiredate
Plan hash value: 3896240783
---------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time |
---------------------------------------------------------------------------
|* 1 | FILTER | | 1 | | 0 |00:00:00.01 |
|* 2 | TABLE ACCESS FULL| EMP | 0 | 1 | 0 |00:00:00.01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(SYSDATE@!-100002 - filter(("E"."HIREDATE">SYSDATE@!-10000 AND
"E"."HIREDATE"
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