[20181015]為什麼是3秒.txt

[20181015]為什麼是3秒.txt

--//以前測試:連線http://blog.itpub.net/267265/viewspace-2144765/=>為什麼是12秒.txt.
--//很奇怪12.1.0.1版本測試12秒(windows版本),而11g是3秒(在使用標量子查詢的情況下).不知道為什麼?
--//在12.2.0.1下測試看看:

1.環境:
SCOTT@test01p> @ver1
PORT_STRING                    VERSION        BANNER                                                                               CON_ID
------------------------------ -------------- -------------------------------------------------------------------------------- ----------
IBMPC/WIN_NT64-9.1.0           12.2.0.1.0     Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production              0

SYS@test01p> grant EXECUTE ON  dbms_lock to scott;
Grant succeeded.

2.建立函式:
CREATE OR REPLACE FUNCTION get_dept (p_deptno dept.deptno%TYPE)
   RETURN dept.dname%TYPE
IS
   l_dname   dept.dname%TYPE;
BEGIN
   DBMS_LOCK.sleep (1);

   SELECT dname
     INTO l_dname
     FROM dept
    WHERE deptno = p_deptno;

   RETURN l_dname;
END;
/

3.測試:
SCOTT@test01p> set timing on
SCOTT@test01p> set feedback only
SCOTT@test01p> select empno, ename, deptno, get_dept(deptno) c20 from emp;
     EMPNO ENAME          DEPTNO C20
---------- ---------- ---------- --------------------

14 rows selected.
Elapsed: 00:00:14.00
--//14秒,這是正確的,14條記錄.呼叫14次需要14秒.

--//換成標量子查詢:
SCOTT@test01p> select empno, ename, deptno, (select get_dept(deptno) from dual )c20 from emp;
     EMPNO ENAME          DEPTNO C20
---------- ---------- ---------- --------------------

14 rows selected.
Elapsed: 00:00:03.03

--//執行時間是3秒,這次是正確的,因為標量子查詢快取結果,而僅僅有3個部門在emp表.這樣3秒就正確了.

4.繼續探究:
SCOTT@test01p> set timing off
SCOTT@test01p> alter session set statistics_level=all;

Session altered.

SCOTT@test01p> select empno, ename, deptno, (select get_dept(deptno) from dual )c20 from emp;
     EMPNO ENAME          DEPTNO C20
---------- ---------- ---------- --------------------

14 rows selected.

SCOTT@test01p> set feedback on
SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  apagxtf1p2puy, child number 1
-------------------------------------
select empno, ename, deptno, (select get_dept(deptno) from dual )c20
from emp
Plan hash value: 1340320406
--------------------------------------------------------------------------------------------------------------------
| Id  | Operation         | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
--------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |      1 |        |       |     9 (100)|          |     14 |00:00:00.01 |       8 |
|   1 |  FAST DUAL        |      |      3 |      1 |       |     2   (0)| 00:00:01 |      3 |00:00:00.01 |       0 |
|   2 |  TABLE ACCESS FULL| EMP  |      1 |     14 |   182 |     3   (0)| 00:00:01 |     14 |00:00:00.01 |       8 |
--------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$2 / DUAL@SEL$2
   2 - SEL$1 / EMP@SEL$1
--//從執行計劃也可以發現fast dual執行了3.再次說明12.1版本有問題.
--//也再次說明oracle任何XX.1版本不能在生產系統使用.

5.繼續測試使用 DETERMINISTIC Functions:
--//一般如果在在某個函式定義索引，需要DETERMINISTIC,表示返回結果固定。其實即使不固定，也可以這樣定義。
CREATE OR REPLACE FUNCTION get_dept (p_deptno dept.deptno%TYPE)
   RETURN dept.dname%TYPE
   DETERMINISTIC
IS
   l_dname   dept.dname%TYPE;
BEGIN
   DBMS_LOCK.sleep (1);

   SELECT dname
     INTO l_dname
     FROM dept
    WHERE deptno = p_deptno;

   RETURN l_dname;
END;
/
SCOTT@test01p> show array
arraysize 200
--//arraysize=200

SCOTT@test01p> set timing on
SCOTT@test01p> select empno, ename, deptno, get_dept(deptno) c20  from emp;
...
14 rows selected.
Elapsed: 00:00:04.06
--//這次正確了4秒.大家可以自行設定array=2等各種情況.
--//為什麼?大家可以看看我寫的.http://blog.itpub.net/267265/viewspace-2138042/=>[20170426]為什麼是4秒.txt

6.最後補充測試RESULT CACHE的情況:

CREATE OR REPLACE FUNCTION get_dept (p_deptno dept.deptno%TYPE)
   RETURN dept.dname%TYPE
   RESULT_CACHE
IS
   l_dname   dept.dname%TYPE;
BEGIN
   DBMS_LOCK.sleep (1);

   SELECT dname
     INTO l_dname
     FROM dept
    WHERE deptno = p_deptno;

   RETURN l_dname;
END;
/

SCOTT@test01p> select empno, ename, deptno, get_dept(deptno) c20  from emp;
14 rows selected.
Elapsed: 00:00:03.07

SCOTT@test01p> select empno, ename, deptno, get_dept(deptno) c20  from emp;
Elapsed: 00:00:00.07

--//第1次執行3秒,第2次執行0秒,因為結果快取了.第二次執行直接取結果.修改如下結果一樣.
CREATE OR REPLACE FUNCTION get_dept (p_deptno dept.deptno%TYPE)
   RETURN dept.dname%TYPE
   RESULT_CACHE
   DETERMINISTIC
IS
   l_dname   dept.dname%TYPE;
BEGIN
   DBMS_LOCK.sleep (1);

   SELECT dname
     INTO l_dname
     FROM dept
    WHERE deptno = p_deptno;

   RETURN l_dname;
END;
/

總結:
--//再次驗證我以前的結論oracle 任何xx.1版本不要在生產系統使用.