[20210301]延迟显示输出.txt

--//一位网友提出一个奇怪要求,就是分段显示输出.我不知道应用是否很好实现.
--//可以在select中加入sleep函数,自己测试看看.

1.环境:
SCOTT@book> @ ver1
PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

CREATE OR REPLACE FUNCTION SCOTT.sleep (seconds IN NUMBER)
   RETURN NUMBER
AS
BEGIN
   sys.DBMS_LOCK.sleep (seconds);
   RETURN seconds;
END;
/

SCOTT@book> set array 3
SCOTT@book> select emp.*,sleep(1) from emp ;
     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO   SLEEP(1)
---------- ---------- --------- ---------- ------------------- ---------- ---------- ---------- ----------
      7369 SMITH      CLERK           7902 1980-12-17 00:00:00        800                    20          1
      7499 ALLEN      SALESMAN        7698 1981-02-20 00:00:00       1600        300         30          1
      7521 WARD       SALESMAN        7698 1981-02-22 00:00:00       1250        500         30          1
      7566 JONES      MANAGER         7839 1981-04-02 00:00:00       2975                    20          1
      7654 MARTIN     SALESMAN        7698 1981-09-28 00:00:00       1250       1400         30          1
      7698 BLAKE      MANAGER         7839 1981-05-01 00:00:00       2850                    30          1
      7782 CLARK      MANAGER         7839 1981-06-09 00:00:00       2450                    10          1
      7788 SCOTT      ANALYST         7566 1987-04-19 00:00:00       3000                    20          1
      7839 KING       PRESIDENT            1981-11-17 00:00:00       5000                    10          1
      7844 TURNER     SALESMAN        7698 1981-09-08 00:00:00       1500          0         30          1
      7876 ADAMS      CLERK           7788 1987-05-23 00:00:00       1100                    20          1
      7900 JAMES      CLERK           7698 1981-12-03 00:00:00        950                    30          1
      7902 FORD       ANALYST         7566 1981-12-03 00:00:00       3000                    20          1
      7934 MILLER     CLERK           7782 1982-01-23 00:00:00       1300                    10          1
14 rows selected.
Elapsed: 00:00:14.02
--//这样可以实现每次显示3条记录.估计这样不符合对方要求.一般更多地采用通过编程实现.

2.不过我发现这个可以满足一个很独特的测试,就是一致性读取测试,通过例子说明问题:

SCOTT@book> create table t as select rownum id1,rownum id2,lpad(rownum||'x',10) vc from dual connect by level<=10;
Table created.

SCOTT@book> create unique index i_t_id1 on t(id1);
Index created.

SCOTT@book> create  index i_t_id2 on t(id2);
Index created.
--//分析略.

3.测试:
--//session 1:
SCOTT@book> select * from t where id1=4 and sleep(12);

--//session 2,以下命令最好事先输入,避免12秒内无法完成:
SCOTT@book> update t set vc=upper(vc) where id1=4;
1 row updated.

SCOTT@book> commit ;
Commit complete.

--//session 1:
SCOTT@book> select * from t where id1=4 and sleep(12)=12;
       ID1        ID2 VC
---------- ---------- --------------------
         4          4 4X4X4X4X4X
--//你可以发现vc输出是大写,而如果按照以前传统的理解,应该输出的是小写.

4.条件换成id2看看.
--//session 1:
SCOTT@book> select * from t where id2=3 and sleep(12)=12;

--//session 2,以下命令最好事先输入,避免12秒内无法完成:
SCOTT@book> update t set vc=upper(vc) where id1=3;
1 row updated.

SCOTT@book> commit ;
Commit complete.

--//session 1:
SCOTT@book> select * from t where id2=3 and sleep(12)=12;
       ID1        ID2 VC
---------- ---------- --------------------
         3          3 3x3x3x3x3x
--//你可以发现输出是小写.

--//Oracle将这种改动称为"RowCR Optimization"，Oracle简单的描述了什么是RowCR Optimization：A brief overview of this
--//optimization is that we try to avoid rollbacks while constructing a CR block if the present block has no uncommitted
--//changes.这里的avoid rollback，意味着在满足特定的条件时，Oracle就不做一致读了.

--//这个提前条件就是索引要唯一.

--//RowCR Optimization通过隐含参数"_row_cr"来控制.
SYS@book> @ hide _row_cr
NAME     DESCRIPTION               DEFAULT_VALUE  SESSION_VALUE SYSTEM_VALUE ISSES ISSYS_MOD
-------- ------------------------- -------------- ------------- ------------ ----- ---------
_row_cr  enable row cr for all sql TRUE           TRUE          TRUE         FALSE IMMEDIATE

SCOTT@book> alter system set "_row_cr"=false scope=memory;
System altered.

--//退出session 1再登录重复测试:
--//sesion 1:
SCOTT@book> select * from t where id1=1 and sleep(12)=12;

--//session 2:
SCOTT@book> update t set vc=upper(vc) where id1=1;
1 row updated.

SCOTT@book> commit ;
Commit complete.

--//sesion 1:
SCOTT@book> select * from t where id1=1 and sleep(12)=12;
       ID1        ID2 VC
---------- ---------- --------------------
         1          1 1x1x1x1x1x
--//现在就输出小写了.

4.收尾:
--//重启还原.