合 Oracle之dual系列

前言部分

导读和注意事项

各位技术爱好者，看完本文后，你可以掌握如下的技能，也可以学到一些其它你所不知道的知识，~O(∩_∩)O~：

① DUAL遭到破坏后的重建（重点）

② 关于参数replication_dependency_tracking简介

③ DUAL简介

本文简介

DUAL系列很早之前是看杨建荣的blog了解到的，最近又看他的书又看见了这块内容，于是决定自己整理一下。在此，小麦苗还是想说的是看书和自己做实验是完全不同的2回事，不多说了。

dual官方文档说明

DUAL is a table automatically created by Oracle Database along with the data dictionary. DUAL is in the schema of the user SYS but is accessible by the name DUAL to all users. It has one column, DUMMY, defined to be VARCHAR2(1), and contains one row with a value X. Selecting from the DUAL table is useful for computing a constant expression with the SELECT statement. Because DUAL has only one row, the constant is returned only once. Alternatively, you can select a constant, pseudocolumn, or expression from any table, but the value will be returned as many times as there are rows in the table. Refer to "About SQL Functions" for many examples of selecting a constant value from DUAL.

Note:

Beginning with Oracle Database 10g Release 1, logical I/O is not performed on the DUAL table when computing an expression that does not include the DUMMY column. This optimization is listed as FAST DUAL in the execution plan. If you SELECT the DUMMY column from DUAL, then this optimization does not take place and logical I/O occurs.

DUAL总结

有关Oracle的DUAL表，我们需要掌握以下几点：

1. DUAL是系统提供的一个虚表，也常称为DUMMY表，临时显示结果的表，里边的内容没有特定的意义，就是为了存在而存在
2. DUAL是属于SYS SCHEMA的一个表,然后以PUBLIC SYNONYM的方式供其他数据库USER使用，这一点可以由如下SQL验证：

SYS@lhrdb> col owner format a15

SYS@lhrdb> col object_name format a15

SYS@lhrdb> SELECT D.OWNER,D.OBJECT_NAME,D.OBJECT_ID,D.OBJECT_TYPE FROM DBA_OBJECTS D WHERE D.OBJECT_NAME='DUAL';

OWNER OBJECT_NAME OBJECT_ID OBJECT_TYPE

--------------- --------------- ---------- -------------------

SYS DUAL 91680 TABLE

PUBLIC DUAL 117 SYNONYM

1. ORACLE对DUAL表的操作做了一些内部处理,尽量保证DUAL表中只返回一条记录，当然这些内部操作是不可见的。我们尝试往DUAL表中插入多条记录，但始终显示的是一条记录。

SYS@lhrdb> INSERT INTO DUAL VALUES('X');

1 row created.

SYS@lhrdb> COMMIT;

Commit complete.

SYS@lhrdb> INSERT INTO DUAL VALUES('X');

1 row created.

SYS@lhrdb> COMMIT;

Commit complete.

SYS@lhrdb> select * from dual;

D

-

X

1. DUAL表出现问题，所有相关的基础操作都会受到影响；若是DUAL表被删除的话，数据库将不能启动，报ORA-01775的错误。
2. DUAL表在数据库启动的不同阶段，里边的字段还是会有一些不同。在数据库OPEN状态下是一行一列的表，在mount和nomount状态下是一行四列的表，验证如下：

SYS@lhrdb> startup force nomount

ORACLE instance started.

Total System Global Area 1720328192 bytes

Fixed Size 2247072 bytes

Variable Size 486540896 bytes

Database Buffers 1224736768 bytes

Redo Buffers 6803456 bytes

SYS@lhrdb> select * from dual;

ADDR INDX INST_ID DU

---------------- ---------- ---------- --

0000000110299728 0 1 X

SYS@lhrdb> alter database mount;

Database altered.

SYS@lhrdb> select * from dual;

ADDR INDX INST_ID D

---------------- ---------- ---------- -

0000000110299728 0 1 X

SYS@lhrdb> alter database open;

Database altered.

SYS@lhrdb> select * from dual;

D

-

X

1. DUAL的常用方式
2. 查看当前连接用户 SELECT USER FROM DUAL；
3. 查看当前日期、时间 SELECT TO_CHAR(SYSDATE,'YYYY-MM-DD HH24:MI:SS') FROM DUAL；
4. 当作计算器用 SELECT 8*9 FROM DUAL；
5. 查看序列值 SELECT MYSEQ.NEXTVAL FROM DUAL；

DUAL表破坏后的恢复

我们首先删除SYS用户下的DUAL表：

SYS@lhrdb> SHOW USER

USER is "SYS"

SYS@lhrdb> SELECT COUNT(*) FROM DBA_OBJECTS D WHERE D.status='INVALID';

COUNT(*)

----------

0

SYS@lhrdb> DROP TABLE DUAL;

Table dropped.

SYS@lhrdb> SELECT * FROM DUAL;

SELECT * FROM DUAL

*

ERROR at line 1:

ORA-01775: looping chain of synonyms

SYS@lhrdb> SELECT COUNT(*) FROM DBA_OBJECTS D WHERE D.status='INVALID';

COUNT(*)

----------

1042

SYS@lhrdb> SET PAGESIZE 9999

SYS@lhrdb> SELECT D.OWNER, COUNT(*) FROM DBA_OBJECTS D WHERE D.status='INVALID' GROUP BY D.OWNER;

OWNER COUNT(*)

------------------------------ ----------

PUBLIC 222

XDB 7

OLAPSYS 43

APEX_030200 157

SYS 403

MDSYS 44

SYSMAN 66

EXFSYS 23

ORACLE_OCM 1

WMSYS 42

ORDSYS 11

IX 6

CTXSYS 15

DBSNMP 1

OE 1

15 rows selected.

SYS@lhrdb>

可以看到删除DUAL表的时候并没有报错，但是查询的时候报错了,并且系统中大约有1000多个对象变为了无效的状态，业务系统中的PL/SQL代码中含有大量的DUAL查询，若是该表被删除，对系统的影响是非常大的。

查看告警日志，也有一些错误报出：

Errors in file /oracle/app/oracle/diag/rdbms/lhrdb/lhrdb/trace/lhrdb_m001_41877662.trc:

ORA-00604: error occurred at recursive SQL level 1

ORA-04063: package body "SYS.PRVT_HDM" has errors

ORA-06508: PL/SQL: could not find program unit being called: "SYS.PRVT_HDM"

ORA-06512: at line 1

Wed Aug 10 10:02:40 2016

Errors in file /oracle/app/oracle/diag/rdbms/lhrdb/lhrdb/trace/lhrdb_j000_11075780.trc:

ORA-12012: error on auto execute of job 4002

ORA-01775: looping chain of synonyms

DUAL的恢复分为2种情况，一种是数据库在没有重启的情况下，第二种是数据库在重启的情况下，但是总的恢复原则都是在数据库OPEN状态下重建DUAL表，下边我们分别测试。

不重启数据库

不重启数据库的情况下我们可以采用重建DUAL表，插入DUMMY的数据。

SYS@lhrdb> CREATE TABLE SYS.DUAL(DUMMY VARCHAR2(1)) STORAGE(INITIAL 1) TABLESPACE SYSTEM;

Table created.

SYS@lhrdb> INSERT INTO DUAL VALUES('X');

1 row created.

SYS@lhrdb> COMMIT;

Commit complete.

SYS@lhrdb> CREATE OR REPLACE PUBLIC SYNONYM DUAL FOR DUAL;

Synonym created.

SYS@lhrdb> GRANT SELECT ON DUAL TO PUBLIC WITH GRANT OPTION;

Grant succeeded.

SYS@lhrdb> SELECT * FROM DUAL;

D

-

X

SYS@lhrdb> SELECT SYSDATE FROM DUAL;

SYSDATE

-------------------

2016-08-10 10:22:37

SYS@lhrdb> SELECT COUNT(*) FROM DBA_OBJECTS D WHERE D.status='INVALID';

COUNT(*)

----------

1041

SYS@lhrdb> SELECT D.OWNER, COUNT(*) FROM DBA_OBJECTS D WHERE D.status='INVALID' GROUP BY D.OWNER;

OWNER COUNT(*)

------------------------------ ----------

PUBLIC 222

XDB 7

OLAPSYS 43

APEX_030200 157

SYS 402

MDSYS 44

SYSMAN 66

EXFSYS 23

ORACLE_OCM 1

WMSYS 42

ORDSYS 11

IX 6

CTXSYS 15

DBSNMP 1

OE 1

15 rows selected.

SYS@lhrdb> col owner format a15

SYS@lhrdb> col object_name format a15

SYS@lhrdb> SELECT D.OWNER,D.OBJECT_NAME,D.OBJECT_ID,D.OBJECT_TYPE FROM DBA_OBJECTS D WHERE D.OBJECT_NAME='DUAL';

OWNER OBJECT_NAME OBJECT_ID OBJECT_TYPE

--------------- --------------- ---------- -------------------

SYS DUAL 91680 TABLE

PUBLIC DUAL 117 SYNONYM

可以看到DUAL表已经重建成功了，但是还有很多的无效对象，下边我们利用脚本@?/rdbms/admin/utlrp.sql来重新编译这些对象。

SYS@lhrdb> @?/rdbms/admin/utlrp.sql

TIMESTAMP

--------------------------------------------------------------------------------

COMP_TIMESTAMP UTLRP_BGN 2016-08-10 10:25:13

DOC> The following PL/SQL block invokes UTL_RECOMP to recompile invalid

DOC> objects in the database. Recompilation time is proportional to the

DOC> number of invalid objects in the database, so this command may take

DOC> a long time to execute on a database with a large number of invalid

DOC> objects.

DOC>

DOC> Use the following queries to track recompilation progress:

DOC>

DOC> 1. Query returning the number of invalid objects remaining. This

DOC> number should decrease with time.

DOC> SELECT COUNT(*) FROM obj$ WHERE status IN (4, 5, 6);

DOC>

DOC> 2. Query returning the number of objects compiled so far. This number

DOC> should increase with time.

DOC> SELECT COUNT(*) FROM UTL_RECOMP_COMPILED;

DOC>

DOC> This script automatically chooses serial or parallel recompilation

DOC> based on the number of CPUs available (parameter cpu_count) multiplied

DOC> by the number of threads per CPU (parameter parallel_threads_per_cpu).

DOC> On RAC, this number is added across all RAC nodes.

DOC>

DOC> UTL_RECOMP uses DBMS_SCHEDULER to create jobs for parallel

DOC> recompilation. Jobs are created without instance affinity so that they

DOC> can migrate across RAC nodes. Use the following queries to verify

后续精彩内容已被站长无情隐藏，请输入验证码解锁本文！

验证码：

获取验证码： 请先关注本站微信公众号，然后回复“验证码”，获取验证码。在微信里搜索“AiDBA”或者“dbaup6”或者微信扫描右侧二维码都可以关注本站微信公众号。