在11.2中，Oracle数据库引入的版本的概念，这为应用程序的升级提供了极大的方便。
这篇简单描述版本的实现和查询方式。
前一篇简单描述了版本，下面接着上面的例子看看Oracle是如何实现这个功能的：
SQL> select synonym_name, table_name
2 from user_synonyms;
SYNONYM_NAME TABLE_NAME
------------------------------ ------------------------------
S_1 T
SQL> select object_id, object_name, object_type, edition_name
2 from user_objects
3 where object_name = 'S_1';
OBJECT_ID OBJECT_NAME OBJECT_TYPE EDITION_NAME
---------- ------------------------------ ------------------- ------------------------------
74125 S_1 SYNONYM ORA$BASE
SQL> select sys_context('USERENV', 'CURRENT_EDITION_NAME')
2 from dual;
SYS_CONTEXT('USERENV','CURRENT_EDITION_NAME')
-------------------------------------------------------------------------------------
ORA$BASE
下面设置版本为MY_EDITION：
SQL> alter session set edition = my_edition;
会话已更改。
SQL> select synonym_name, table_name
2 from user_synonyms;
SYNONYM_NAME TABLE_NAME
------------------------------ ------------------------------
S_1 T1
SQL> select object_id, object_name, object_type, edition_name
2 from user_objects
3 where object_name = 'S_1';
OBJECT_ID OBJECT_NAME OBJECT_TYPE EDITION_NAME
---------- ------------------------------ ------------------- ------------------------------
74128 S_1 SYNONYM MY_EDITION
可以看到在USER_SYNONYMS视图中可以只能看到当前版本的同义词定义。而在USER_OBJECTS中可以看到，两个同义词的名称相同，但是两个对象的版本名称不同，而且OBJECT_ID并不相同，也就是说，二者实际上不是同一个对象。
SQL> select obj#, owner#, name, type#
2 from sys.obj$
3 where name = 'S_1';
OBJ# OWNER# NAME TYPE#
---------- ---------- ------------------------------ ----------
74125 84 S_1 5
74128 86 S_1 5
检查SYS.OBJ$发现，两个对象不但OBJECT_ID不同，连OWNER也是不同的。
SQL> select user#, name, type#, ext_username
2 from sys.user$
3 where user# in (84, 86);
USER# NAME TYPE# EXT_USERNAME
---------- ------------------------------ ---------- ------------------------------
84 YANGTK 1
86 SYS_LNLQ7CWEC5SUF_0IC_Q_ONI8GO 2 YANGTK
当切换到不同的版本时，Oracle实际上切换到了不同的用户。
而用户查询的是当前版本的对象，这时由于很多的静态数据字典都发生了变化：
SQL> select text
2 from dba_views
3 where view_name = 'DBA_SYNONYMS';
TEXT
--------------------------------------------------------------------------------
select u.name, o.name, s.owner, s.name, s.node
from sys.user$ u, sys.syn$ s, sys."_CURRENT_EDITION_OBJ" o
where o.obj# = s.obj#
and o.type# = 5
and o.owner# = u.user#
可以看到，这些支持版本对象的数据字典信息，访问的系统表不再是SYS.OBJ$，而是另外一个对象：SYS._CURRENT_EDITION_OBJ。
SQL> select owner, object_name, object_id, object_type
2 from dba_objects
3 where object_name = '_CURRENT_EDITION_OBJ';
OWNER OBJECT_NAME OBJECT_ID OBJECT_TYPE
------------------------------ ------------------------------ ---------- -------------------
SYS _CURRENT_EDITION_OBJ 3070 VIEW
这个对象是一个视图，查看视图的定义：
SQL> select text
2 from dba_views
3 where view_name = '_CURRENT_EDITION_OBJ';
TEXT
--------------------------------------------------------------------------------
select o."OBJ#",o."DATAOBJ#",o."OWNER#",o."NAME",o."NAMESPACE",o."SUBNAME",o."TY
PE#",o."CTIME",o."MTIME",o."STIME",o."STATUS",o."REMOTEOWNER",o."LINKNAME",o."FL
AGS",o."OID$",o."SPARE1",o."SPARE2",o."SPARE3",o."SPARE4",o."SPARE5",o."SPARE6",
o.spare3,
case when (o.type# not in (4,5,7,8,9,10,11,12,13,14,22,87) or
bitand(u.spare1, 16) = 0) then
null
when (u.type# = 2) then
(select eo.name from obj$ eo where eo.obj# = u.spare2)
else
'ORA$BASE'
end
from obj$ o, user$ u
where o.owner# = u.user#
and ( /* non-versionable object */
( o.type# not in (4,5,7,8,9,10,11,12,13,14,22,87,88)
or bitand(u.spare1, 16) = 0)
/* versionable object visible in current edition */
or ( o.type# in (4,5,7,8,9,10,11,12,13,14,22,87)
and ( (u.type# <> 2 and
sys_context('userenv', 'current_edition_name') = 'ORA$BASE')
or (u.type# = 2 and
u.spare2 = sys_context('userenv', 'current_edition_id'))
or exists (select 1 from obj$ o2, user$ u2
where o2.type# = 88
and o2.dataobj# = o.obj#
and o2.owner# = u2.user#
and u2.type# = 2
and u2.spare2 =
sys_context('userenv', 'current_edition_id'))
)
)
)
ORACLE就是通过将这个视图代替了数据字典中的OBJ$对象，从而使得用户可以查询当前的版本信息。
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