在10g中，由于解决了9I ALL_SYNONYMS的bug，导致查询ALL_SYNONYMS性能下降。

上一篇介绍了9i中，如果创建一个指向同义词的同义词，则在ALL_SYNONYMS视图中可能看不到这个同义词。10g中则解决了这个问题。可以看到，9i和10g的ALL_SYNONYMS视图的定义发生了变化：

SQL> select

dbms_metadata.get_ddl('VIEW', 'ALL_SYNONYMS') from dual;

DBMS_METADATA.GET_DDL('VIEW','ALL_SYNONYMS')

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

CREATE OR REPLACE FORCE VIEW

"SYS"."ALL_SYNONYMS" ("OWNER",

"SYNONYM_NAME", "TABLE_OWNER", "TABLE_NAME",

"DB_LINK") A

S

select u.name, o.name, s.owner, s.name, s.node

from sys.user$ u, sys.syn$ s, sys.obj$ o

where o.obj# = s.obj#

and o.type# = 5

and o.owner# = u.user#

and (

o.owner# in (USERENV('SCHEMAID'), 1 /* PUBLIC */) /* user's private, any public

*/

or /* user has any privs on base object */

exists

(select null from sys.objauth$ ba, sys.obj$ bo, sys.user$ bu

where bu.name = s.owner

and bo.name = s.name

and bu.user# = bo.owner#

and ba.obj# = bo.obj#

and ( ba.grantee# in (select kzsrorol from x$kzsro)

or ba.grantor# = USERENV('SCHEMAID')

)

)

or /* user has system privileges */

exists (select null from v$enabledprivs

where priv_number in (-45 /* LOCK ANY TABLE */,

-47 /* SELECT ANY TABLE */,

-48 /* INSERT ANY TABLE */,

-49 /* UPDATE ANY TABLE */,

-50 /* DELETE ANY TABLE */)

)

)

SQL> select * from v$version;

BANNER

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

Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production

PL/SQL Release 9.2.0.4.0 - Production

CORE 9.2.0.3.0 Production

TNS for Linux: Version 9.2.0.4.0 - Production

NLSRTL Version 9.2.0.4.0 - Production

而10g在9i的基础上添加了新的部分：

SQL> select

dbms_metadata.get_ddl('VIEW', 'ALL_SYNONYMS') from dual;

DBMS_METADATA.GET_DDL('VIEW','ALL_SYNONYMS')

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

CREATE OR REPLACE FORCE VIEW

"SYS"."ALL_SYNONYMS" ("OWNER",

"SYNONYM_NAME", "TABLE_OWNER", "TABLE_NAME",

"DB_LINK") A

S

select u.name, o.name, s.owner, s.name, s.node

from sys.user$ u, sys.syn$ s, sys.obj$ o

where o.obj# = s.obj#

and o.type# = 5

and o.owner# = u.user#

and (

o.owner# in (USERENV('SCHEMAID'), 1 /* PUBLIC */) /* user's private, any public

*/

or /* user has any privs on base object in local database */

exists

(select null

from sys.objauth$ ba, sys.obj$ bo, sys.user$ bu

where s.node is null /* don't know accessibility if syn for db link */

and bu.name = s.owner

and bo.name = s.name

and bu.user# = bo.owner#

and ba.obj# = bo.obj#

and ( ba.grantee# in (select kzsrorol from x$kzsro)

or ba.grantor# = USERENV('SCHEMAID')

)

)

or /* local object, and user has system privileges */

(s.node is null /* don't know accessibility if syn is for db link */

and exists (select null from v$enabledprivs

where priv_number in (-45 /* LOCK ANY TABLE */,

-47 /* SELECT ANY TABLE */,

-48 /* INSERT ANY TABLE */,

-49 /* UPDATE ANY TABLE */,

-50 /* DELETE ANY TABLE */)

)

)

)

union

select u.name, o.name, s.owner, s.name, s.node

from sys.user$ u, sys.syn$ s, sys.obj$ o, sys."_ALL_SYNONYMS_TREE" st

where o.obj# = s.obj#

and o.type# = 5

and o.owner# = u.user#

and o.obj# = st.syn_id /* syn is in tree pointing to accessible base obj */

and s.obj# = st.syn_id /* syn is in tree pointing to accessible base obj */

SQL> select * from v$version;

BANNER

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

Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bi

PL/SQL Release 10.2.0.3.0 - Production

CORE 10.2.0.3.0 Production

TNS for Linux: Version 10.2.0.3.0 - Production

NLSRTL Version 10.2.0.3.0 - Production

正是增加的这部分UNION语句，解决了9i中bug的同时，也带来了性能影响。

问题是不仅仅是用户访问ALL_SYNONYMS同义词变慢，而很多工具会导致系统递归调用中访问ALL_SYNONYMS视图，从而导致系统整体性能下降。

对于用户访问这个视图的性能下降，最简单的办法是仿照9i的语法，在当前用户建立一个视图，比如ALL_SYNONYMS_V92，然后创建一个当前用户下的同义词ALL_SYNONYMS指向当前用户的ALL_SYNONYMS_V92视图。由于用户下的同义词优先级高于PUBLIC SYNONYMS，用户在访问的时候就会访问用户下的ALL_SYNONYMS_V92视图，不过9i中指向同义词的同义词就可能在ALL_SYNONYMS中无法访问。

不过这种方式对于一些系统调用或者工具产生的调用无效，因为这些SQL可能访问的并不是PUBLIC同义词，而是SYS.ALL_SYNONYMS对象，这就导致了用户创建的同义词无效。如果是这种情况，那么为了解决性能问题，只要仿照9i的语法重建10g的ALL_SYNONYMS视图，同样9i中的bug就会在10g中再次出现。

Oracle在文档ID 377037.1详细描述了这个问题。看来有的时候正确性和性能很难完全兼顾。