在10g中，由于解決了9I ALL_SYNONYMS的bug，導(dǎo)致查詢ALL_SYNONYMS性能下降。

上一篇介紹了9i中，如果創(chuàng)建一個指向同義詞的同義詞，則在ALL_SYNONYMS視圖中可能看不到這個同義詞。10g中則解決了這個問題。可以看到，9i和10g的ALL_SYNONYMS視圖的定義發(fā)生了變化：

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的基礎(chǔ)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

正是增加的這部分UNION語句，解決了9i中bug的同時，也帶來了性能影響。

問題是不僅僅是用戶訪問ALL_SYNONYMS同義詞變慢，而很多工具會導(dǎo)致系統(tǒng)遞歸調(diào)用中訪問ALL_SYNONYMS視圖，從而導(dǎo)致系統(tǒng)整體性能下降。

對于用戶訪問這個視圖的性能下降，最簡單的辦法是仿照9i的語法，在當(dāng)前用戶建立一個視圖，比如ALL_SYNONYMS_V92，然后創(chuàng)建一個當(dāng)前用戶下的同義詞ALL_SYNONYMS指向當(dāng)前用戶的ALL_SYNONYMS_V92視圖。由于用戶下的同義詞優(yōu)先級高于PUBLIC SYNONYMS，用戶在訪問的時候就會訪問用戶下的ALL_SYNONYMS_V92視圖，不過9i中指向同義詞的同義詞就可能在ALL_SYNONYMS中無法訪問。

不過這種方式對于一些系統(tǒng)調(diào)用或者工具產(chǎn)生的調(diào)用無效，因為這些SQL可能訪問的并不是PUBLIC同義詞，而是SYS.ALL_SYNONYMS對象，這就導(dǎo)致了用戶創(chuàng)建的同義詞無效。如果是這種情況，那么為了解決性能問題，只要仿照9i的語法重建10g的ALL_SYNONYMS視圖，同樣9i中的bug就會在10g中再次出現(xiàn)。

Oracle在文檔ID 377037.1詳細(xì)描述了這個問題。看來有的時候正確性和性能很難完全兼顧。

總結(jié)

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