v$和v_$的一些玄机

悠悠 2022-12-27 12:38 19阅读 0赞

Oracle的普通用户执行dbms\_xplan包查看执行计划，有时会提示错误，

SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced'));
    User has no SELECT privilege on V$SESSION

原因是该用户缺少一些动态性能视图的访问权限，但是当我们尝试将提示的v$session授予用户(例如hr)时，提示了这个错，

SQL> grant select on v$session to hr;
    grant select on v$session to hr
                    *
    ERROR at line 1:
    ORA-02030: can only select from fixed tables/views

这就有些玄机了，我是最早从eygle的书中看到的这个问题，一个普通用户，在未授权v$session访问权限的前提下，执行desc v$session，他提示的是v\_$session，不是v$session，v\_$session和v$session，有什么区别？结合上面的grant错误，应该如何授权？

SQL> desc v$session;
    ERROR:
    ORA-04043: object "SYS"."V_$SESSION" does not exist

首先，我们常用的v$session其实是个同义词，

SQL> select object_name, object_type, owner from dba_objects where object_name='V$SESSION';
    OBJECT_NAME               OBJECT_TYPE             OWNER
    ------------------------- ----------------------- ---------------
    V$SESSION                 SYNONYM                 PUBLIC

看下他的定义，他是v\_$session的同义词，

SQL> select dbms_metadata.get_ddl('SYNONYM','V$SESSION','PUBLIC') from dual;
    DBMS_METADATA.GET_DDL('SYNONYM','V$SESSION','PUBLIC')
    --------------------------------------------------------------------------------
    CREATE OR REPLACE NONEDITIONABLE PUBLIC SYNONYM "V$SESSION" FOR "SYS"."V_$SESSION"

而v\_$session是个视图，

SQL> select object_name, object_type, owner from dba_objects where object_name='V_$SESSION';
    OBJECT_NAME               OBJECT_TYPE             OWNER
    ------------------------- ----------------------- ---------------
    V_$SESSION                VIEW                    SYS

这是他的定义，我们看到结尾，v\_$session是从v$session视图得到的，

SQL> select dbms_metadata.get_ddl('VIEW','V_$SESSION','SYS') from dual;
    DBMS_METADATA.GET_DDL('VIEW','V_$SESSION','SYS')
    --------------------------------------------------------------------------------
      CREATE OR REPLACE FORCE NONEDITIONABLE VIEW "SYS"."V_$SES
    SION"  CONTAINER_DATA
     ("SADDR", "SID", "SERIAL#", "AUDSID", "PADDR", "USER#",
    "USERNAME", "COMMAND", "OWNERID", "TADDR
    ", "LOCKWAIT", "STATUS", "SERVER", "SCHE
    MA#", "SCHEMANAME", "OSUSER", "PROCESS",
     "MACHINE", "PORT", "TERMINAL", "PROGRAM
    ", "TYPE", "SQL_ADDRESS", "SQL_HASH_VALU
    E", "SQL_ID", "SQL_CHILD_NUMBER", "SQL_E
    XEC_START", "SQL_EXEC_ID", "PREV_SQL_ADD
    R", "PREV_HASH_VALUE", "PREV_SQL_ID", "P
    REV_CHILD_NUMBER", "PREV_EXEC_START", "P
    REV_EXEC_ID", "PLSQL_ENTRY_OBJECT_ID", "
    PLSQL_ENTRY_SUBPROGRAM_ID", "PLSQL_OBJEC
    T_ID", "PLSQL_SUBPROGRAM_ID", "MODULE",
    "MODULE_HASH", "ACTION", "ACTION_HASH",
    "CLIENT_INFO", "FIXED_TABLE_SEQUENCE", "
    ROW_WAIT_OBJ#", "ROW_WAIT_FILE#", "ROW_W
    AIT_BLOCK#", "ROW_WAIT_ROW#", "TOP_LEVEL
    _CALL#", "LOGON_TIME", "LAST_CALL_ET", "
    PDML_ENABLED", "FAILOVER_TYPE", "FAILOVE
    R_METHOD", "FAILED_OVER", "RESOURCE_CONS
    UMER_GROUP", "PDML_STATUS", "PDDL_STATUS
    ", "PQ_STATUS", "CURRENT_QUEUE_DURATION"
    , "CLIENT_IDENTIFIER", "BLOCKING_SESSION
    _STATUS", "BLOCKING_INSTANCE", "BLOCKING
    _SESSION", "FINAL_BLOCKING_SESSION_STATU
    S", "FINAL_BLOCKING_INSTANCE", "FINAL_BL
    OCKING_SESSION", "SEQ#", "EVENT#", "EVEN
    T", "P1TEXT", "P1", "P1RAW", "P2TEXT", "
    P2", "P2RAW", "P3TEXT", "P3", "P3RAW", "
    WAIT_CLASS_ID", "WAIT_CLASS#", "WAIT_CLA
    SS", "WAIT_TIME", "SECONDS_IN_WAIT", "ST
    ATE", "WAIT_TIME_MICRO", "TIME_REMAINING
    _MICRO", "TIME_SINCE_LAST_WAIT_MICRO", "
    SERVICE_NAME", "SQL_TRACE", "SQL_TRACE_W
    AITS", "SQL_TRACE_BINDS", "SQL_TRACE_PLA
    N_STATS", "SESSION_EDITION_ID", "CREATOR
    _ADDR", "CREATOR_SERIAL#", "ECID", "SQL_
    TRANSLATION_PROFILE_ID", "PGA_TUNABLE_ME
    M", "SHARD_DDL_STATUS", "CON_ID", "EXTER
    NAL_NAME", "PLSQL_DEBUGGER_CONNECTED") A
    S
      select "SADDR","SID","SERIAL#","AUDSID","PADDR","USER#"
    ,"USERNAME","COMMAND","OWNERID","TADDR",
    "LOCKWAIT","STATUS","SERVER","SCHEMA#","
    SCHEMANAME","OSUSER","PROCESS","MACHINE"
    ,"PORT","TERMINAL","PROGRAM","TYPE","SQL
    _ADDRESS","SQL_HASH_VALUE","SQL_ID","SQL
    _CHILD_NUMBER","SQL_EXEC_START","SQL_EXE
    C_ID","PREV_SQL_ADDR","PREV_HASH_VALUE",
    "PREV_SQL_ID","PREV_CHILD_NUMBER","PREV_
    EXEC_START","PREV_EXEC_ID","PLSQL_ENTRY_
    OBJECT_ID","PLSQL_ENTRY_SUBPROGRAM_ID","
    PLSQL_OBJECT_ID","PLSQL_SUBPROGRAM_ID","
    MODULE","MODULE_HASH","ACTION","ACTION_H
    ASH","CLIENT_INFO","FIXED_TABLE_SEQUENCE
    ","ROW_WAIT_OBJ#","ROW_WAIT_FILE#","ROW_
    WAIT_BLOCK#","ROW_WAIT_ROW#","TOP_LEVEL_
    CALL#","LOGON_TIME","LAST_CALL_ET","PDML
    _ENABLED","FAILOVER_TYPE","FAILOVER_METH
    OD","FAILED_OVER","RESOURCE_CONSUMER_GRO
    UP","PDML_STATUS","PDDL_STATUS","PQ_STAT
    US","CURRENT_QUEUE_DURATION","CLIENT_IDE
    NTIFIER","BLOCKING_SESSION_STATUS","BLOC
    KING_INSTANCE","BLOCKING_SESSION","FINAL
    _BLOCKING_SESSION_STATUS","FINAL_BLOCKIN
    G_INSTANCE","FINAL_BLOCKING_SESSION","SE
    Q#","EVENT#","EVENT","P1TEXT","P1","P1RA
    W","P2TEXT","P2","P2RAW","P3TEXT","P3","
    P3RAW","WAIT_CLASS_ID","WAIT_CLASS#","WA
    IT_CLASS","WAIT_TIME","SECONDS_IN_WAIT",
    "STATE","WAIT_TIME_MICRO","TIME_REMAININ
    G_MICRO","TIME_SINCE_LAST_WAIT_MICRO","S
    ERVICE_NAME","SQL_TRACE","SQL_TRACE_WAIT
    S","SQL_TRACE_BINDS","SQL_TRACE_PLAN_STA
    TS","SESSION_EDITION_ID","CREATOR_ADDR",
    "CREATOR_SERIAL#","ECID","SQL_TRANSLATIO
    N_PROFILE_ID","PGA_TUNABLE_MEM","SHARD_D
    DL_STATUS","CON_ID","EXTERNAL_NAME","PLS
    QL_DEBUGGER_CONNECTED" from v$session

一会v$session，一会v\_$session，一会又是v$session，有些凌乱了?

用eygle书中说的，概括一下，

"通常大部分用户访问的v$对象，并不是视图，而是指向v\_$视图的同义词，而v\_$视图是基于真正的v$视图(这个试图是基于x$表创建的)创建的。"

用代码模拟，

(1) 假设x$bisal是从dba\_tables复制过来的，

create table x$bisal as select \* from dba\_tables;

(2) 创建视图v$bisal，他的基表是x$bisal，

create view v$bisal as select \* from x$bisal;

(3) 创建视图v\_$bisal，他是基于v$bisal视图创建的，

create view v\_$bisal as select \* from v$bisal;

(4) 创建公共同义词v$bisal，他表示的是v\_$bisal视图，

create public synonym v$bisal for v\_$bisal;

这说明了什么？说明用户常用的v$bisal并不是真正的视图，他只是个同义词，相当于通过v\_$视图将真正底层v$视图和普通用户进行了隔离，v\_$视图可以授权给用户，但是真正的v$视图不能直接授权，这就是最开始，授权v$session提示错误的原因，按eygle所说，真正v$视图访问的限制是通过软件机制实现的，不是数据库权限控制的，

SQL> grant select on v$session to hr;
    grant select on v$session to hr
                    *
    ERROR at line 1:
    ORA-02030: can only select from fixed tables/views

再者，之所以执行desc提示这个错误，就是因为用户对v$session同义词的检索会解析为对底层视图v\_$session的访问，真正v$session视图不允许SYS之外的用户直接访问的，

SQL> desc v$session;
    ERROR:
    ORA-04043: object "SYS"."V_$SESSION" does not exist

了解了这些原因，我们知道如果让用户能访问v$session(同义词)，需要授权的是他所代表的v\_$session视图，针对执行dbms\_xplan的场景，不仅仅是上述提示的v$session，如下这些视图，都需授权，hr用户才可以使用dbms\_xplan检索执行计划，

SQL> grant select on v_$sql_plan to hr;
    Grant succeeded.
    
    
    SQL> grant select on v_$session to hr;   
    Grant succeeded.
    
    
    SQL> grant select on v_$sql_plan_statistics_all to hr;
    Grant succeeded.
    
    
    SQL> grant select on v_$sql to hr;
    Grant succeeded.

Oracle的这种隔离保护基表的设计，确实值得学习，看似轻描淡写，实则内藏玄机。

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