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标签归档:_smu_debug_mode
设置_smu_debug_mode实现指定session级别使用特定回滚段
通过设置_smu_debug_mode值来实现指定session级别使用特定的回滚段
_smu_debug_mode为默认值
--测试数据库版本 SQL> select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production PL/SQL Release 11.2.0.3.0 - Production CORE 11.2.0.3.0 Production TNS for Solaris: Version 11.2.0.3.0 - Production NLSRTL Version 11.2.0.3.0 - Production --_smu_debug_mode值 SQL> select a.ksppinm name,b.ksppstvl value 2 from x$ksppi a,x$ksppcv b 3 where a.inst_id = USERENV ('Instance') 4 and b.inst_id = USERENV ('Instance') 5 and a.indx = b.indx 6 and upper(a.ksppinm) LIKE upper('%¶m%') 7 order by name; Enter value for param: _smu_debug_mode old 6: and upper(a.ksppinm) LIKE upper('%¶m%') new 6: and upper(a.ksppinm) LIKE upper('%_smu_debug_mode%') NAME VALUE -------------------------------- ------------------------ _smu_debug_mode 0 --undo管理模式 SQL> show parameter undo; NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ undo_management string AUTO undo_retention integer 900 undo_tablespace string UNDOTBS1 --指定回滚段(查询dba_rollback_segs得到回滚段名称) SQL> set transaction use rollback segment "_SYSSMU7_1887299474$"; Transaction set. SQL> delete from t where rownum<10; 9 rows deleted. --查询使用回滚段 SQL> select XIDUSN from V$TRANSACTION; XIDUSN ---------- 9
这里可以看到在undo自动管理模式下,我们手工指定了回滚段但是被数据库给忽略,还是使用了系统自动分配的回滚段。例如这里我指定的回滚段7,但是使用了系统自动分配的回滚段9
_smu_debug_mode=45
SQL> select * from v$version; BANNER ------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - Production PL/SQL Release 11.2.0.3.0 - Production CORE 11.2.0.3.0 Production TNS for 32-bit Windows: Version 11.2.0.3.0 - Production NLSRTL Version 11.2.0.3.0 - Production SQL> show parameter undo; NAME TYPE VALUE ------------------------------------ ------- ----------------- undo_management string AUTO undo_retention integer 900 undo_tablespace string undo_new SQL> select a.ksppinm name,b.ksppstvl value 2 from x$ksppi a,x$ksppcv b 3 where a.inst_id = USERENV ('Instance') 4 and b.inst_id = USERENV ('Instance') 5 and a.indx = b.indx 6 and upper(a.ksppinm) LIKE upper('%¶m%') 7 order by name; Enter value for param: _smu_debug_mode old 6: and upper(a.ksppinm) LIKE upper('%¶m%') new 6: and upper(a.ksppinm) LIKE upper('%_smu_debug_mode%') NAME VALUE -------------------------------- ------------------------ _smu_debug_mode 45 /* 使用alter system set "_smu_debug_mode" = 45;配置 注意:该参数只能在system级别配置 */ --测试表 SQL> create table t_xifenfei 2 as 3 select * from dba_objects; Table created. --指定回滚段 SQL> set transaction use rollback segment "_SYSSMU15_1680736333$"; Transaction set. SQL> delete from t_xifenfei where rownum<10; 9 rows deleted. --查询事务回滚段 SQL> select XIDUSN from V$TRANSACTION; XIDUSN ---------- 15 SQL> commit; Commit complete. --再次指定回滚段 SQL> set transaction use rollback segment "_SYSSMU17_527554872$"; Transaction set. SQL> delete from t_xifenfei where rownum<10; 9 rows deleted. --查询事务回滚段 SQL> select XIDUSN from V$TRANSACTION; XIDUSN ---------- 17
这里可以看出来通过设置”_smu_debug_mode” = 45可以很好的实现在undo自动管理模式下,指定事务在特定的回滚段,在某些极限情况下,可以通过该操作来减少回滚段争用.