scn_to_timestamp的scn和time的对应关系从何而来?
是来自于系统表smon_scn_time吗?
用10046去trace"select scn_to_timestamp(xxxxxxx) from dual"
并没有发现访问smon_scn_time.

但是对scn_to_timestamp来说，最小的有效的scn是"select min(scn_bas) from smon_scn_time"

SYS@bbb>select min(scn_bas) SCN_BASE from smon_scn_time;

  SCN_BASE
----------
3771579140

SYS@bbb>select scn_to_timestamp(3771579140-1) from dual;
select scn_to_timestamp(3771579140-1) from dual
       *
ERROR at line 1:
ORA-08181: specified number is not a valid system change number
ORA-06512: at "SYS.SCN_TO_TIMESTAMP", line 1

比"select min(scn_bas) SCN_BASE from smon_scn_time"小的scn会导致
ORA-08181: specified number is not a valid system change number

对scn_to_timestamp来说，最大的有效的scn并不是"select max(scn_bas) from smon_scn_time"
可以比"select max(scn_bas) SCN_BASE from smon_scn_time"大一些

SYS@bbb>select max(scn_bas) SCN_BASE from smon_scn_time;

  SCN_BASE
----------
3771603459


SYS@bbb>select scn_to_timestamp(3771603459+1) from dual;


SCN_TO_TIMESTAMP(3771603459+1)
---------------------------------------------------------------------------
19-FEB-17 03.35.55.000000000 AM

删除smon_scn_time中最小的scn=3771579140
SYS@bbb>delete smon_scn_time where scn_bas=3771579140;

1 row deleted.

SYS@bbb>commit;

Commit complete.

当前session仍然可以查scn=3771579140

SYS@bbb>select scn_to_timestamp(3771579140) from dual;


SCN_TO_TIMESTAMP(3771579140)
---------------------------------------------------------------------------
18-FEB-17 09.30.08.000000000 PM


应该是pga里有smon_scn_time的缓存
在新的session里scn=3771579140已不可用


SYS@bbb>select scn_to_timestamp(3771579140) from dual;
select scn_to_timestamp(3771579140) from dual
       *
ERROR at line 1:
ORA-08181: specified number is not a valid system change number
ORA-06512: at "SYS.SCN_TO_TIMESTAMP", line 1