我們知道ORACLE中有SCN(System Change
Number)和Checkpoint_Change#,那這兩者的關係是什麼呢,其實Checkpoint_Change#是來源於SCN,SCN是時刻在變化的,Checkpoint_Change#是在數據發生了檢查點的時候才改變的,它的值來源於SCN.下面通過一個例子來說明.
1.獲取當前的SCN
SQL> select dbms_flashback.get_system_change_number() from dual;
DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER()
-----------------------------------------
1275075
2.產生檢查點
SQL> alter system checkpoint;
System altered.
3.從數據文件和數據頭文件中查看檢查點
SQL> column name format a50;
SQL> select name,checkpoint_change# from v$datafile;
NAME CHECKPOINT_CHANGE#
-------------------------------------------------- ------------------
E:APPADMINISTRATORORADATAORCLSYSTEM01.DBF 1275080
E:APPADMINISTRATORORADATAORCLSYSAUX01.DBF 1275080
E:APPADMINISTRATORORADATAORCLUNDOTBS01.DBF 1275080
E:APPADMINISTRATORORADATAORCLUSERS01.DBF 1275080
SQL> select name, checkpoint_change# from v$datafile_header;
NAME CHECKPOINT_CHANGE#
-------------------------------------------------- ------------------
E:APPADMINISTRATORORADATAORCLSYSTEM01.DBF 1275080
E:APPADMINISTRATORORADATAORCLSYSAUX01.DBF 1275080
E:APPADMINISTRATORORADATAORCLUNDOTBS01.DBF 1275080
E:APPADMINISTRATORORADATAORCLUSERS01.DBF 1275080
從以上的輸出可以看出Checkpoint_Change#非常接近SCN的值,說明Checkpoint_Change#來源於SCN,因爲SCN時刻在變化,無法捕獲到Checkpoint_Change#等於SCN的時刻.
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