為什么bytes的值比maxbytes的值要大

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目的：
本文解釋了DBA_DATA_FILES中bytes的值要比maxbytes的值要大的原因。

描述：
當(dāng)一個(gè)datafile 手工resize到一個(gè)大于maxsize（maxbytes）的值之后，DBA_DATA_FILES中bytes的值要比maxbytes的值要大
當(dāng)該datafile被設(shè)置為autoextend on之后，dba_data_files中的maxbytes 值被自動(dòng)結(jié)算為一個(gè)非零值。
dba_data_files中的bytes是該datafile的當(dāng)前的大?。ㄒ詁ytes為單位）

下面是一個(gè)示例：

解釋?zhuān)?/strong>
當(dāng)datafile 被resize的時(shí)候，dba_data_files中的maxbytes 值 不會(huì)被更新，此時(shí)被更新的列只是bytes列。
dba_data_files中的maxbytes 值是在使用ALTER DATABASE command with MAXSIZE option 時(shí)才會(huì)被更新的。

Step-1: Create a New Tablespace  
======  ===================================================

SQL> create tablespace tst datafile '/u01/app/oracle/oradata/DBdb/tst.dbf' size 5m autoextend on;

Tablespace created.

SQL> col file_name for a60
SQL> set lines 200
SQL>  select file_name, bytes, maxbytes, autoextensible from dba_data_files;  

FILE_NAME                                                              BYTES   MAXBYTES AUT
----------------------------------------------------------------- ---------- ---------- ---
/u01/app/oracle/oradata/DBdb/users01.dbf                          3207331840 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/undotbs01.dbf                        2710568960 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/sysaux01.dbf                          744488960 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/system01.dbf                         2936012800 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/example01.dbf                         355205120 3.4360E+10 YES
/u01/app/oracle/fast_recovery_area/DBDB/newback/ts_xxf_01.dbf       10485760          0 NO
/u01/app/oracle/oradata/DBdb/tst.dbf                                 5242880 3.4360E+10 YES

7 rows selected.


Step-2: Alter the MAXSIZE value from default value  
======  ===================================================

SQL> alter database datafile '/u01/app/oracle/oradata/DBdb/tst.dbf' autoextend on maxsize 10m;  

Database altered.

SQL> select file_name, bytes, maxbytes, autoextensible from dba_data_files;

FILE_NAME                                                              BYTES   MAXBYTES AUT
----------------------------------------------------------------- ---------- ---------- ---
/u01/app/oracle/oradata/DBdb/users01.dbf                          3207331840 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/undotbs01.dbf                        2710568960 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/sysaux01.dbf                          744488960 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/system01.dbf                         2936012800 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/example01.dbf                         355205120 3.4360E+10 YES
/u01/app/oracle/fast_recovery_area/DBDB/newback/ts_xxf_01.dbf       10485760          0 NO
/u01/app/oracle/oradata/DBdb/tst.dbf                                 5242880   10485760 YES

7 rows selected.

Step-3: Resize the datafile to a value higher than MAXBYTES  
======  ===================================================
SQL> alter database datafile '/u01/app/oracle/oradata/DBdb/tst.dbf' resize 20m;

Database altered.

SQL> select file_name, bytes, maxbytes, autoextensible from dba_data_files;

FILE_NAME                                                              BYTES   MAXBYTES AUT
----------------------------------------------------------------- ---------- ---------- ---
/u01/app/oracle/oradata/DBdb/users01.dbf                          3207331840 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/undotbs01.dbf                        2710568960 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/sysaux01.dbf                          744488960 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/system01.dbf                         2936012800 3.4360E+10 YES
/u01/app/oracle/oradata/DBdb/example01.dbf                         355205120 3.4360E+10 YES
/u01/app/oracle/fast_recovery_area/DBDB/newback/ts_xxf_01.dbf       10485760          0 NO
/u01/app/oracle/oradata/DBdb/tst.dbf                                20971520   10485760 YES

7 rows selected.

SQL>

表空間tst(/u01/app/oracle/oradata/DBdb/tst.dbf  )能達(dá)到的最大大小是dba_data_files.bytes的值。

感謝各位的閱讀，以上就是“為什么bytes的值比maxbytes的值要大”的內(nèi)容了，經(jīng)過(guò)本文的學(xué)習(xí)后，相信大家對(duì)為什么bytes的值比maxbytes的值要大這一問(wèn)題有了更深刻的體會(huì)，具體使用情況還需要大家實(shí)踐驗(yàn)證。這里是億速云，小編將為大家推送更多相關(guān)知識(shí)點(diǎn)的文章，歡迎關(guān)注！