为什么bytes的值比maxbytes的值要大

蜗牛互联网技术资讯2021-12-222310

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

描述：
当一个datafile 手工resize到一个大于maxsize（maxbytes）的值之后，DBA_DATA_FILES中bytes的值要比maxbytes的值要大
当该datafile被设置为autoextend on之后，dba_data_files中的maxbytes 值被自动结算为一个非零值。
dba_data_files中的bytes是该datafile的当前的大小（以bytes为单位）

下面是一个示例：

解释：
当datafile 被resize的时候，dba_data_files中的maxbytes 值不会被更新，此时被更新的列只是bytes列。
dba_data_files中的maxbytes 值是在使用ALTER DATABASE command with MAXSIZE option 时才会被更新的。

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>

表空间tst(/u01/app/oracle/oradata/DBdb/tst.dbf )能达到的最大大小是dba_data_files.bytes的值。