Difference between the user_bytes and the bytes of dba_data_files

Question

What's the difference between the user_bytes and the bytes column of the dba_data_file view?

Answer 1

Hi,
>>Bytes : Size Occupied
No. The size of the datafile. Doesn't matter free or used space.
>>User Bytes: Size available for Occupying.
No and Yes. Keep in mind that it shows the total size of the file available for user data since it was creation minus the related metadata information.
In resume, take a look at below:
You can see below, that datafile has 1,500 MB of size but it has 1,499.9375 MB available for user data.
LEGATTI@ORACLE10> select file_id,file_name,bytes,user_bytes from dba_data_files where tablespace_name='USERS2';
   FILE_ID FILE_NAME                                         BYTES USER_BYTES
         7 /u02/oradata/DB01/user02.dbf                 1572864000 1572798464
LEGATTI@ORACLE10> SELECT Substr(df.tablespace_name,1,20) "Tablespace Name",
  2         Substr(df.file_name,1,40) "File Name",
  3         Round(df.bytes/1024/1024,2) "Size (M)",
  4         df.increment_by "Increment By",
  5         Round(e.used_bytes/1024/1024,2) "Used (M)",
  6         Round(f.free_bytes/1024/1024,2) "Free (M)"
  7  FROM   DBA_DATA_FILES DF,
  8         (SELECT file_id,
  9                 Sum(Decode(bytes,NULL,0,bytes)) used_bytes
10          FROM dba_extents
11          GROUP by file_id) E,
12         (SELECT Max(bytes) free_bytes,
13                 file_id
14          FROM dba_free_space
15          GROUP BY file_id) f
16  WHERE  e.file_id (+) = df.file_id
17  AND    df.file_id  = f.file_id (+)
18  AND    df.file_id = 7
19  ORDER BY df.tablespace_name,
20           df.file_name;
Now, you can see that just 874 MB has been used from 1,499.9375 MB real space available.
Tablespace Name      File Name                                  Size (M) Increment By   Used (M)   Free (M)
USERS2               /u02/oradata/DB01/user02.dbf                  1,500            0     874.88     625,05Now, is that clear?
Cheers
Legatti