一：Oracle官方给出的描述
Oracle Database assigns each datafile two associated file numbers, an absolute file
number and a relative file number, that are used to uniquely identify it
Absolute：Uniquely identifies a datafile in the database. This file number can be used in many SQL statements that reference datafiles in place of using the file name. The absolute file number can be found in the FILE# column of the V$DATAFILE or V$TEMPFILE view, or in the FILE_ID column of the DBA_DATA_FILES or DBA_TEMP_FILES view
Relative：Uniquely identifies a datafile within a tablespace. For small and medium size databases, relative file numbers usually have the same value as the absolute file number. However, when the
