A data file is created when a 'create tablespace' or 'alter tablespace' command is executed. Creating a data file of size 10MB actually allocates 10MB space on the hard disk. If a tablespace is declared to contain 100 data files, one slot per data file is reserved in the control file. There are four types of data files -- system data files, application (or user) data files, temporary data files (used by oracle to store temp data while executing complex SQL statements) and rollback data files.It is possible to resize a datafile. The command which will serve this purpose is