If a datafile exists in the data dictionary but not in the new control file, Oracle7 creates a placeholder entry in the control file under the name MISSINGnnnn (where nnnn is the file number in decimal). MISSINGnnnn is flagged in the control file as being offline and requiring media recovery.
In the following two cases only, the actual datafile corresponding to MISSINGnnnn can be made accessible by renaming MISSINGnnnn to point to it.
Case 1: The new control file was created using the CREATE CONTROLFILE command with the NORESETLOGS option, thus allowing the database to be opened without using the RESETLOGS option. This would be possible only if all online redo logs are available.
Case 2: It was necessary to use the RESETLOGS option on the CREATE CONTROLFILE command, thus forcing the database to be opened using the RESETLOGS option, but the actual datafile corresponding to MISSINGnnnn was read-only or offline normal.
If, on the other hand, it was necessary to open the database using the RESETLOGS option, and MISSINGnnnn corresponds to a datafile that was not read-only or offline normal, then the rename operation cannot be used to make the datafile accessible (since the datafile requires media recovery that is precluded by the results of RESETLOGS). In this case, the tablespace containing the datafile must be dropped.
In contrast, if a datafile indicated in the control file is not present in the data dictionary, Oracle7 removes references to it from the new control file. In both cases, Oracle7 includes an explanatory message in the ALERT file to let you know what it found.