How To Drop A Datafile From A Tablespace

How to drop a datafile from a tablespace by Jeff Hunter, Sr. Database Administrator I have been asked on several occasions about how to drop a datafile from a tablespace. Much of the confusion comes from the ALTER DATABASE DATAFILE OFFLINE DROP command. This month I attempt to erase the confusion surrounding this command and how to successfully drop a datafile from a tablespace. There are two reasons why a DBA would want to remove a datafile from a tablespace: • •

The DBA may have mistakenly added a file to a tablespace, or perhaps made the file much larger than intended and now want to remove it. The DBA may be involved in a recovery scenario and the database won't start because a datafile is missing.

Database recovery (scenario 2) is beyond the scope of this months article. I will attempt to discuss scenario 1. Oracle unfortunately does not provide an interface for dropping datafiles in the same way you would drop a schema object such as a table or a user. Once the DBA creates a datafile for a tablespace, the datafile cannot be removed, although this article discusses several workarounds. As with any critical operation like dropping datafiles, ensure you have a full backup of the database. Determining how many datafiles make up a tablespace To determine how many and which datafiles make up a tablespace, you can use the following query: SELECT file_name , tablespace_name FROM dba_data_files WHERE tablespace_name ='';

Scenario 1 If the datafile you wish to remove is the only datafile in that tablespace, you can simply drop the entire tablespace using the following: DROP TABLESPACE INCLUDING CONTENTS;

The above command will remove the tablespace, the datafile, and the tablespace's contents from the data dictionary. All of the objects that where contained in that tablespace are permanently removed. One point to keep in mind is that Oracle will not drop the physical datafile after the DROP TABLESPACE command. This action needs to be performed at the operating system. Depending on the OS, you may have to completely shut down (For example, on Windows NT, you may have to shutdown Oracle AND stop the associated service before the operating system will allow you to delete the file - in some cases, file locks are still held by Oracle)

Scenario 2 If you have more than one datafile in the tablespace, and you do not need the information contained in that tablespace, or if you can easily recreate the information in this tablespace, then use the same command as above: DROP TABLESPACE INCLUDING CONTENTS;

Again, this will remove the tablespace, the datafiles, and the tablespace's contents from the data dictionary. Oracle will no longer have access to any object that was contained in this tablespace. You can then use CREATE TABLESPACE and re-import the appropriate objects back into the tablespace. Scenario 3 If you have more than one datafile in the tablespace and you wish to keep the objects that reside in the other datafile(s) which are part of this tablespace, then you must export all the objects inside the tablespace. Gather information on the current datafiles within the tablespace by running the following query in SQL*Plus: SELECT file_name , tablespace_name FROM dba_data_files WHERE tablespace_name ='';

Make sure you specify the tablespace name in capital letters. You now need to identify which objects are inside the tablespace for the purpose of running an export. To do this, run the following query: SELECT owner , segment_name , segment_type FROM dba_segments WHERE tablespace_name=''

Now, export all the objects that you wish to keep. Once the export is done, issue the DROP TABLESPACE INCLUDING CONTENTS. Note that this PERMANENTLY removes all objects in this tablespace. Delete the datafiles belonging to this tablespace using the operating system (see the comment above about possible problems in doing this) Recreate the tablespace with the datafile(s) desired, then import the objects into that tablespace. (This may have to be done at the table level, depending on how the tablespace was organized.) NOTE: The ALTER DATABASE DATAFILE OFFLINE DROP command, is not meant to allow you to remove a datafile. What the command really means is that you are offlining the datafile with the intention of dropping the tablespace.

If you are running in archivelog mode, you can also use: ALTER DATABASE DATAFILE OFFLINE; instead of OFFLINE DROP. Once the datafile is offline, Oracle no longer attempts to access it, but it is still considered part of that tablespace. This datafile is marked only as offline in the controlfile and there is no SCN comparison done between the controlfile and the datafile during startup (This also allows you to startup a database with a non-critical datafile missing). The entry for that datafile is not deleted from the controlfile to give us the opportunity to recover that datafile. Scenario 4 If you do not wish to follow any of these procedures, there are other things that can be done besides dropping the tablespace. • •

If the reason you wanted to drop the file is because you mistakenly created the file of the wrong size, then consider using the RESIZE command. If you really added the datafile by mistake, and Oracle has not yet allocated any space within this datafile, then you can use ALTER DATABASE DATAFILE RESIZE; command to make the file smaller than 5 Oracle blocks. If the datafile is resized to smaller than 5 oracle blocks, then it will never be considered for extent allocation. At some later date, the tablespace can be rebuilt to exclude the incorrect datafile.