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DUL(Data UnLoading) & Desperation

Imagine, if you will, a situation where your Oracle database has become corrupted and your backup could either not be restored or was too out-of-date to be worth restoration. Unfortunately, this situation isn’t as hypothetical or as rare as one would think.
While poor backup strategies set the stage for this situation, hardware failures are an all-too-common occurrence which can wreak havoc on your database. So, what is a DBA to do when confronted with such a vexing situation? If the corruption is substantial or the data must be recovered, the only solution is Data Unloading (DUL).

What is Data Unloading?
DUL is the process of extracting (unloading) data from Oracle data files directly; completely bypassing the Oracle Kernel. Unloading does not even require Oracle to be installed.

How is DUL different from exporting data?
While DUL does export data, it does not require a running server; as does EXP, EXPDP, etc. DUL is strictly an offline operation.

What about database security?
As DUL reads data files directly, user and role-level permissions are completely ineffective. Currently, the only way to protect data from an unloader is to use encryption. However, Oracle’s DUL (and soon DUDE) can read a file which uses Transparent Data Encryption (TDE).

Who can perform DUL?
Technically, anyone who knows the data type and file storage formats used by Oracle can extract the data. Though, DUL is generally performed by specialized utilities; a software category of which only a handful of tools exist.

Basically, these utilities first attempt to open the SYSTEM tablespace data files and retrieve the USER$, OBJ$, TAB$ and COL$ data for use in extracting the data. If this file is also corrupt, you may have to specify object information manually. Though, several of the utilities are able to scan the data files and heuristically determine objects to extract.

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