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Quoting from Oracle documentation about logical storage structures:

A database must have the SYSTEM and SYSAUX tablespaces:

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Why is it compulsory to have these tablespaces?

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Tradition and the ability for elegant failure degradation.

As the database has evolved over 11 versions, important views and tables that maintain and support the database have been programmed into the two tablespaces. Furthermore, they represent a fantastic logical separation between the "NEVER EVER TOUCH" and the "third parties can put stuff here."

From the docs:

The SYSAUX tablespace was installed as an auxiliary tablespace to the SYSTEM tablespace when you created your database. Some database components that formerly created and used separate tablespaces now occupy the SYSAUX tablespace.

If the SYSAUX tablespace becomes unavailable, core database functionality will remain operational. The database features that use the SYSAUX tablespace could fail, or function with limited capability.

Thus, while it's critical to have the SYSTEM tablespace never ever fail, one doesn't need to be nearly as paranoid for SYSAUX, this allows the designers to keep the size of SYSTEM down, while allowing third-party auxiliary features to be part of the "core" database.

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