Expert Cube Development with Microsoft SQL Server 2008

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Chapter 1
Therefore, the conclusion is that although surrogate keys are very useful and we are
not saying that there is something wrong with them, in some well-defined situations
it makes sense to deviate from the standard recommendations for surrogate keys and
use different forms instead.
The last consideration is that – even in cases where we deviate from the
standard – the usage of surrogate keys of the smallest integer type possible is always
strongly advised. The Analysis Services engine is optimized for the handling of
integer values and does not handle string and date values anywhere near as well.
Unknown members, key errors, and
NULLability
When designing a data mart, questions often arise about the relationship between
the fact table and the dimensions. Should the foreign keys be NULLable or not?
Should we use the built-in foreign keys of SQL Server to handle the relationship?
What about key errors?
Since these are very interesting topics, let us discuss them in more detail.
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Can we use NULLable foreign keys columns? The answer is definitely no.
If we do, when the data is moved into the cube, there will be no relationship
between the dimension and the facts containing the NULL key, leading to
either processing errors or situations where the user will see partial data. It
is much better to add a new member to the dimension, and relate the facts
with missing key values to that member in our ETL. Although the Unknown
Members feature in Analysis Services does this for us, we will have more
flexibility if we handle the issue ourselves.
Should we use SQL Server FOREIGN KEYS? The correct technique to adopt
here is as follows: we should define the foreign keys in the data model and
activate them during debugging, in order to detect errors. When the system
is in production, the foreign key can be disabled in order to speed up the
ETL code.
Key errors: there should be no key errors at all. If we enable foreign key
checking then we will end up with a table without key errors. This is
important: even though we can leverage Analysis Services' key error
handling functionality during processing, we advise not using it because
the presence of a key error should be detected and resolved during the
ETL process, as with NULL keys.
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