> That's another interesting argument. Again, I had in mind the space
> efficiency principle and I decided to use null IDs for dimension tables if
> I don't have the information. I noticed though that in those cases I can't
> use any index and performances result very poor.

For one thing, this is false optimization; a NULL isn't saving you any table 
size on an INT or BIGINT column.    NULLs are only smaller on variable-width 
columns.  If you're going to start counting bytes, make sure it's an informed 

More importantly, you should never, ever allow null FKs on a star-topology 
database.    LEFT OUTER JOINs are vastly less efficient than INNER JOINs in a 
query, and the difference between having 20 outer joins for your data view, 
vs 20 regular joins, can easily be a difference of 100x in execution time.


Josh Berkus
Aglio Database Solutions
San Francisco

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