Gabriele,
> 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
count.
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
Josh Berkus
Aglio Database Solutions
San Francisco