Hi,
Am Mittwoch, den 19.01.2005, 15:02 -0800 schrieb J. Greenlees:
> Roman Neuhauser wrote:
> > # alex@xxxxxxxxxxxxxxx / 2005-01-20 01:35:32 +1100:
> >
> >>i have a unique index on a table over multiple columns. If now one of
> >>the records has a null value in one of the indexed columns i can insert
> >>the same record multiple times.
> >>
> >>Is this a problem within postgres or expected?
> >
> >
> > In SQL, NULL means "unknown value". How could you assert that two
> > NULLs are equal?
> >
> which doesn't make mathematical sense.
> mathwise null is an empty result.
> so setting the logic up using the math logic, null values are always equal.
What kind of mathematics you are speaking?
For example you have "infinity" where infinity is never
equal to infinity.
Same with null. Which is "unknown" or "undefined"
So if x is undefined and y is undefined you cannot
assume x=y - because if you assume this, then
they would not be undefined anymore.
q.e.d.
Regards
Tino
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
