On Mon, 2009-07-06 at 11:56 +0100, Simon Riggs wrote:
> I think it will be useful to separate the concepts of a constraint from
> the concept of an index. It seems possible to have a UNIQUE constraint
> that doesn't help at all in locating rows, just in proving that the rows
> are unique.
That would be interesting. Do you have a use case? Checking the
constraint would surely be slower in a lot of cases.
I could imagine different constraint-checking schemes that could be fast
against a heap. For instance, if it's greater than the max or less than
the min value, that would be cheap to check. That might be an
interesting way to handle the constraint for a sequence-generated
column, or timestamp column that is always ascending.
However, the problem is I don't see a lot of room for a practical use
case. In the above situations, you'd almost certainly want indexes
anyway: what's the point of a sequence number unless you're going to do
lookups? And if you have an ascending timestamp column, I would think
that you might do range lookups occasionally (which will be even better
because the heap will be clustered).
Regards,Jeff Davis