Re: Partial Indices vs. mixing columns and functions

Tom Lane wrote:
>
> Mike Mascari <mascarm@mascari.com> writes:
> > To enforce uniqueness because
> > deactive is NULL, I cannot just create an index like:
>
> > CREATE UNIQUE INDEX i_foo (value, deactive);
>
> It's not clear to me what you are really after here.  You *can* create a
> unique index, even though 'deactive' is allowed to be NULL --- what will
> happen is that rows containing NULL will never conflict with other
> entries.  Is that what you want, or are you trying to say that you don't
> want more than one row with 'deactive' NULL for any given 'value' value?

The latter.

>
> > Or will Martijn van Oosterhout's new Partial Indices work allow me to
> > create a unique index like:
>
> > CREATE UNIQUE INDEX i_foo ON foo(value)
> > WHERE deactive IS NULL;
>
> This would seem to imply that you want the latter.

Yes.

>
> As Martijn remarks elsewhere, the above would not be allowed by the
> existing code for partial indexes.  But there is no good reason for
> that.  The reason for the restriction is that the planner's code for
> determining whether a partial index can be used in a query is pretty
> limited (with good reason; we don't want to be letting loose a full-tilt
> automated theorem prover on every query...).  But the above example
> demonstrates that an index can be useful even if it's never used in
> a query!
>
> I would say that this example shows that we should rip out the
> restrictions on the form of the predicate, and just ensure that the
> planner code will give up cleanly if the predicate is not of a form
> it can handle.

Fantastic! If left in its current state, I would have to use a fake
deactive value (some arbitrary date in the past), or add another column
to enforce uniqueness amongst 'active' records.

>
>                         regards, tom lane

Thanks,

Mike Mascari
mascarm@mascari.com