Re: UNIQUE predicate

On Sat, Jul 06, 2002 at 05:32:53PM -0400, Tom Lane wrote:
> nconway(at)klamath(dot)dyndns(dot)org (Neil Conway) writes:
> > The attached patch implements the SQL92 UNIQUE predicate.
>
> The implementation seems to be well short of usefulness in a production
> setting, for two reasons: (1) you're accumulating all the tuples into
> memory --- what if they don't fit? (2) the comparison step is O(N^2),
> which renders the first point rather moot ... a test case large enough
> to risk memory exhaustion will not complete in your lifetime.

That's true -- I probably should have noted in the original email that
my implementation was pretty much "the simplest thing that works".

> I think a useful implementation will require work in the planner to
> convert the UNIQUE predicate into a SORT/UNIQUE plan structure (somewhat
> like the way DISTINCT is implemented, but we just want a boolean
> result).

Hmmm... that's certainly possible, but I'm not sure the feature is
important enough to justify that much effort.

Cheers,

Neil

--
Neil Conway <neilconway(at)rogers(dot)com>
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