Re: Unique constraints and indexes.

Steve Rogerson <steve(dot)pg(at)yewtc(dot)demon(dot)co(dot)uk> writes:
> On 05/01/16 19:47, Tom Lane wrote:
>> That's operating as designed. A unique constraint needs an index,
>> but not vice versa.

> I can see that might be plausible , hence the question but as a "unique index"
> imposes as constraint they seem equivalent. What's the functional difference
> between the two situations?

There is none so far as uniqueness-enforcement is concerned, because the
index is the same either way, and that's what enforces it.

The main reason we don't automatically create a constraint for every
unique index is that not all index declarations can be represented
by SQL-standard constraints.

regards, tom lane