Re: Difference between UNIQUE constraint vs index

>>> I would imagine that other DBMSes also enforce uniqueness by means of
>>> indexes, because it'd be awful darn expensive to enforce the constraint
>>> without one; but I'm only guessing here, not having looked. Can anyone
>>> point to a real system that enforces unique constraints without an
>>> underlying index?
>
> In Rdb/VMS (which does not use MVCC), PK (and it's alias UNIQUE)
> constraints are independent of whether you have a unique index on
> the table.

PK is NOT an alias for UNIQUE. Yes it does have the same functional
operation but it is technically incorrect to consider them the same.

>
> Now, 99.44% of the time you will *not* have a PK constraint, but
> simply a unique index.

Then you have designed your database incorrectly.

Sincerely,

Joshua D. Drake

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