Naz Gassiep <naz@xxxxxxxx> writes: > I would like more information on this deficiency and what causes it so I > know when to anticipate it. The uniqueness constraint is checked on a row-by-row basis, so if you update one row to hold the same value as another row holds, you get an error immediately. It doesn't matter that if the query had been allowed to finish, it would have updated that other row to some non-conflicting value. (You might be able to work around this if you could control the order in which rows are updated, but you can't.) This is not what the SQL spec says should happen, but so far no one has proposed a reimplementation that doesn't give up unreasonable amounts of performance. It's on the TODO list ... regards, tom lane
