ACIDity of transactions as rescue...

Let's examine what all of the A, the C, the I and the D have for implications on our current subject.

Transactions are atomic (A)

The database should not really care about validating the constraints all the time Only the beginning and end state is really important. Inside a transaction, the data may be inconsistent : Dust hasn't yet settled.

Let's study an example. You have an unique index on an ordering.

TABLE ITEMS (
    ITEM_ID SERIAL,
    GROUP_ID INTEGER,
    ORDER INTEGER,
    LABEL VARCHAR
)

ADD UNIQUE INDEX ON ITEMS(GROUP_ID, ORDER)

Here, if you want to swap 2 items, you have to use an unused temporary value, otherwise the check won't be valid at all times.

Transactions are consistent (C)

The check needs to be done at least when the transaction ends. No special need here except that you need to re-enable before ending the transaction.

Transactions are isolated (I)

We can also easily imagine that the check only needs to be done when the transaction ends, just before the commit. Normally no one should be able to see the changing data meanwhile it's not completed.

One very important thing to know is that if you are using a READ UNCOMMITTED isolation for other transactions you will see inconsistent data.

Anyway, if you are doing that, you know what you are doing and are obviously taking special care about it, don't you ?

Transactions are durable (D)

This final property has nothing to do with our current issue. Good.

... But sometimes early warning is quite nice

When interacting with external systems, that don't participate in our transaction, extra care should be taken in order to cope with the exceptional case of a constraint failure. Distributed transactions is a very complex subject, and usually it's not supported done[1].

Conclusion

With the principle of least surprise in mind, we can easily understand why deferred is not the default behavior, but it makes a very nice addition to our toolbox.

So now there isn't any good reason anymore not to use (and abuse) constraints in your databases.

Remember, your data is you most precious asset, protect it at all cost from evil misbehaved bug-ridden software[2] !

Wow... Just in time before the end of the month... I would have failed my motto on URLs ;-)

Notes

[1] Do you always implement a double-phase commit on every SOAP/REST/XML-HTTP/POST interface that you expose ?

[2] Yeah, I have chosen my side in the coming war...