Deferrable constraints

Are there any plans to implement deferrable unique constraints?

I have two identical tables a1 and a2. Each of them has a primary key
'id' and unique constraint on 'prod' and 'code'.
Now, imagine the following situation:
a1:

id prod code
1 1 abc1
2 1 abc2

a2:

id prod code
1 1 abc2
2 1 abc1

Basically, a1 and a2 have code values swapped.
Now, I want to update 'code' values in table a1 with values from table
a2 relying on equal 'id' values. This throws a 'can not insert duplicate
value' exception.
Is there a way to deal with this situation? I would prefer not to update
'id' values based on 'prod' and 'code' values though.

Thanks.

Oleg

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