Re: unique constraint

On Wed, 2010-10-20 at 16:10 -0400, Michael Glaesemann wrote:
> Upgrade if you can. Otherwise you can do two updates when rearranging paragraphs. The first updates them to an
"invalid"range which isn't used in "correct" data and so won't conflict with other values, and the second moves them
backto the valid range of positions. 
> Negative numbers are often used for this middle range.

The reason for the upgrade suggestion I believe was DEFERRABLE unique
constraints. These allow you to temporarily violate a unique constraint,
as long as it's correct at the end.

See:

  http://www.postgresql.org/docs/9.0/static/sql-createtable.html

and look for "DEFERRABLE".

You can also consider a constraint trigger:

  http://www.postgresql.org/docs/9.0/static/sql-createconstraint.html

Be careful to lock appropriately, however.

Regards,
    Jeff Davis