On Wed, Feb 14, 2007 at 10:01:58AM +0100, Steven Lambert wrote:
> You are right, the index has more columns and one of them have most of the
> time the value NULL.
> It is the date when the record has been "deleted"
See "Unique Constraints" in the documentation, in particular the
last paragraph:
http://www.postgresql.org/docs/8.1/interactive/ddl-constraints.html#AEN2016
"However, null values are not considered equal in this comparison.
That means even in the presence of a unique constraint it is possible
to store duplicate rows that contain a null value in at least one
of the constrained columns."
This has been discussed before; search the archives for words like
"null", "unique", and "comparison" or "compare".
--
Michael Fuhr