Unique indexes can be partial, i.e. defined with a where clause (that must be included in a query so that PostgreSQL knows to use that index) whereas unique constraints cannot. JORGE MALDONADO wrote > I have search for information about the difference between "unique index" > and "unique constraint" in PostgreSQL without getting to a specific > answer, > so I kindly ask for an explanation that helps me clarify such concept.
A constraint says what valid data looks like. An index stores data in such a way as to enhance search performance. Uniqueness is a constraint. It happens to be implemented via the creation of a unique index since an index is quickly able to search all existing values in order to determine if a given value already exists. PostgreSQL has chosen to allow a user to create a unique index directly, instead of only via a constraint, but one should not do so. The uniqueness property is a constraint and so a "unique index" without a corresponding constraint is an improper model. If you look at the model without any indexes (which are non-model objects) you would not be aware of the fact that duplicates are not allowed yet in the implementation that is indeed the case. Logically the constraint layer sits on top of an index and performs its filtering of incoming data so that the index can focus on its roles of storing and retrieving. Extending this thought the underlying index should always be non-Unique and a unique filter/constraint would use that index for validation before passing the new value along. However, practicality leads to the current situation where the index takes on the added role of enforcing uniqueness. This is not the case for any other constraint but the UNIQUE constraints case is so integral to PRIMARY KEY usage that the special case behavior is understandable and much more performant. Conceptually the index is an implementation detail and uniqueness should be associated only with constraints. David J. -- View this message in context: http://postgresql.1045698.n5.nabble.com/Unique-index-VS-unique-constraint-tp5773386p5773427.html Sent from the PostgreSQL - sql mailing list archive at Nabble.com. -- Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-sql