Originally posted by dbafreak
Let me tell the major difference:

Unique constraint only takes care to see that only unique values are entered, whereas Unique Index creates a seperate Index Block in which data is indexed as well. In the latter case, when the column is referenced in the where clause -- the BTREE search is instantiated so that the execution is very fast. With a unique key alone, its not a base for a good tuned system.

Does this mean that a unique constraint does not create an Index. (but primary key constraint does)