title: How does PostgreSQL enforce the UNIQUE constraint / what type of index does it use?
body: <p>I've been trying to sort out the relationship between unique and index in Postgres after reading the <a href="http://www.postgresql.org/docs/8.4/interactive/indexes-unique.html" rel="noreferrer">docs on index uniqueness being an implementation detail</a>:</p>
  
  <blockquote>
    <p>The preferred way to add a unique constraint to a table is ALTER TABLE ... ADD CONSTRAINT. The use of indexes to enforce unique
    constraints could be considered an implementation detail that should
    not be accessed directly. One should, however, be aware that there's
    no need to manually create indexes on unique columns; doing so would
    just duplicate the automatically-created index.</p>
  </blockquote>
  
  <p>So taking the docs at their word I'm going to just declare things as unique and use the implicit index - or - create an index and not assume that the values are unique. Is this a mistake? </p>
  
  <p>What kind of index will I be getting from unique? Given that only a btree will accept the unique constraint and unique implicitly creates an index is it true that UNIQUE creates a btree index? I don't want to be running ranges on a hash index inadvertently. </p>