title: Syntax error at or near "user" when adding Postgres constraint
body: <p>I'm running Postgres 8.4.13, and trying to add a constraint to an existing table. According to <a href="http://www.postgresql.org/docs/8.4/static/sql-altertable.html">the docs</a>, this should be possible:</p>
  
  <pre><code>alter table indexed_friends add constraint no_duplicate_user_friends unique (user, friend);
  </code></pre>
  
  <p>Yet when I run this I get the following error:</p>
  
  <pre><code>ERROR:  syntax error at or near "user"
  </code></pre>
  
  <p>I'm confused because I'm following an unique constraint example listed in the documentation almost exactly. I can provide the table schema, but since it's complaining about a syntax error, I'm not sure that's necessary.</p>