title: Can I add a UNIQUE constraint to a PostgreSQL table, after it's already created?
body: <p>I have the following table:</p>
  
  <pre><code> tickername | tickerbbname  | tickertype
  ------------+---------------+------------
   USDZAR     | USDZAR Curncy | C
   EURCZK     | EURCZK Curncy | C
   EURPLN     | EURPLN Curncy | C
   USDBRL     | USDBRL Curncy | C
   USDTRY     | USDTRY Curncy | C
   EURHUF     | EURHUF Curncy | C
   USDRUB     | USDRUB Curncy | C
  </code></pre>
  
  <p>I don't want there to ever be more than one column for any given <code>tickername</code>/<code>tickerbbname</code> pair. I've already created the table and have lots of data in it (which I have already ensured meets the unique criteria). As it gets larger, though, room for error creeps in.</p>
  
  <p>Is there any way to add a <code>UNIQUE</code> constraint at this point?</p>