title: PostgreSQL: Should I alter an existing index or create a new one?
body: <p>I have a billing_infos table that has a column called <code>order_id</code>. It already has an index.</p>
  
  <pre><code>\d billing_infos
  ...
  Indexes:
      "billing_infos_pkey" PRIMARY KEY, btree (id)
      "billing_infos_address_id_idx" btree (address_id)
      "index_billing_infos_on_order_id" btree (order_id) //&lt;--this one
  </code></pre>
  
  <p>However, I am not sure if this is a unique index. I have the task of making a unique index, but I'm not sure if I should change the one that's already there or create a new one. The order_id values should all be unique.</p>
  
  <p>Should I create a new index or alter the existing one?<br>
  And how do I check to see if the existing indexes are unique?</p>