title: PostgreSQL Alter column from array of varchar to array of pair(varchar, integer)
body: <p>Now I have a array of varchar (nullable &amp; no default value),
  and wanting to convert it into a array of pair(varchar, integer).
  For the integer value, just set 0 for now.</p>
  
  <p>For example, <code>["a", "b"]</code> will be <code>[("a", 0), ("b", 0)]</code>.</p>
  
  <p>I already know that I can create pair type via:</p>
  
  <pre><code>CREATE TYPE keycount AS (key VARCHAR, count INTEGER);
  </code></pre>
  
  <p>but have no idea how to use <code>SET</code> to alter the column.</p>
  
  <p>Thank you for any advices!</p>