title: issue with string_agg with distinct in postgres
body: <p>I am getting error for below query. Basically to use <code>||</code> &amp; <code>distinct</code> together.</p>
  
  <pre><code>select string_agg( 'pre' || distinct user.col, 'post')
  </code></pre>
  
  <p>It works fine like this</p>
  
  <pre><code>select string_agg( 'pre' || user.col, 'post')
  </code></pre>
  
  <p>&amp; this</p>
  
  <pre><code>select string_agg(distinct user.col, 'post')
  </code></pre>