title: postgres - unrecognized configuration parameter
body: <p>I use the following query in my trigger to store user's ID for further use:</p>
  <pre><code>set_config('session.my_username', 'john', false);
  </code></pre>
  <p>Later, in other trigger I use it this way</p>
  <pre><code>select current_setting('session.my_username')
  </code></pre>
  <p>It works like a charm when parameter is set (first query is executed), which happens when user uses the application.</p>
  <p>The point is, that IF developer / supporter tries to modify data that calls second trigger USING some database tool, first trigger does not get executed. In this case, developer / supporter gets 'unrecognized configuration parameter' error message.</p>
  <p>My question is, how to avoid such situation? Is there any way to know that configuration parameter is not set before I call 'select current_setting'?</p>