title: get array of all json field keys in postgresql
body: <p>I have a table called user, and inside the table there is a field called friends, this field is a json type has a value as in the following example</p>
  
  <pre><code>{"blockList": {"199": {"date": 1453197190, "status": 1}, "215": {"date": 1459325611, "status": 1}, "219": {"date": 1454244074, "status": 1}, "225": {"date": 1453981312, "status": 1}, "229": {"date": 1459327685, "status": 1}}, "followers": {"211": {"date": 1452503369}, "219": {"date": 1452764627}, "334": {"date": 1456396375}}, "following": {"215": {"date": 1459325619}, "219": {"date": 1453622322}, "226": {"date": 1454244887}, "229": {"date": 1459327691}}, "friendList": {"213": {"date": 1453622410, "type": 2, "status": 1}, "214": {"date": 1452763643, "status": 1}, "215": {"date": 1455606872, "type": 2, "status": 2}, "218": {"date": 1453280047, "status": 1}, "219": {"date": 1453291227, "status": 2}, "221": {"date": 1453622410, "type": 2, "status": 1}, "224": {"date": 1453380152, "type": 2, "status": 1}, "225": {"date": 1453709357, "type": 2, "status": 2}, "226": {"date": 1454244088, "type": 2, "status": 1}, "229": {"date": 1454326745, "type": 2, "status": 2}}}
  </code></pre>
  
  <p>this record has a blockList object that is containning objects for blocked users.
  what I need, is to return an array of all block list keys like this</p>
  
  <pre><code>["199", "215", "219", "225", "229"]
  </code></pre>
  
  <p>any help how can I write a plpgsql function to do that (return all object keys in an array)?
  I'm a beginner in psotgresql, and need a help please.</p>