title: postgres change jsonb[] to jsonb
body: <p>I use <code>postgres9.4</code>, and there exists relation "Patients" has column "contact" with type <code>jsonb[]</code>, how to transfer type <code>jsonb[]</code> to <code>jsonb</code>?</p>
  
  <p>The following is on record.</p>
  
  <pre><code>=&gt;select name, contact from "Patients" where contact is not null;
  
  name  |                                               contact                                               
  --------+-----------------------------------------------------------------------------------------------------
  "tom" | {"{\"name\": \"tom\", \"phone\": \"111111\", \"address\": \"shanghai\", \"relation\": \"your_relation\"}"}
  </code></pre>
  
  <p>I have tried as the followings, <code>contact4</code> is column with type <code>jsonb</code></p>
  
  <pre><code>alter table "Patients" alter column contact4 type jsonb using contact4::text::jsonb;
  
  ERROR:  invalid input syntax for type json
  DETAIL:  Expected ":", but found "}".
  CONTEXT:  JSON data, line 1: ...ress\": \"shanghai\", \"relation\": \"your_relation\"}"}
  </code></pre>