title: Parse JSON from Presto varchar column fails
body: <p>I am attempting to convert my <code>varchar</code> column data, which is stringifed <code>JSON</code>, to <code>MAP</code> datatype so I can reference the data as elements.</p>
  
  <pre class="lang-sql prettyprint-override"><code>WITH
  data(c) AS (
      SELECT message from mydb.mytable
  ),
  parsed AS (
      SELECT cast(json_parse(c) as map(varchar, varchar)) AS m
      FROM data
  )
  SELECT m['action'], m['uuid']
  FROM parsed
  </code></pre>
  
  <p>Sample data looks like:
  <code>{"action":"send","timestamp":1566432054,"uuid":"1234"}</code></p>
  
  <p>I tried solution provided here: <a href="https://stackoverflow.com/questions/55279784/how-to-cast-varchar-to-mapvarchar-varchar-in-presto">How to cast varchar to MAP(VARCHAR,VARCHAR) in presto</a>, which is where I got the query from replacing <code>values</code> with a <code>select</code> statement but it did not work. I get error:</p>
  
  <p><code>INVALID_CAST_ARGUMENT: Value cannot be cast to map(varchar,varchar)</code></p>