title: Presto identify unique values
body: <p>New to <code>presto</code>, I have a table with a column contains a lot key value pairs. I can extract each row's keys using `map_keys', but I am wondering is there a function to combine and list all unique keys in one row?</p>
  
  get all the keys
  
  <pre><code>SELECT
      DISTINCT MAP_KEYS(col)
  FROM tbl
  </code></pre>
  
  what it looks like:
  
  <pre><code>_col0
  [key1, key2]
  [key2, key3]
  </code></pre>
  
  <h3>desired outputs:</h3>
  
  <pre><code>_col0
  [key1, key2, key3]
  </code></pre>