title: Presto map(varchar,varchar) : How to get all the possible keys for it?
body: <p>I am trying to search a column having the data type map(varchar,varchar). Now one way to access the column is to use this structure, name_of_column['<strong>key</strong>'], which will give the value for that key. But I want to know what are possible <strong>keys</strong> and then apply group by and other operations based on those <strong>keys</strong>. </p>
  
  <p>I tried searching on the internet but was not able to find appropriate solution. </p>