title: How to use map datatype in clickhouse? How to turn on experimental features?
body: <p>I am trying to create a table with map datatype in clickhouse but it is giving this error.</p>
  <pre><code>query: CREATE TABLE table_map (a Map(String, UInt64)) ENGINE=Memory;
  
  Received exception from server (version 21.3.4):
  Code: 44. DB::Exception: Received from localhost:9000. DB::Exception: Cannot create table with column 'a' which type is 'Map(String,UInt64)' because experimental Map type is not allowed. Set 'allow_experimental_map_type = 1' setting to enable.
  </code></pre>
  <p>How do I turn this feature on ? I am not able to figure it out.</p>