title: Append new column to a dataframe by reading parquet files first
body: <p>Assume there is a dataframe with multiple columns, which looks smth like this (I omitted unnecessary columns):</p>
  
  <pre><code>+----------------------------------------+
  |path                                    |
  +----------------------------------------+
  |/tmp/some_folder/2020-04-02/blabla1.parq|
  |/tmp/some_folder/2020-05-14/bla2bla.parq|
  +----------------------------------------+
  </code></pre>
  
  <p>Where path is some parquet file in hdfs, which has only one row and a structure like this:</p>
  
  <pre><code>+-----------+
  |value      |
  +-----------+
  |some value |
  +-----------+
  </code></pre>
  
  <p>How can I read those files and add a column ('value') to the initial dataframe? As a result, I want a structure like this:</p>
  
  <pre><code>+----------------------------------------+----------+
  |path                                    |value     |
  +----------------------------------------+----------+
  |/tmp/some_folder/2020-04-02/blabla1.parq|some value|
  |/tmp/some_folder/2020-05-14/bla2bla.parq|bla blah  |
  +----------------------------------------+----------+
  </code></pre>
  
  <p>For instance, I can turn the 'path' column into a list, read into datframes by iterating it and join with the initial dataframe. Are there any other solutions? Preferably faster performance-wise.</p>