title: Convert spark dataframe having list<Values> to rows using sparklyR
body: <p>I have a spark data frame that has list(values) in each row. I want to flatten the data by converting list of values to rows using <strong>sparklyr</strong>. Here is the sample data. </p>
  
  <pre><code>  id   colA     colB  colC
  1: 1 list&lt;&gt; 4b,8b,2b list&lt;&gt;
  2: 2 list&lt;&gt; 7b,2b,2b list&lt;&gt;
  </code></pre>
  
  <p>My output should look like this:</p>
  
  <pre><code>   id colA colB  colC
  1:  1    1   4b FALSE
  2:  1    2   8b FALSE
  3:  1    3   2b FALSE
  4:  2    1   7b FALSE
  5:  2    2   2b FALSE
  6:  2    3   2b FALSE
  </code></pre>
  
  <p>My data is of medium size(around 200M records). <strong>So I don't want to collect the data into R memory to perform this operation</strong>.  </p>
  
  <p>Here is the reproducible dataset  </p>
  
  <pre><code>data.table(structure(list(id = list(1,2), 
                 colA = list(list(1, 2, 3),list(1, 2, 3)), 
                 colB = list(as.raw(c(0x4b, 0x8b, 0x2b)),as.raw(c(0x7b, 0x2b, 0x2b))), 
                 colC = list(list(FALSE, FALSE, FALSE),list(FALSE, FALSE, FALSE))
                ),
            .Names = c("id", "colA", "colB", "colC"), 
            row.names = c(NA, -1L),
            class = c("data.frame","data.table")))
  </code></pre>