title: Create dataframe from values/columns from another dataframe
body: <p>I hava a dataframe like this:</p>
  <pre><code>df1 = pd.DataFrame({'A': [1, 2, 3], 'B': ['a', 'b', 'c'], 'C': [4, 5, 6], 'D': ['e', 'f', 'g'], 'E': [7, 8, 9], id: [25, 15, 30]})
  </code></pre>
  <p><a href="https://i.stack.imgur.com/uSJDX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uSJDX.png" alt="enter image description here" /></a></p>
  <p>I would like to use the values of df1 (and their respective columns) as a basis for filling in df2.</p>
  <p>Expected:</p>
  <pre><code>expected = pd.DataFrame({'column': ['A', 'B', 'C', 'D', 'E', 'A', 'B', 'C', 'D', 'E'], 'value': [1, 'a', 4, 'e', 7, 2, 'b', 5, 'f', 8], 'id': [25, 15]})
  </code></pre>
  <p><a href="https://i.stack.imgur.com/4ajM4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4ajM4.png" alt="enter image description here" /></a></p>
  <p>I tried using iterrows, but as I need to use it for a large amount of data, the performance results were not positive. Can you help me?</p>