title: Date_range between 2 columns
body: <p>Im kinda new to Python and Datascience.</p>
  
  <p>I have a Dataset with 2 datetime columns <code>A</code> and <code>B</code> :</p>
  
  <pre><code>                     A                    B
  0  2019-03-13 08:12:20  2019-03-13 08:12:25
  1  2019-03-15 10:02:18  2019-03-13 10:02:20
  </code></pre>
  
  <p>For each row, i want to generate the date range in seconds between column A and column B, so as a result i should get this :</p>
  
  <pre><code>                    A
  0 2019-03-13 08:12:20
  1 2019-03-13 08:12:21
  2 2019-03-13 08:12:22
  3 2019-03-13 08:12:23
  4 2019-03-13 08:12:24
  5 2019-03-13 08:12:25
  </code></pre>
  
  <p>I made it work with this :</p>
  
  <pre><code>import pandas as pd, numpy as np
  
  df=pd.DataFrame({'A': ["2019-03-13 08:12:20", "2019-03-15 10:02:18"], 'B': ["2019-03-13 08:12:25", "2019-03-13 10:02:20"]})
  l=[pd.date_range(start=df.iloc[i]['A'], end=df.iloc[i]['B'], freq='S') for i in range(len(df))]
  df1=(pd.DataFrame(l).T)[0]
  print(df1)
  </code></pre>
  
  <p>But as i have like 1M rows, it's taking too much time to run and i know that this solution isn't really the best, can you please guys show me whats the best way to do this ?</p>