title: Find missing dates of each product where multiple products are stored in the same table
body: <p>I have a table structure similar to this which tracks some product data in a daily base:</p>
  <pre><code>product_id|columnA|columnB|my_date|
     1      |  a1   |  a2   |2021-03-03|
     1      |  a1   |  a2   |2021-03-04|
     1      |  a1   |  a2   |2021-03-06|
     1      |  a1   |  a2   |2021-03-07|
     1      |  a1   |  a2   |2021-03-10|
     2      |  a1   |  a2   |2021-06-01|
     2      |  a1   |  a2   |2021-06-03|
    ...
   (more product_id)
  </code></pre>
  <p>As you can see, <code>|2021-03-05|</code>, <code>|2021-03-08|</code> and <code>|2021-03-09|</code> are missing for <code>product_id</code> 1 and <code>|2021-06-02|</code>  is missing for <code>product_id</code>
  2 .</p>
  <p>I want to get all the missing dates for each <code>product_id</code>, the result table should look like:</p>
  <pre><code>product_id|mssing_date|
     1      |2021-03-05|
     1      |2021-03-08|
     1      |2021-03-09|
     2      |2021-06-02|
    ...          ....
  other_ids |other_missing dates|
  </code></pre>