title: Column name is ambiguous on UPSERT action
body: <p>I am trying to run an <code>UPSERT</code> query string on a postgres db -</p>
  <pre><code>insert into business_data_test 
  (select * from business_data_tmp) 
  on conflict(phone_number) 
  do update set 
  average_expense=(average_expense*expense_count + excluded.average_expense*excluded.expense_count)/(expense_count + excluded.expense_count), expense_count=(expense_count + excluded.expense_count);
  </code></pre>
  <p>I am basically trying to update the column <code>average_expense</code> if there is conflicting data, but I think there is something wrong with the query as I am running into the following error -</p>
  <pre><code>ERROR:  column reference &quot;average_expense&quot; is ambiguous
  LINE 1: ...lict(phone_number) do update set average_expense=(average_ex...
                                                               ^
  SQL state: 42702
  Character: 123
  </code></pre>
  <p>I believe we have to add some table name alias somewhere but I am not sure how to fix this.</p>