title: Big Query specify _PARTITIONTIME when inserting from one table into another
body: <p>I have two Big Query tables, <code>t1</code> and <code>t2</code>, both of which contain <code>_PARTITIONTIME</code> columns.</p>
  
  <p>My goal is to insert rows from <code>t2</code> into <code>t1</code> in such a way that maintains the <code>_PARTITIONTIME</code> </p>
  
  <p>Is there a way of <code>INSERT</code>ing the <code>_PARTITIONTIME</code> as a selected variable? </p>
  
  <p>Or is there a way of retrospectively defining the <code>_PARTITIONTIME</code> column after the rows have been inserted?</p>