title: How to Handle BQ GA Export Changes?
body: <p>I'm trying to reprocess ga_sessions_yyyymmdd data but am finding the ga_sessions never used to have a field called [channelGrouping] but it does in more recent data. </p>
  
  <p>So my jobs work fine for the latest version of ga_sessions but when i try reprocess earleir ga_sessions data the job fails as it's missing the [channelGrouping] field. </p>
  
  <p>Obviously usually this is what you want, but in this case it's not. I want to make sure i'm sticking to the latest ga_sessions schema and would like the job to just set missing cols to null for when they did not exist. </p>
  
  <p>Is there any way around this?</p>
  
  <p>Perhaps i need to make an empty table called ga_sessions_template_latest and union it on to whatever ga_sessions_ daily table i'm handling - maybe this will 'upgrade' the old ga_sessions to the new structure. </p>
  
  <p>Attached is a screenshot of exactly what i mean (my union idea will actually be horrible due to nested fields in ga_sessions). <a href="https://i.stack.imgur.com/MEk4b.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MEk4b.jpg" alt="enter image description here"></a></p>