title: Google Bigquery: Join of two external tables fails if one of them is empty
body: <p>I have 2 external tables in BiqQuery, created on top of JSON files on Google Cloud Storage. The first one is a fact table, the second is errors data - and it might or might not be empty.</p>
  
  <ul>
  <li>I can query each table separately just fine, even an empty one - here is an 
  <a href="https://i.stack.imgur.com/nIdiH.png" rel="nofollow noreferrer">empty table query result example</a></li>
  <li>I'm also able to left join them if both of them are not empty.</li>
  </ul>
  
  <p>However, if errors table is empty, my query fails with the following error:</p>
  
  <blockquote>
    <p>The query specified one or more federated data sources but not all of them were scanned. It usually indicates incorrect uri specification or a 'limit' clause over a union of federated data sources that was satisfied without having to read all sources.</p>
  </blockquote>
  
  <p>This situation isn't covered anywhere in the docs, and it's not related to this versioning issue - <a href="https://stackoverflow.com/questions/36193519/reading-bigquery-federated-table-as-source-in-dataflow-throws-an-error">Reading BigQuery federated table as source in Dataflow throws an error</a></p>
  
  <p>I'd rather avoid converting either of this tables to native, since they are used in just one step of the ETL process, and this data is dropped afterwards. One of them being empty doesn't look like an exceptional situation, since plain select works just fine. </p>
  
  <p>Is some workaround possible?</p>
  
  <p>UPD: raised an issue with Google, waiting for response - <a href="https://issuetracker.google.com/issues/145230326" rel="nofollow noreferrer">https://issuetracker.google.com/issues/145230326</a></p>