title: Is it possible to write a windowed unbounded PCollection to a specific BQ partition?
body: <p>Given a Windowed PCollection in streaming, I would like to write to a specific partition in a BQ table based on the window. </p>
  
  <p>When specifying a <code>tableSpecFunction</code> for <code>BigQueryIO.Write.Bound.to(SerializableFunction&lt;BoundedWindow,String&gt; tableSpecFunction)</code> <a href="https://cloud.google.com/dataflow/java-sdk/JavaDoc/com/google/cloud/dataflow/sdk/io/BigQueryIO.Write.html#to-com.google.cloud.dataflow.sdk.transforms.SerializableFunction" rel="nofollow noreferrer">(docs)</a> it seems I am only able to specify an alphanumeric name for the table reference string. </p>
  
  <p>With no table decorator specified, it seems the data is written to a partition ID of <code>__UNPARTITIONED__</code>. AFAICT this will eventually resolve to <a href="https://cloud.google.com/dataflow/java-sdk/JavaDoc/com/google/cloud/dataflow/sdk/io/BigQueryIO.Write.html#to-com.google.cloud.dataflow.sdk.transforms.SerializableFunction" rel="nofollow noreferrer">today's partition</a>.</p>
  
  <p>For my use case, I would like the data to be immediately accessible in the correct partition. Additionally it would also be great to be able to account for late data and write that to the correct partition as well.</p>
  
  <p>So to restate my question:</p>
  
  <p>Is it possible to write a windowed unbounded PCollection to a specific BQ partition?</p>