Allow overriding flattenResults when reading with BigQueryIO.Read.fromQuery

[nevillelyh]

Just realized that BigQueryIO now supports reading fromQuery. It'll be nice if it allows overriding flattenResults to false so that we can retain nested/repeated records in some cases, e.g. mapping to Avro.

https://github.com/GoogleCloudPlatform/DataflowJavaSDK/blob/master/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/BigQueryTableRowIterator.java#L334

[nevillelyh]

Also I'm creating a view to get output schema without executing the query, as suggested in this ticket, and realize that there's no way to flatten result when creating the view. So with a query like SELECT field1, field2.f2a.f2b ... I'll get different schema from the view vs actual result:

View schema: {"field1": ..., "field2": {"f2a": ..., "f2b": ...}}
Output schema: {"field1": ..., "field2_f2a_f2b": ...}

[davorbonaci]

I agree that having this flag exposed would be nice. It is a fairly trivial change too.

Some background: there are several BigQuery flags that could be beneficial to Dataflow users. So far, we have been exposing one-by-one on-demand. It would be even better if we had a general way of interacting with BigQuery through Dataflow in a way that exposes all the richness of the BigQuery API.

[dhalperi]

Fixed in today's push by b08ec20