[SQL] Create External Schema

[Anton Kedin]

Hi,

I am planning to work on implementing a support for external schema
providers for Beam SQL and wanted to share a high level idea how I think
this can work.

*Short Version*
Implement CREATE FOREIGN SCHEMA statement:

CREATE FOREIGN SCHEMA

 TYPE 'bigquery'

 LOCATION 'dataset_example'

 AS bq;

CREATE FOREIGN SCHEMA

 TYPE 'hcatalog'

 LOCATION 'hive-server:2341'

 AS hive;

SELECT *

 FROM

      bq.table_example_bq AS bq_table1

    JOIN

      hive.table_example_hive AS hive_table1

    ON
      bq_table1.some_field = hive_table1.some_other_field;

*A Bit Longer Version: *
https://docs.google.com/document/d/1Ilk3OpDxrp3bHNlcnYDoj29tt9bd1E0EXt8i0WytNmQ

Thoughts, ideas?

Regards,
Anton