Thanks for your interest in the BigQuery Data Source, and for your question.
Before I answer directly on the migration question, I’d like to understand your use case a bit more, and give a little bit of background on how we would expect BigQuery to work in terms of Cloud Database Use Cases.
First: Are you truly “Migrating” from Cloud SQL to BigQuery, or are you “Copying data regularly from Cloud SQL to BigQuery”.
My understanding of the primary use case for BigQuery is the latter. It should be used as “Read-only” copy of data from some other Database, which can then be used for Reporting and Data Analytics types of use cases (ie: a data warehouse). It is NOT expected to be used as an OLTP (online transaction processing) DB.
Therefore, our plan is to provide a Read-only capability with the BigQuery data source. If your apps are using read/write capabilities (adding rows and/or updating existing rows), then I would recommend you re-evaluate your decision to move from CloudSQL to BigQuery.
If you do have read/write use cases for Big Query that you would want to use an AppSheet app for, I would be very interested to understand the use case and we can also re-evaluate our decision to do read-only. However, this opens up a “can of worms” which may be hard to unwind later, so we want to be cautious here.
As stated in some of the other comments below, we would expect a migration (if you have read-only use cases) to be done in the same way as moving from Sheets to Cloud SQL…copy the app and point it to the new data source.
Let me know your thoughts. I’d love to hear about your use cases and plans for BigQuery.