[GitHub] [beam] damccorm opened a new issue, #19875: Queries that attempt to write to pubsub publish time should fail at construction time

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damccorm opened a new issue, #19875:
URL: https://github.com/apache/beam/issues/19875

   Currently it's possible to perform a query like:
   
   ```
   
   CREATE TABLE pubsub (
     event_timestamp TIMESTAMP,
     id VARCHAR
   ) ...
   
   INSERT INTO pubsub (event_timestamp,
   id) VALUES (...)
   
   ```
   
   
   But when this is executed, the event_timestamp will be dropped, because on read it will be instead be populated with pubsub's publish time.
   
   A couple of ideas:
   - We could indicate that this is a VIRTUAL GENERATED column, and is therefore read-only. Calcite seems to have some support for this concept, see [ColumnStrategy.java](https://github.com/apache/calcite/blob/master/core/src/main/java/org/apache/calcite/schema/ColumnStrategy.java).
   - We could just throw an exception in the Pubsub JSON Table Provider if the query's output schema contains event_timestamp.
   
   Imported from Jira [BEAM-8741](https://issues.apache.org/jira/browse/BEAM-8741). Original Jira may contain additional context.
   Reported by: bhulette.


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