Redshift Streaming Ingestion From Kinesis: New Column in Materialized View?

You're correct that you can't directly alter the schema of the existing materialized view. Instead, you'll need to create a new materialized view that includes the additional column.

The most straight-forward approach would be to create a new table in Redshift with the same structure as the materialized view. Then, copy data from the materialized view into the newly created table. You may want to do this in batches depending on the size so you don't run into resource constraints. Once the data are set up, keep you data in sync with automatic inserts or unions from the Kinesis stream. You should then be able to run queries on the table with the updated column.