According to the documentation at https://materialize.io/docs/overview/architecture/

> Streaming sources must receive all of their data from the stream itself; there is no way to “seed” a streaming source with static data. However, you can union streaming and file sources in views, which accomplishes a similar outcome.

What I find odd is that the documentation doesn't show UNION support in the SELECT documentation : https://materialize.io/docs/sql/select/

Perhaps just an oversight in the documentation?