Since we have a canonical copy of the data in a separate store, I’m not too concerned with retaining previous versions of the data. Importing the data into a second copy of the tables and then switching over would indeed work, but would require much more logic, as well as manual cleanup (if there is an error, have to truncate the half-imported tables manually instead of just rolling back the transaction).
In any case, the issue of how to do a large transaction in Ecto which may involve more than one process remains, and there may be other use cases for this. The code I have now works, and save for perhaps adding some timeout handling for robustness, seems like it does the job—I was just wondering if there was a more straightforward solution.
