The address-splitting example is the clearest way to make this concrete, "123 Main St, New York, NY" parsing into street/city/state sounds trivial until you remember real-world addresses are inconsistent enough (apartment numbers, missing states, international formats) that the extraction step itself is often where a backfill actually gets hard, not the schema change that triggered it.

The concept-vs-execution distinction at the end is the most useful framing here. "Backfill" as an idea is genuinely simple, but that simplicity is exactly what makes production backfills dangerous, it's easy to underestimate a task that sounds conceptually trivial, right up until you're the one handling batching, idempotency, and retries on a table with millions of rows under real traffic.