Most of Your Backfills Didn't Have to Happen

A teammate finds a bug that's been quietly corrupting the last month of data. A two-line fix. To clean up the history, they re-run the pipeline over the last thirty days and head to lunch. They come back to doubled revenue — what a productive lunch huh?? Almost seems too good to be true! Ok, what actually happened — the job wrote with INSERT in append mode, so re-running it didn't replace the old rows, it stacked new ones on top of them. Month of data, counted twice. Every dashboard downstream now shows a company that overnight started selling twice as much as it did yesterday. The bug fix was two lines. Cleaning up after the fix, however, was a full day of deleting duplicate partitions, re-running with the right write mode, and explaining to people why the whole weekend's numbers were fiction. The job that was supposed to fix an incident instead became one. That's a backfill: re-running a pipeline over old data to rewrite pa