Data engineering·Moving data out of the source
you re-ran the load after a failure and the rows updated in place instead of appearing twice.
Upsert
Also calledmerge, insert or update, MERGE INTO
A write that inserts a row if its key is new and updates it if the key is already there. It is the single most load-bearing operation in a pipeline, because it is what makes a re-run safe: the same batch applied twice leaves the same table. Its cost is that it needs a real key and a way to decide which of two versions is newer, and getting either wrong turns a harmless retry into silent data loss.