src holds two kinds of rows: "hot" (kind = 'hot') and "cold" (kind = 'cold'). Cold rows must be atomically moved into dest — no race window between deleting from one and inserting into the other. src(id, kind) holds four rows: a pair of hot and a pair of cold. dest(id, kind) has the same structure and is empty. In a single statement, delete all cold rows from src and insert them straight into dest. After the operation src keeps only hot rows, while dest receives the cold ones.