Emit / output mode

Emit / output mode - which rows the sink actually sees: a streaming output mode of append delivers each result once, when it is final; update delivers only the rows that changed; complete redelivers the whole result table every time. The names vary by engine, the three shapes do not.

Append only works when a result will never change again, which for a windowed aggregate means waiting for the watermark to close the window. Update is a changelog keyed by the result key, the same stream-of-updates-to-a-table idea Kafka Streams is built on, and the consumer reads latest-per-key. Complete re-emits everything on every refresh, which is fine for ten groups and absurd for ten million.

Do you need it? Don't panic: the scary dropdown is chosen for you by the shape of the query. In ubik a windowed query appends, one final row per closed window, and a continuous view updates, an upsert changelog keyed by the group that republishes only the groups that changed. There is no complete mode, because a consumer holding latest-per-key already has the whole table.