Global Aggregation repeat emit

[chenziliang]

Use case

For global aggregation , if there is no update (no new events), it will stop emitting previous emitted results (emit periodic 1s for example) for now. There are use cases, user likes to repeatedly emit previous emitted results when there are no new events.

A new syntax like this will do the work

SELECT key, sum(n), max(k) ... FROM stream EMIT PERIODIC 1s REPEAT;

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Additional context