For aggregations, things are a little more complex. A full table scan is also required unless the user is grouping by the primary key and sets optimize_aggregation_in_order=1. In this case, a propagation signal is sent once sufficient results are acquired. Provided previous steps of the query are capable of streaming the data, e.g., filter, then this mechanism will work, and the query will terminate early. Normally, however, an aggregation must consume all table data before returning and applying the LIMIT as the final stage.
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