Aggregate multiple aggregation intermediate results into one. The type of the result is agg_state, and the function signature is consistent with the input parameter.
AGGREGATE_FUNCTION_UNION(agg_state)
mysql [test]>select avg_merge(t) from (select avg_union(avg_state(1)) as t from d_table group by k1)p; +----------------+ | avg_merge(`t`) | +----------------+ | 1 | +----------------+
AGG_STATE, UNION