Add 3 counters for ExecNode:
ExecTime - Total execution time(excluding the execution time of children).
OutputBytes - The total number of bytes output to parent.
BlockCount - The total count of blocks output to parent.
select c_name from customer union select c_name from customer
this sql used agg node to get distinct row of c_name,
so it's no need to wait for inserted all data to hash map,
could output the data which it's inserted into hash map successed.
