The original logic is to first deserialize the ColumnString into a HashSet (insert the deserialized elements into the hashset), and then traverse all the HashSet elements into the target HashSet during the merge phase.
After optimization, when deserializing, elements are directly inserted into the target HashSet, thereby reducing unnecessary hashset insert overhead.
In one of our internal query tests, 30 hashsets were merged in second phase aggregation(the average cardinality is 1,400,000), and the cardinality after merging is 42,000,000. After optimization, the MergeTime dropped from 5s965ms to 3s375ms.
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