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Parallelizing ClickHouse aggregation merge for fixed hash map


I recently worked on https://github.com/ClickHouse/ClickHouse/pull/87366. The idea is simple but learned a lot of ClickHouse aggregation, so want to jot it down. Background The original issue stated it clear, if you run almost identical queries, their performances varies a lot! milovidov-pc :) SELECT number % 10000

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Photo of fixed hash map

fixed hash map