SET max_bytes_before_external_group_by = 200000000; SET max_memory_usage = 1500000000; SET max_threads = 12; SET min_bytes_to_use_mmap_io = 0; SELECT URL, uniq(SearchPhrase) AS u FROM test.hits GROUP BY URL ORDER BY u DESC, URL LIMIT 10; SET max_memory_usage = 300000000; SET max_threads = 2; SET aggregation_memory_efficient_merge_threads = 1; SELECT URL, uniq(SearchPhrase) AS u FROM test.hits GROUP BY URL ORDER BY u DESC, URL LIMIT 10;