create table matview_1 ( a String, b_count AggregateFunction(uniq, UInt64) ) Engine=MergeTree partition by tuple() ORDER by a SETTINGS index_granularity = 1024; create table matview_10000 ( a String, b_count AggregateFunction(uniq, String) ) Engine=MergeTree partition by tuple() ORDER by a SETTINGS index_granularity = 1024; DROP TABLE IF EXISTS matview_1 DROP TABLE IF EXISTS matview_10000 INSERT INTO matview_10000 SELECT a, uniqState(b) b_count FROM ( SELECT toString(intDiv(number, 20000)) a, toString(number % 10000) b FROM numbers_mt(20000000) ) GROUP BY a SETTINGS max_insert_threads=8; OPTIMIZE TABLE matview_10000 FINAL INSERT INTO matview_1 SELECT '1', uniqState(number) b_count FROM ( SELECT * FROM numbers_mt(2000000) ) GROUP BY number SETTINGS max_insert_threads=8; OPTIMIZE TABLE matview_1 FINAL select a, uniqMerge(b_count) as b_count from matview_10000 prewhere a='55' group by a FORMAT Null SETTINGS max_threads=1; select uniqMerge(b_count) as b_count from matview_10000 FORMAT Null SETTINGS max_threads=1; select uniqMerge(b_count) as b_count FROM matview_1 FORMAT Null SETTINGS max_threads=1;