create table z(a date, b Int64) Engine=MergeTree Partition by toYYYYMM(a) order by a;
insert into z select today(), number from numbers(1000000000);
insert into z select yesterday(), number from numbers(1000);

create table mv_z_store(a date, max_b AggregateFunction(MAX,Int64)) ENGINE = AggregatingMergeTree Partition by toYYYYMM(a) order by a;
create table temp(a date, b Int64) Engine=Null;
create MATERIALIZED VIEW mv_z to mv_z_store AS SELECT a, maxState(b) AS max_b FROM temp GROUP BY a;
insert into temp select * from z;
drop table mv_z;
drop table temp;
create MATERIALIZED VIEW mv_z to mv_z_store AS SELECT a, maxState(b) AS max_b FROM z GROUP BY a;

insert into z select yesterday()-1, number from numbers(100);
select a, maxMerge(max_b) from mv_z group by a
a	maxMerge(max_b)
2018-08-07	99
2018-08-08	999
2018-08-09	999999999