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Showing content from https://github.com/ClickHouse/ClickHouse/issues/24778 below:

Projections inconsistent results for ReplacingMergeTree/CollapsingMergeTree · Issue #24778 · ClickHouse/ClickHouse · GitHub

drop table if exists tp;

create table tp (type Int32, eventcnt UInt64,
projection p (select sum(eventcnt), type group by type)) 
engine = ReplacingMergeTree order by type;

insert into tp select number%3, 1 from numbers(3);
insert into tp select number%3, 2 from numbers(3);

optimize table tp final;

set allow_experimental_projection_optimization = 0, force_optimize_projection = 0;

select sum(eventcnt) eventcnt, type
from tp
group by type
┌─eventcnt─┬─type─┐
│        2 │    0 │
│        2 │    2 │
│        2 │    1 │
└──────────┴──────┘

set allow_experimental_projection_optimization = 1, force_optimize_projection = 1;

select sum(eventcnt) eventcnt, type
from tp
group by type
┌─eventcnt─┬─type─┐
│        3 │    0 │
│        3 │    2 │
│        3 │    1 │
└──────────┴──────┘

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