AggregatingMergeTree + projection

AggregatingMergeTree_projection.sql

create table test ( 
   A Int64, 
   B String, 
   SomeID AggregateFunction(uniq, Int64), 
   projection p1 (select B, uniqMergeState(SomeID) group by B)
) 
Engine=AggregatingMergeTree order by (A, B);

insert into test select number A, number%3 B, uniqState(toInt64(rand64())) from numbers(1e7) group by A,B;
        
select B, finalizeAggregation(uniqMergeState(SomeID)) from test group by B 
settings allow_experimental_projection_optimization = 0;
┌─B─┬─finalizeAggregation(uniqMergeState(SomeID))─┐
│ 2 │                                     3315471 │
│ 1 │                                     3329550 │
│ 0 │                                     3325098 │
└───┴─────────────────────────────────────────────┘
3 rows in set. Elapsed: 0.364 sec. Processed 10.00 million rows, 1.46 GB (27.49 million rows/s., 4.00 GB/s.)

select B, finalizeAggregation(uniqMergeState(SomeID)) from test group by B;
┌─B─┬─finalizeAggregation(uniqMergeState(SomeID))─┐
│ 1 │                                     3329550 │
│ 0 │                                     3325098 │
│ 2 │                                     3315471 │
└───┴─────────────────────────────────────────────┘
3 rows in set. Elapsed: 0.014 sec.