example.txt

:) SELECT number % 3 AS key, number AS x FROM system.numbers LIMIT 10

SELECT 
    number % 3 AS key, 
    number AS x
FROM system.numbers 
LIMIT 10

┌─key─┬─x─┐
│   0 │ 0 │
│   1 │ 1 │
│   2 │ 2 │
│   0 │ 3 │
│   1 │ 4 │
│   2 │ 5 │
│   0 │ 6 │
│   1 │ 7 │
│   2 │ 8 │
│   0 │ 9 │
└─────┴───┘

10 rows in set. Elapsed: 0.008 sec. 

:) SELECT key, sum(x) FROM (SELECT number % 3 AS key, number AS x FROM system.numbers LIMIT 10) GROUP BY key

SELECT 
    key, 
    sum(x)
FROM 
(
    SELECT 
        number % 3 AS key, 
        number AS x
    FROM system.numbers 
    LIMIT 10
) 
GROUP BY key

┌─key─┬─sum(x)─┐
│   0 │     18 │
│   1 │     12 │
│   2 │     15 │
└─────┴────────┘

3 rows in set. Elapsed: 0.014 sec.

...

2017.08.28 21:05:03.886232 [ 19 ] <Trace> Aggregator: Aggregation method: key8

...

Aggregator.h

struct AggregatedDataVariants

...

std::unique_ptr<AggregationMethodOneNumber<UInt8, AggregatedDataWithUInt8Key>> key8;

AggregationMethodOneNumber<UInt8, AggregatedDataWithUInt8Key>

using AggregatedDataWithUInt8Key = HashMap<UInt64, AggregateDataPtr, TrivialHash, HashTableFixedGrower<8>>;

В HashMap будет лежать три значения.

AggregateDataPtr будет указывать на AggregateFunctionSumData<UInt64>
(Которые будут расположены в Arena.)

А в них будут просуммированы числа: 18, 12, 15.