ColumnLowCardinalityT fail in constructor

[sabudilovskiy]

I'm trying to create a clickhouse::ColumnLowCardinalityTclickhouse::ColumnInt32 to insert into low_cardinality(Int32), but in this code:

template <typename T>
using LCC = clickhouse::ColumnLowCardinalityT<T>;

using column_int32 = LCC<clickhouse::ColumnInt32>;

std::shared_ptr<column_int32> abc = std::make_shared<column_int32>(); // throws ItemView value size mismatch for Int32 expected: 4, got: 0

I don't understand what I'm doing wrong and what I need to pass to the constructor.

[slabko]

Hi @sabudilovskiy

Unfortunately, low-cardinality types other than String, FixedString, and Nullable are not supported by clickhouse-cpp. While ClickHouse itself allows wrapping integer types in LowCardinality columns, in most cases this leads to performance degradation due to an additional level of indirection when mapping from the value index (which is itself an integer) to the value (also an integer). It is way faster and easier avoid this mapping in the first place.

However, your example seems to work fine for LCC<clickhouse::ColumnString>, so you have not done anything wrong here. We probably need to find a way to provide a friendlier error message for unsupported types.

[sabudilovskiy]

@slabko UUID must be supported?

using column_uuid = LCC<clickhouse::ColumnUUID>;
std::shared_ptr<column_uuid> abc2 = std::make_shared<column_uuid>();

[build] /usr/include/clickhouse/columns/lowcardinality.h:128:11: ошибка: нет имени типа «ValueType» в «class clickhouse::ColumnUUID»
[build]   128 |     using ValueType = typename DictionaryColumnType::ValueType;

ValueType is not represented in ColumnUUID. Is this intentional?

[slabko]

@sabudilovskiy, unfortunately, LowCardinality(UUID) is not supported by clickhouse-cpp either.
The absence of ValueType in clickhouse::ColumnUUID is only part of the problem; even if you address that, there are still missing pieces in the library required to fully support clickhouse::ColumnUUID.
For example: https://github.com/ClickHouse/clickhouse-cpp/blob/master/clickhouse/columns/factory.cpp#L197