Reading LowCardinality(Nullable(String)) column is not supported (for the data received from ClickHouse)

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> Please see https://github.com/ClickHouse/clickhouse-cpp/pull/339
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> [This one confirms that it works](https://github.com/ClickHouse/clickhouse-cpp/pull/339/files#diff-5ba05c8bd7182b90fb7e5b5b010e00c29c723612eb084a0b24b6f897fb45c531L215)
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> Perhaps, it would be helpful to know the real type of column at run-time with:
>
> ```c++
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> column->GetType().GetName()
>
> ```

_Originally posted by @Enmk in [#326](https://github.com/ClickHouse/clickhouse-cpp/issues/326#issuecomment-1759450216)_

The [test](https://github.com/ClickHouse/clickhouse-cpp/blob/v2.6.1/ut/roundtrip_tests.cpp#L279) _Map_LowCardinalityTString_LowCardinalityTNullableString_ might succeed, but this is only true because you create the column with the type `ColumnLowCardinalityT>` explicitly in the test.

However, for the real data received from _ClickHouse_ the type of the underlying column will be simply `ColumnLowCardinality`. This is probably due to the [logic](https://github.com/ClickHouse/clickhouse-cpp/blob/v2.6.1/clickhouse/columns/factory.cpp#L226) implemented in https://github.com/ClickHouse/clickhouse-cpp/pull/182:
```
case Type::Nullable:
return std::make_shared(
std::make_shared(
CreateColumnFromAst(GetASTChildElement(nested, 0), settings),
std::make_shared()
)
);
```
Therefore the `dynamic_cast` inside `column->As>>()` will yield _nullptr_.

This is easily illustrated with a simple **typeid** check (assume that `column` is an instance of a `ColumnRef` pointer).

This is what we have for `LowCardinality(String)` column:
```
// ColumnRef column;

auto type_name = column->GetType().GetName(); // "LowCardinality(String)"

auto type_id = typeid(*column).name(); // clickhouse::ColumnLowCardinalityT ("N10clickhouse21ColumnLowCardinalityTINS_12ColumnStringEEE")
```
And this is the result for `LowCardinality(Nullable(String))`:
```
// ColumnRef column;

auto type_name = column->GetType().GetName(); // "LowCardinality(Nullable(String))"

auto type_id = typeid(*column).name(); // clickhouse::ColumnLowCardinality ("N10clickhouse20ColumnLowCardinalityE")
```

The conclusion is that it's not possible to read `LowCardinality(Nullable(String))` column in the current version of _clickhouse-cpp_ (_v2.6.1_):
1. `column->As>>()` doesn't work for the reasons explained above.
2. While `column->As()` does work, it's of no help: trying to wrap the result into `ColumnLowCardinalityT>` will also fail because the nested type is `ColumnNullable` (not `ColumnNullableT`).
3. Declaring `ColumnLowCardinalityT` is not possible because there's no `ValueType` alias in `ColumnNullable`.

Commencez par clickhouse/columns/factory.cpp, autour de la gestion de Nullable, et comparez-la avec ut/roundtrip_tests.cpp au niveau de Map_LowCardinalityTString_LowCardinalityTNullableString_. Examinez ColumnRef::As et GetType().GetName() pour les colonnes reçues de ClickHouse, plutôt que de vous limiter aux colonnes construites localement. Le travail est terminé lorsque des données LowCardinality(Nullable(String)) peuvent être lues depuis une réponse réelle de ClickHouse et qu’un test de régression existe.

Rédigé par le modèle d'indexation à partir du texte de l'issue.