Method getDynamicSubcolumnData

src/DataTypes/DataTypeMap.cpp:187–234

Resolves a dynamic subcolumn like `map['key']` by parsing the key from the subcolumn name, creating a `SerializationMapKeyValue` that knows how to read only the relevant bucket, and optionally pre-extracting the values from an existing column. The subcolumn name must start with "key_" followed by the text-serialized key value.

/// and optionally pre-extracting the values from an existing column.
/// The subcolumn name must start with "key_" followed by the text-serialized key value.
std::unique_ptr<IDataType::SubstreamData> DataTypeMap::getDynamicSubcolumnData(std::string_view subcolumn_name, const SubstreamData & data, size_t /*initial_array_level*/, bool throw_if_null) const
{
    /// Only subcolumns of the form "key_<serialized_key>" are supported.
    if (!subcolumn_name.starts_with(KEY_SUBCOLUMN_PREFIX))
    {
        if (throw_if_null)
            throw Exception(ErrorCodes::ILLEGAL_COLUMN, "Type {} doesn't have subcolumn {}", getName(), subcolumn_name);
        return nullptr;
    }

    /// Parse the key value from the subcolumn name.
    std::string_view key_string = subcolumn_name.substr(KEY_SUBCOLUMN_PREFIX.size());
    auto key_column = key_type->createColumn();
    auto key_serialization = key_type->getDefaultSerialization();
    ReadBufferFromString buf(key_string);
    try
    {
        key_serialization->deserializeWholeText(*key_column, buf, FormatSettings{});
    }
    catch (...)
    {
        if (throw_if_null)
            throw Exception(ErrorCodes::ILLEGAL_COLUMN, "Type {} doesn't have subcolumn {}", getName(), subcolumn_name);
        return nullptr;
    }

    /// Create a serialization that reads only the bucket containing the requested key.
    const auto & map_serialization = assert_cast<const SerializationMap &>(*removeNamedSerialization(data.serialization));
    auto key_value_serialization = SerializationMapKeyValue::create(
        map_serialization.getValueSerialization(),
        map_serialization.getNestedSerialization(),
        map_serialization.getMapSerializationVersion(),
        key_column->getPtr(),
        nested);
    std::unique_ptr<SubstreamData> res = std::make_unique<SubstreamData>(key_value_serialization);
    res->type = value_type;

    /// If a column is available, pre-extract the values for the requested key.
    if (data.column)
    {
        const auto & column_map = assert_cast<const ColumnMap &>(*data.column);
        auto value_column = value_type->createColumn();
        extractKeyValueFromMap(*column_map.getNestedColumnPtr(), *key_column->getPtr(), *value_column, 0, data.column->size());
        res->column = std::move(value_column);
    }

    return res;
}

static DataTypePtr create(const ASTPtr & arguments)
{