As discussed with @kitaisreal 
{
    return partition_key;
}
bool StorageInMemoryMetadata::isPartitionKeyDefined() const
{
    return partition_key.definition_ast != nullptr;
}
bool StorageInMemoryMetadata::hasPartitionKey() const
{
    return !partition_key.column_names.empty();
}
Names StorageInMemoryMetadata::getColumnsRequiredForPartitionKey() const