class query_builder {
private:
    std::unique_ptr<sql_query_builder> sql_builder_;      // Always allocated
    std::unique_ptr<mongodb_query_builder> mongo_builder_;// Always allocated  
    std::unique_ptr<redis_query_builder> redis_builder_;  // Always allocated
    database_types db_type_;
};

class query_builder {
public:
    explicit query_builder(database_types type) : db_type_(type) {}
    
    // Get appropriate builder lazily
    sql_query_builder& sql() {
        if (!sql_builder_ && is_sql_type(db_type_)) {
            sql_builder_ = std::make_unique<sql_query_builder>();
        }
        return *sql_builder_;
    }
    
    mongodb_query_builder& mongo() {
        if (!mongo_builder_ && db_type_ == database_types::mongodb) {
            mongo_builder_ = std::make_unique<mongodb_query_builder>();
        }
        return *mongo_builder_;
    }
    
    redis_query_builder& redis() {
        if (!redis_builder_ && db_type_ == database_types::redis) {
            redis_builder_ = std::make_unique<redis_query_builder>();
        }
        return *redis_builder_;
    }

private:
    std::unique_ptr<sql_query_builder> sql_builder_;
    std::unique_ptr<mongodb_query_builder> mongo_builder_;
    std::unique_ptr<redis_query_builder> redis_builder_;
    database_types db_type_;
};

class query_builder {
private:
    std::variant<
        std::unique_ptr<sql_query_builder>,
        std::unique_ptr<mongodb_query_builder>,
        std::unique_ptr<redis_query_builder>
    > builder_;
};