如何设计一个可维护的MySQL表结构来实现在线预订功能？

-- 用户表
CREATE TABLE `user` (
  `user_id` INT(10) UNSIGNED AUTO_INCREMENT NOT NULL COMMENT '用户ID',
  `username` VARCHAR(50) NOT NULL COMMENT '用户名',
  `password` VARCHAR(50) NOT NULL COMMENT '密码',
  `phone_number` VARCHAR(20) NOT NULL COMMENT '手机号',
  PRIMARY KEY (`user_id`),
  UNIQUE KEY `username_UNIQUE` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

-- 商品表
CREATE TABLE `product` (
  `product_id` INT(10) UNSIGNED AUTO_INCREMENT NOT NULL COMMENT '商品ID',
  `product_name` VARCHAR(100) NOT NULL COMMENT '商品名称',
  `description` TEXT COMMENT '商品描述',
  `price` DECIMAL(10, 2) NOT NULL COMMENT '价格',
  PRIMARY KEY (`product_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

-- 订单表
CREATE TABLE `order` (
  `order_id` INT(10) UNSIGNED AUTO_INCREMENT NOT NULL COMMENT '订单ID',
  `user_id` INT(10) UNSIGNED NOT NULL COMMENT '用户ID',
  `product_id` INT(10) UNSIGNED NOT NULL COMMENT '商品ID',
  `booking_time` DATETIME NOT NULL COMMENT '预订时间',
  PRIMARY KEY (`order_id`),
  KEY `fk_order_user_idx` (`user_id`),
  KEY `fk_order_product_idx` (`product_id`),
  CONSTRAINT `fk_order_user` FOREIGN KEY (`user_id`) REFERENCES `user` (`user_id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT `fk_order_product` FOREIGN KEY (`product_id`) REFERENCES `product` (`product_id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

-- 为用户表的用户名字段设置唯一索引
CREATE UNIQUE INDEX `username_UNIQUE` ON `user` (`username`);

-- 为订单表的用户ID字段设置普通索引
CREATE INDEX `fk_order_user_idx` ON `order` (`user_id`);

-- 为订单表的商品ID字段设置普通索引
CREATE INDEX `fk_order_product_idx` ON `order` (`product_id`);