计算机类毕业外文翻译

To implement the relational model for the same partial LIMS schema presented in Fig. 1 for the hierarchical model, tables would need to be created for each entity relationship in the schema. Fig. 3 shows the mapping required in the relational model for the Customer-Plant and Plant-Grade relationships of the LIMS schema, where each customer (company) has one or more plant sites and each plant purchases one or more grades of products. A table is created for the Customer, which includes fields for Customer ID and Customer Name. A table is also created for the Plant, which contains the Customer ID, Plant ID, Plant Name, and Plant Address. In this situation, the Customer ID would be the primary key in the customer table, but the secondary key in the plant table. It links the information contained within the two tables and implicitly forms a relationship between the two entities. To describe the Plant-Grade relationship, Grade and Plant-Grade tables are added.
Instead of direct navigation as described in the hierarchical and network models, the relational model requires joins for the Customer, Plant, and Grade tables to find out which Customer purchases a particular Product-Grade. Fig. 3 shows that in order to navigate to a related record, the secondary key is used to find the primary key in an index file, which contains a sorted list of keys and physical addresses for each table record. Multiple database accesses are usually required to locate one record in an index, and as the table grows larger, the more database accesses are required. This is why relational databases slow down as they get larger.
Unlike the hierarchical and network databases, which depend on the physical structure of data, a relational database provides data independence, which means that the physical and logical structures are separated so that the user applications do not have to worry about the actual physical address of data. Other features of relational databases include concurrency, recovery, transaction, and an ad hoc query facility.

实施关系模型为同样部份LIMS 图解提出了在图1为等级模型, 桌会需要被创造为各个个体关系在图解。图3 显示映射必需在关系模型为LIMS 图解的顾客植物和植物品级关系, 各名顾客(公司) 有一个或更多厂址和各棵植物购买产品一个或更多等级。桌被创造为顾客, 包括领域为顾客身份证和客户名称。桌并且被创造为植物, 包含顾客身份证, 植物身份证, 植物名字, 和植物地址。在这个情况, 顾客身份证会是主关键字在顾客桌中, 但次关键字在植物桌中。它连接信息包含在二张桌之内和含蓄地形成一个关系在二个体之间。描述植物品级关系, 等级和植物品级桌增加。 代替直接航海如所描述在等级制度和网络模型, 关系模型要求加入为顾客, 植物, 并且顾客购买一个特殊产品品级的等级桌对发现。图3 表示, 为了驾驶对一个相关纪录, 次关键字使用发现主关键字在索引文件, 包含钥匙和物理地址一张被排序的名单为各个桌纪录。多数据库存取通常必需查出一记录索引, 并且当桌增长更大, 更多数据库存取必需。这就是为什么关系数据库减速如同他们得到更大。 不同于等级制度和网络数据库, 取决于数据物理结构, 关系数据库提供数据独立, 意味物理和逻辑结构被分离以便用户应用不必须担心数据实际物理地址。关系数据库其它特点包括并发、补救、交易, 和一种特别询问设施。

死锁 死锁可以用一个经典的，某程度上说有点不卫生的“哲学家吃饭”问题来阐述. 5位哲学家外出围在一张桌子上吃中国菜.可那里只有5根筷子(不是5双), 每1根分别放在就餐者与就餐者之间. 哲学家们轮流着思考和吃饭.每人都需要获得2根筷子才能吃饭,然后又放下筷子继续思考问题.现在有个“筷子管理法则”几乎能让每人都可以及时地吃上东西(其中一位饥饿的哲学家试图抢占和自己临近的筷子,但如果他还缺一根, 就放下手头上的那根筷子然后等待大约1分钟后再尝试抢占),这样，就可能会导致某些甚者所有的哲学家都因饥饿而渴望得到筷子的情况了(每位哲学家都快速抢得他左手边的那根筷子，并在他们放下左手边的筷子之前等待他右边的筷子). 最后, 他们每人都占据着一个为其他人所需要的资源，并一直等待其他人所占有的资源,而且又不释放自己所占据的，直到获得了自己所没有的资源.这就是死锁的阐述了. 当一个线程一直占据着一个锁，那么其他试图获得该锁的线程就会陷入一直阻塞等待的状态。当线程a占据锁l并试图获取锁m，但与此同时线程b又占据m而且也试图获取l，那么两个线程就会一直等下去.这种情况就是最简单的死锁了(或者叫“死锁”【翻译者注】英文中“死锁”一词有多种写法，但中文里统一称为“死锁”), 即多个线程因这种循环的锁而一直等待下去. (试想一下，让线程作为有向图的结点，该有向图的边代表“线程a正在等待线程b所占据的资源”的关系.如果该图是循环的，那么这就是一个死锁了.) 【以上绝对是非机器翻译的，“死锁”以前看过了，因为我也是读计算机的哟】

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