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CHAPTER 2 THE ANATOMY OF A DATABASE SYSTEM
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Accessing data in this configuration involves scanning the sets at the highest level to access and scan only those sets that are necessary to obtain the desired information. This process significantly reduces the number of elements to be scanned. Keeping the data items to be scanned close together minimizes search time. The arrangement of data on disk into structured files is called file organization. The goal is to design an access method that provides a way of immediately processing transactions one by one, thereby allowing us to keep an up-to-thesecond stored picture of the real-world situation. File-organization techniques were revised as operating systems evolved in order to ensure greater efficiency of storage and retrieval. Modern database systems create new challenges for which currently accepted methods may be inadequate. This is especially true for systems that execute on hardware with increased disk speeds with high data throughput. Additionally, understanding database design approaches, not only as they are described in textbooks but also in practice, will increase the requirements levied against database systems and thus increase the drive for further research. For example, the recent adoption of redundant and distributed systems by industry has given rise to additional research in these areas to make use of new hardware and/or the need to increase data availability, security, and recovery. Since accessing data from disk is expensive, the use of a cache mechanism, sometimes called a buffer, can significantly improve read performance from disk, thus reducing the cost of storage and retrieval of data. The concept involves copying parts of the data either in anticipation of the next disk read or based on an algorithm designed to keep the most frequently used data in memory. The handling of the differences between disk and main memory effectively is at the heart of a good-quality database system. The trade-off between the database system using disk or using main memory should be understood. See Table 2-3 for a summary of the performance trade-offs between physical storage (disk) and secondary storage (memory).
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Table 2-3. Performance Trade-offs
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Speed Storage space Persistence Access time Block size
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Main Memory vs. Disk
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Main memory is at least 1,000 times faster than disk. Disk can hold hundreds of times more information than memory for the same cost. When the power is switched off, disk keeps the data, and main memory forgets everything. Main memory starts sending data in nanoseconds, while disk takes milliseconds. Main memory can be accessed one word at a time, and disk one block at a time.
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Advances in database physical storage have seen much of the same improvements with regard to storage strategies and buffering mechanisms, but little in the way of exploratory examination of the fundamental elements of physical storage has occurred. Some have explored the topic from a hardware level and others from a more pragmatic level of what exactly it is we need to store. The subject of persistent storage is largely forgotten due to the capable and efficient mechanisms available in the host operating system.
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CHAPTER 2 THE ANATOMY OF A DATABASE SYSTEM
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File-access mechanisms are used to store and retrieve the data that is encompassed by the database system. Most file-access mechanisms have additional layers of functionality that permit locating data within the file more quickly. These layers are called index mechanisms. Index mechanisms provide access paths (the way data will be searched for and retrieved) designed to locate specific data based on a subpart of the data called a key. Index mechanisms range in complexity from simple lists of keys to complex data structures designed to maximize key searches. The goal is to find the data we want quickly and efficiently, without having to request and read more disk blocks than absolutely necessary. This can be accomplished by saving values that identify the data (or keys) and the location on disk of the record to form an index of the data. Furthermore, reading the index data is faster than reading all of the data. The primary benefit of using an index is that it allows us to search through large amounts of data efficiently without having to examine or in many cases read every item until we find the one we are searching for. Indexing therefore is concerned with methods of searching large files containing data that is stored on disk. These methods are designed for fast random access of data as well as sequential access of the data. There are many kinds of index mechanisms. Most involve a tree structure that stores the keys and the disk block addresses. Examples include B-trees, B+trees, and hash trees. The structures are normally traversed by one or more algorithms designed to minimize the time spent searching the structure for a key. Most database systems use one form or another of the B-tree in their indexing mechanisms. These tree algorithms provide very fast search speeds without requiring a large memory space. During the execution of the query, interpretative query execution methods access the assigned index mechanism and request the data via the access method specified. The execution methods then read the data, typically a record at a time; analyze the query for a match to the predicate by evaluating the expressions; and then pass the data through any transformations and finally on to the transmission portion of the server to send the data back to the client.
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