TAILIEUCHUNG - HandBooks Professional Java-C-Scrip-SQL part 155

Tham khảo tài liệu 'handbooks professional java-c-scrip-sql part 155', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Footnotes 1. Obviously this is a simplification normally we would want to be able to find a customer s record by his name or other salient characteristic. However that part of the problem can be handled by for example a hash coded lookup from the name into a record number as we will see in the next chapter. Here we are concerned with what happens after we know the record number. 2. This figure could just as easily be considered a layout for a single record with variable-length fields however the explanation is valid either way. 3. The problem of changing the length of an existing record can be handled by deleting the old version of the record and adding a new version having a different length. 4. I am indebted for this extremely valuable algorithm to its inventor Henry Beitz who generously shared it with me in the mid-1970 s. 5. In general I use the terms quantum and block interchangeably in the few cases where a distinction is needed I will note it explicitly. 6. In the current implementation the default block size is 16K. However it is easy to change that size in order to be able to handle larger individual items or to increase storage efficiency. 7. Some blocks used to store internal tables such as the free space list are not divided into items but consist of an array of fixed-size elements sometimes preceded by a header structure describing the array. 8. For simplicity in our sample application each user record is stored in one item however since any one item must fit within a quantum applications dealing with records that might exceed the size of a quantum should store each potentially lengthy field as a separate item. 9. Four of these bytes are used to hold the index in the IRA to which this item corresponds this information would be very helpful in reconstructing as much as possible of the file if it should become corrupted. Another two bytes are used to keep track of the type of the item. These entries are for error trapping and file reconstruction if the .

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