Companion to the Weekly Problem Papers by John Milne

By John Milne

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3. Binary search still requires more than one or two seeks to find a record, even on moderately sized files. Is it worth incurring these costs? If a file was unlikely to change after it is created, and we often need to search the file, it might be appropriate to incur the overhead of building a sorted file to obtain the benefit of significantly faster searching. This is a classic tradeoff between the initial cost of construction versus the savings after construction. Another possible solution might be to read the file into memory, then sort it prior to processing search requests.

This informs the secondary index that the record it’s searching for no longer exists in the data file. If this approach is adopted, at some point in the future the secondary index will need to be “cleaned up” by removing all entries that reference non-existent primary key values. Update. During update, since the secondary index references through the primary key index, a certain amount of buffering is provided. There are three “types” of updates that a secondary index needs to consider. 1. The secondary key value is updated.

This assumes that the cost of an in-memory sort whenever the file is opened is cheaper than the cost of maintaining the file in sorted order on disk. Unfortunately, even if this is true, it would only work for small files that can fit entirely in main memory. 5 FILE MANAGEMENT Files are not static. In most cases, their contents change over their lifetime. ” Addition is straightforward, since we can store new data either at the first position in a file large enough to hold the data, or at the end of the file if no suitable space is available.

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