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    Question

    In a dense index, why might an index entry not appear for every search key value in the file?

    A Because the dense index only stores pointers to unique search keys. Correct Answer Incorrect Answer
    B Because the dense index only stores pointers to the first occurrence of each search key. Correct Answer Incorrect Answer
    C Because the dense index stores pointers to every block in the file, not every key. Correct Answer Incorrect Answer
    D Because the dense index is used only for primary keys, not for secondary keys. Correct Answer Incorrect Answer
    E Because the dense index is a sparse index with reduced entries. Correct Answer Incorrect Answer

    Solution

    In database indexing, a dense index is an index where there is an index entry for every single search key value in the file. However, in the context of this question, if an index entry does not appear for every search key value, it is likely because the index is not truly "dense" as traditionally defined but rather acting more like a sparse index. In a sparse index , the index entries do not appear for every search key value but only for some of the key values. For example, a sparse index might store pointers only to the first occurrence of each search key in a block or a page, not every occurrence within the file. This reduces the number of index entries and thus the storage overhead, but it means that not every search key is directly represented in the index.

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