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Unformatted text preview: 106 Chapter 11 1. Explain why local depth and global depth are needed. 2. After an insertion that causes the directory size to double, how many buckets have exactly one directory entry pointing to them? If an entry is then deleted from one of these buckets, what happens to the directory size? Explain your answers brieﬂy. 3. Does Extendible Hashing guarantee at most one disk access to retrieve a record with a given key value? 4. If the hash function distributes data entries over the space of bucket numbers in a very skewed (non-uniform) way, what can you say about the size of the directory? What can you say about the space utilization in data pages (i.e., non-directory pages)? 5. Does doubling the directory require us to examine all buckets with local depth equal to global depth? 6. Why is handling duplicate key values in Extendible Hashing harder than in ISAM? Answer 11.3 The answer to each question is given below. 1. Extendible hashing allows the size of the directory to increase and decrease de- pending on the number and variety of inserts and deletes. Once the directory size changes, the hash function applied to the search key value should also change. So there should be some information in the index as to which hash function is to be applied. This information is provided by the global depth . An increase in the directory size doesn’t cause the creation of new buckets for each new directory entry. All the new directory entries except one share buckets with the old directory entries. Whenever a bucket which is being shared by two or more directory entries is to be split the directory size need not be doubled. This means for each bucket we need to know whether it is being shared by two or more directory entries. This information is provided by the local depth of the bucket. The same information can be obtained by a scan of the directory, but thisbucket....
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This note was uploaded on 01/17/2012 for the course EGN 4302 taught by Professor Dr.vishak during the Fall '12 term at University of Central Florida.
- Fall '12