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1 Linear Hashing Appendix for Chapter 1

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2 Linear Hashing Allow a hash file to expand and shrink dynamically without needing a directory. Suppose the file starts with M buckets numbered 0,1,…,M -1 and used h(K) = K mod M. This hash function is called initial hash function h i. Overflow can be handled by maintaining individual overflow chains for each bucket. When a collision leads to an overflow record in any file bucket, bucket 0 is split into 2 buckets: the original bucket 0 and a new bucket M at the end of the file. The records originally in bucket 0 are distributed between the two buckets based on h i+1 (K)=K mod 2M. Any records that hashed to bucket 0 based on h i will hash to either bucket 0 or bucket M based on h i+1.

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3 Linear Hashing (cont.) With further overflow records, additional buckets are slit in the linear order 1,2, 3, 4,… If enough overflows occur, all the original buckets 0,1,…,M-1 will have been split, so the file now has 2M instead of M buckets and all buckets use the hash function h i+1. Hence, the records in overflow are redistributed into regular buckets, using h i+1. There is a value n – which is initially set to 0 and is incremented by 1 whenever a split occurs – is needed to determine which buckets have been split. To retrieve a record with hash key value K, first apply h i to K; if h i (K) < n, then apply the function h i+1 on K because the bucket is already split. Initially, n = 0, indicating h i applies to all buckets. When n = M, this means that all the original buckets have been split and h i+1 applies to all records in the file. At this point, n is reset to 0, and any collisions lead to overflow lead to the use of a new hash function h i+2 (K) = K mod 4M.

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4 Linear Hashing (cont.) In general, h i+j (K) = K mod(2 j M), where j =0,1,2…. A hash function is needed whenever all the buckets 0,1,2,…,(2 j M)-1 have been split and n is reset to 0. Splitting can be controlled by monitoring the file load factor: l = r/(bfr*N) where r is the current number of records, bfr is the max number of records that can fit in a bucket and N is the current number of file buckets. Split can be triggered when the load of the file exceeds a certain threshold (e.g. 0.9) Buckets that have been split can also be combined if the load of the file falls below a given threshold (e.g. 0.7)

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12 The search procedure for linear hashing if n = 0 then m h j (k) else begin m h j (k); if m < n then m h j+1 (k) end; search the bucket whose hash value is m (and its overflow, if any);

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External Memory Hashing. Hash Tables Hash function h: search key [0…B-1]. Buckets are blocks, numbered [0…B-1]. Big idea: If a record with search key.

External Memory Hashing. Hash Tables Hash function h: search key [0…B-1]. Buckets are blocks, numbered [0…B-1]. Big idea: If a record with search key.

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