Database System Concepts
Database System Concepts
7th Edition
ISBN: 9780078022159
Author: Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher: McGraw-Hill Education
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One of the goals of a hash table is to get O(1) for set operations add() and remove().
With linear probing, one checks each hash table index (starting at index 0) and continues until the end of the array (index length-1) looking for an open "slot".
Lazy deletion is a mechanism where you "mark" items as being removed, rather than removing them.
A collision happens when two items hash to the same position in the hash table. The more collisions there are, the more efficient the hash table.
The load factor is the fraction of the table that is full (for open addressing).
Primary clustering happens when large clusters are formed by performing quadratic probing.
Quadratic probing probes from the original probe point, looking at +1^2, +2^2, etc.
If the table size is non-prime and the load is greater than 0.5 you can always find a place to insert with quadratic probing.
When rehashing, the table size is expanded and items can be copied with a System.arraycopy() in the same way as a dynamic array.
The textbook has a full HashMap implementation which I can look at to understand the code implementation.
Separate chaining supports higher load factors than open addressing and a load factor of 1 is acceptable.
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Transcribed Image Text:One of the goals of a hash table is to get O(1) for set operations add() and remove(). With linear probing, one checks each hash table index (starting at index 0) and continues until the end of the array (index length-1) looking for an open "slot". Lazy deletion is a mechanism where you "mark" items as being removed, rather than removing them. A collision happens when two items hash to the same position in the hash table. The more collisions there are, the more efficient the hash table. The load factor is the fraction of the table that is full (for open addressing). Primary clustering happens when large clusters are formed by performing quadratic probing. Quadratic probing probes from the original probe point, looking at +1^2, +2^2, etc. If the table size is non-prime and the load is greater than 0.5 you can always find a place to insert with quadratic probing. When rehashing, the table size is expanded and items can be copied with a System.arraycopy() in the same way as a dynamic array. The textbook has a full HashMap implementation which I can look at to understand the code implementation. Separate chaining supports higher load factors than open addressing and a load factor of 1 is acceptable.
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