Linear hashing: Difference between revisions

Revision as of 20:00, 26 May 2024

Revision as of 21:42, 15 July 2024

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The file structure of a dynamic hashing data structure adapts itself to changes in the size of the file, so expensive periodic file reorganization is avoided.<ref name=RD/> A Linear Hashing file expands by splitting a predetermined bucket into two and shrinks by merging two predetermined buckets into one. The trigger for a reconstruction depends on the flavor of the scheme; it could be an overflow at a bucket or [[load factor (computer science)|'''load factor''']] (i.e., the number of records divided by the number of buckets) moving outside of a predetermined range.<ref name=WL80 /> In Linear Hashing there are two types of buckets, those that are to be split and those already split. While extendible hashing splits only overflowing buckets, [[spiral Hashing|spiral hashing]] (a.k.a. spiral storage) distributes records unevenly over the buckets such that buckets with high costs of insertion, deletion, or retrieval are earliest in line for a split.<ref name=AL/>

Linear Hashing has also been made into a scalable distributed data structure, '''LH*'''. In LH*, each bucket resides at a different server.<ref name=WL93>

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Linear hashing algorithms may use only controlled splits or both controlled and uncontrolled splits.

'''Controlled splitting''' occurs if a split is performed whenever the [[load factor (computer science)|load factor]], which is monitored by the file, exceeds a predetermined threshold.<ref name="LMS" /> If the hash index uses controlled splitting, the buckets are allowed to overflow by using linked overflow blocks. When the ''load factor'' surpasses a set threshold, the ''split pointer's'' designated bucket is split. Instead of using the load factor, this threshold can also be expressed as an occupancy percentage, in which case, the maximum number of records in the hash index equals (occupancy percentage)*(max records per non-overflowed bucket)*(number of buckets).<ref name=":0">{{Cite book |last=Silberschatz |first=Abraham |title=Database system concepts |last2=Korth |first2=Henry F. |last3=Sudarshan |first3=S. |date=2020 |publisher=McGraw-Hill Education |isbn=978-1-260-08450-4 |edition=Seventh |___location=New York, NY}}</ref>

An '''uncontrolled split''' occurs when a split is performed whenever a bucket overflows, in which case that bucket would be split into two separate buckets.