Chapter: Fundamentals of Database Systems - File Structures, Indexing, and Hashing - Disk Storage, Basic File Structures, and Hashing

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Hashing Techniques

Another type of primary file organization is based on hashing, which provides very fast access to records under certain search conditions.

Hashing Techniques

 

Another type of primary file organization is based on hashing, which provides very fast access to records under certain search conditions. This organization is usually called a hash file. The search condition must be an equality condition on a single field, called the hash field. In most cases, the hash field is also a key field of the file, in which case it is called the hash key. The idea behind hashing is to provide a func-tion h, called a hash function or randomizing function, which is applied to the hash field value of a record and yields the address of the disk block in which the record is stored. A search for the record within the block can be carried out in a main memory buffer. For most records, we need only a single-block access to retrieve that record.

 

Hashing is also used as an internal search structure within a program whenever a group of records is accessed exclusively by using the value of one field. We describe the use of hashing for internal files in Section 17.8.1; then we show how it is modi-fied to store external files on disk in Section 17.8.2. In Section 17.8.3 we discuss techniques for extending hashing to dynamically growing files.


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