What is Hash Cluster ?

A row is stored in a hash cluster based on the result of applying a hash function to the row's cluster key value. All rows with the same hash key value are stores together on disk.

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partho sarathi konar

  • Oct 1st, 2006

Hash Clusters cluster table data in a manner similar to normal, index clusters.

To find or store a row in a hash cluster, Oracle applies a hash function to the row's cluster key value. The resulting hash value corresponds to a data block in the cluster.
All rows with the same key value are stored together on disk.

Hash clusters are a better choice than using an indexed table or index cluster when a table is often queried with equality queries (for example, WHERE product_id=123). For such queries, the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the rows.

This reduces the amount of I/Os that must be performed to locate and read/write a row of data.

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