What is hash cluster in Oracle?

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Hash cluster is a technique to store a data in hash table and improve the performance of data retrieval. Hash function is applied on table row’s cluster key value and store in hash cluster.

In Oracle, a hash cluster is a type of table cluster that uses a hash function to distribute rows among a fixed number of clusters. The purpose of a hash cluster is to evenly distribute data across the clusters, which can help improve performance in certain types of queries.

Here’s a breakdown of the key components and concepts related to hash clusters in Oracle:

  1. Hash Function:
    • A hash function is used to determine the cluster where a particular row will be stored. The hash function takes one or more columns from the table and produces a hash value. This hash value is then used to identify the cluster.
  2. Fixed Number of Clusters:
    • Unlike other types of clusters, a hash cluster has a fixed number of clusters. This number is determined when the cluster is created.
  3. Even Data Distribution:
    • The goal of using a hash cluster is to evenly distribute the data across the clusters. This helps in preventing hotspots (unevenly distributed data) that can occur in other types of clusters.
  4. Query Performance:
    • Hash clusters are designed to improve the performance of certain types of queries, especially those that involve range scans or equality conditions on the columns used in the hash function.
  5. Choosing Columns for Hash Function:
    • When creating a hash cluster, careful consideration must be given to choosing the columns for the hash function. The goal is to distribute data evenly and avoid clustering related rows together.
  6. Limitations:
    • Hash clusters may not be suitable for all types of applications. They are most effective when queries involve the hash key. If queries frequently involve other columns, a different type of clustering might be more appropriate.

To create a hash cluster in Oracle, you would use the CREATE CLUSTER statement with the HASHKEYS clause to specify the number of clusters and the columns for the hash function.

CREATE CLUSTER your_cluster
(column1 datatype1, column2 datatype2, ...)
HASHKEYS 8; -- Specify the desired number of clusters

This example creates a hash cluster named your_cluster with a hash function based on the specified columns and 8 clusters.