Oracle: Table Cluster with example

Clustering is a method of storing tables that are intimately related and often joined together into the same area on disk. The cluster key is the column or columns by which the tables are usually joined in a query

By storing the field comprising the Cluster Key once instead of multiple times, storage is saved. The arguably more significant advantage to Clustering is to expidite join queries. When a query is fired that joins these 2 tables by Cluster Key, the joined rows would be fetched with a single IO operation.
A cluster is a data structure that improves retrieval performance

Example:
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create cluster empdept (did number(2));
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create index empdept_indx on cluster empdept;
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create table emp
(
eid number(10),
ename varchar2(100),
did number(2)
)
cluster empdept(did);
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create table dept
(
did number(2),
dname varchar2(100)
)
cluster empdept(did);
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