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Creating a cluster and cluster index

Can I add a B*Tree cluster to my existing tables? After creating a cluster, how do I create a cluster index on that table?

I have already created tables. Now I want to add a B*Tree cluster to the existing tables to improve performance in the long run. Can I add a cluster to the existing tables, and after creating a cluster, how do I create a cluster index on that table? Will my SQL queries run in the same manner as they were before clustering?
Instead of a cluster, have you considered just adding a B-tree index to the table? This document shows how to create an index on a table.

An Oracle "cluster" is used to prejoin two (or more) tables on a specific column(s), called the cluster key. Almost every time I have determined that a cluster is beneficial, I have just denormalized the tables and created one table out of the two tables. There are exceptions to this rule. If you do need a cluster, you have to create the cluster first and then create the tables as part of the cluster.

Without knowing more about your exact situation, I would guess that what you really want is just a normal index on the table to speed up query processing.

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