ORA-00951: cluster not empty

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A cluster can be dropped only if the participating tables have been dropped first, otherwise you'll get:

ORA-00951: cluster not empty

Alternatively, if you are absolutely sure, you can add the INCLUDING TABLES clause:

drop CLUSTER dummy_cluster INCLUDING TABLES
/

If there are any tables whose constraints point to the cluster table and are preventing you from dropping the cluster and the tables, you can also add the CASCADE CONSTRAINTS clause to the statement:

drop CLUSTER dummy_cluster INCLUDING TABLES CASCADE CONSTRAINTS
/

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