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The figure shows how an IOT (index-organized table) storage table for a nested table works when the COMPRESS clause is used. The storage table has two columns:
The NESTED_TABLE_ID column identifies the rows in the parent table to which the nested table's rows belong. In an IOT storage table, the parent row IDs are grouped together instead of being dispersed throughout the storage table. When, additionally, the COMPRESS clause is used, only one parent row ID is stored for the entire group of rows that have that ID. This saves space.