What is a data segment?

What is a data segment?

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A data segment is a set of extents allocated for a table. A single data segment holds all data of a non-partitioned or non-clustered table, a partition of a partitioned table, or a cluster of tables. A data segment is created for a table as soon as the table is created. A data segment can belong to only one tablespace. However, it can be spread across multiple data files belonging to the tablespace, as extents allocated to it can belong to multiple data files.

When creating a table, the name of the tablespace where the data segment will be created can be specified. However, the name of the data file where the data segment will be created cannot be specified. Oracle automatically chooses the data file. This is because a data file is a physical structure and can be managed only by Oracle. However, a tablespace, being a logical structure, can be managed by database users.

For maintenance purposes, the header block of each data segment contains a directory of all extents in that segment.

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