Concepts and Architecture - What is an Extent ?

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What is an Extent ?

An Extent is a specific number of contiguous data blocks, obtained in a single allocation, and used to store a specific type of information.

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Last Update: January 10, 2007

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Submitted by: gaurav_varma

An extent is a logical unit of database storage space allocation made up of a number of contiguous data blocks. One or more extents in turn make up a segment. When the existing space in a segment is completely used, Oracle allocates a new extent for the segment.

When you create a table, Oracle allocates to the table's data segment an initial extent of a specified number of data blocks. Although no rows have been inserted yet, the Oracle data blocks that correspond to the initial extent are reserved for that table's rows.

If the data blocks of a segment's initial extent become full and more space is required to hold new data, Oracle automatically allocates an incremental extent for that segment. An incremental extent is a subsequent extent of the same or greater size than the previously allocated extent in that segment. (The next section explains the factors controlling the size of incremental extents.)

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

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rnraomaddula

January 10, 2007 00:54:19

gaurav_varma

Member Since: January 2007 Contribution: 3

RE: What is an Extent ?

When you create a table Oracle allocates to the table's data segment an initial extent of a specified number of data blocks. Although no rows have been inserted yet the Oracle data blocks that correspond to the initial extent are reserved for that table's rows.

If the data blocks of a segment's initial extent become full and more space is required to hold new data Oracle automatically allocates an incremental extent for that segment. An incremental extent is a subsequent extent of the same or greater size than the previously allocated extent in that segment. (The next section explains the factors controlling the size of incremental extents.)

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

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