Data blocks, extents, and segments

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Introduction to data blocks, extents, and segments

Oracle allocates logical database space for all data in a database. The units of database space allocation are data blocks, extents, and segments. Figure 2-1 shows the relationships among these data structures:

Figure 2-1 the relationships among segments, extents, and data blocks

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At the finest level of granularity, Oracle stores data inData blocks(Also calledLogical Blocks,Oracle Blocks, OrPages). One data block corresponds to a specific number of bytes of physical database space on disk.

The next level of logical database space isExtent. An extent is a specific number of contiguous data blocks allocated for storing a specific type of information.

The level of logical database storage above an extent is calledSegment. A segment is a set of extents, each of which has been allocated for a specific data structure and all of which are stored in the same tablespace. for example, each table's data is stored in its ownData Segment, While each index's data is stored in its ownIndex segment. If the table or index is partitioned, each partition is stored in its own segment.

Oracle allocates space for segments in units of one extent. when the existing extents of a segment are full, Oracle allocates another extent for that segment. because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk.

A segment and all its extents are stored in one tablespace. within a tablespace, a segment can include extents from more than one file; that is, the segment can span datafiles. however, each extent can contain data from only one datafile.

Although you can allocate additional extents, the blocks themselves are allocated separately. if you allocate an extent to a specific instance, the blocks are immediately allocated to the Free List. however, if the extent is not allocated to a specific instance, then the blocks themselves are allocated only when the high water mark moves. theHigh water markIs the boundary between used and unused space in a segment.