Introduction to Data Blocks, Extents, and Segments(一)

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Introduction to Data Blocks, Extents, and Segments

1. Oracle stores data in data blocks (also called logical blocks, Oracle blocks, or pages)

2. An extent is a specific number of contiguous data blocks allocated for storing a specific type of information.

3. A segment is a set of extents,are stored in the same tablespace.each table's data is stored in its own data segment, while each index's data is stored in its own index segment. If the table or index is partitioned, each partition is stored in its own segment.

4. 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.the extents of a segment may or may not be contiguous on disk.

5. 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.

6. 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. The high water mark is the boundary between used and unused space in a segment.

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