Jun 22, 2023
Tablespaces- Files
As noted earlier, a tablespace is a container—it holds segments. Each segment belongs to exactly one tablespace. A tablespace may have many segments within it. All of the extents for a given segment will be found in the tablespace associated with that segment. Segments never cross tablespace boundaries.
A tablespace itself has one or more datafiles associated with it. An extent for any given segment in a tablespace will be contained entirely within one datafile. However, a segment may have extents from many different datafiles.
Graphically, a tablespace might look like Figure 3-3.
Figure 3-3. A tablespace containing two datafiles, three segments, and four extents
Figure 3-3 shows a tablespace named USER_DATA. It consists of two datafiles, user_data01.dbf and user_data02.dbf. It has three segments allocated to it: T1, T2, and I1 (probably two tables and an index).
The tablespace has four extents allocated in it, and each extent is depicted as a logically contiguous set of database blocks. Segment T1 consists of two extents, one extent in each file. Segments T2 and I1 each have one extent depicted. If we need more space in this tablespace, we could either resize the datafiles already allocated to the tablespace or we could add a third datafile to it.
A tablespace is a logical storage container in Oracle. As developers, we will create segments in tablespaces. We will never get down to the raw file level—we don’t specify that we want our extents to be allocated in a specific file (we can, but in general we don’t). Rather, we create objects in tablespaces and Oracle takes care of the rest. If at some point in the future, the DBA decides to move our datafiles around on disk to more evenly distribute I/O, that is OK with us. It will not affect our processing at all.
Storage Hierarchy Summary
In summary, the hierarchy of storage in Oracle is as follows:
\ 1.\ A database is made up of one or more tablespaces.
\ 2.\ A tablespace is made up of one or more datafiles. These files might be cooked files in a file system, raw partitions, ASM-managed database files, or a file on a clustered file system. A tablespace contains segments.
\ 3.\ A segment (TABLE, INDEX, and so on) is made up of one or more extents. A segment exists in a tablespace, but may have data in many datafiles within that tablespace.
\ 4.\ An extent is a logically contiguous set of blocks on disk. An extent is in a single tablespace and, furthermore, is always in a single file within that tablespace.
\ 5.\ A block is the smallest unit of allocation in the database. A block is the smallest unit of I/O used by a database on datafiles.
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