Database Logical Architecture
Database objects such as table, index are stored as segments in tablespaces. Oracle uses the term “segment” to describe any structure that contains data. Typical segment is a table, containing rows of data, but there are more than a dozen possible segment types in an Oracle database. Of particular interest are table segments, index segments, and undo segments. Tables contain rows of information; that indexes are a mechanism for giving fast access to any particular row; and that undo segments are data structures used for storing information that might be needed to reverse, or roll back, any transactions that you do not wish to make permanent.
> System administrators see physical datafiles; programmers see logical segments. Oracle abstracts the logical storage from the physical storage by means of the tablespace.
> A tablespace is logically a collection of one or more segments, and physically a collection of one or more datafiles.
> One tablespace can belong to only one database at a time.
> One tablespace consist of one or more data files.
> Tablespaces are further divided into logical units of storage called Segments.
> Segments exist within a tablespace.
> Segments are made of a collection of extents.
> Extents are a collection of data blocks.
> Data blocks are mapped to OS blocks.
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