SEGMENT vs EXTENT vs BLOCK

[m_d_marian]

Hi all,

It's not clear in my mind, the difference between a segment and a extent. Somebody can give a definition of an SEGMENT versus an EXTENT versus an BLOCK?

Thanks.

[rotem_fo]

Hi,
read the article at:
[url]http://www.alise.lv/alise/technolog.nsf/62c9b2530fea3cb8c22566ef0041c1e3/cfef6b0993adbf4a42256982004eb44f/$FILE/defrag.pdf[/url]

[kmesser]

Segments are the physical representation of the logical database objects. A segment is thus a subunit of a tablespace, and will be contained in a datafile. extents are subunits of a segment, and are defined as contiguous sets of Oracle blocks. Successive extents are not guaranteed to be contiguous, but within each extent the Oracle blocks are contiguous. A block is the unit of data that can be read or written to an Oracle data file, and is managed using PCT_FREE and PCT_USED, among others.

[m_d_marian]

Thanks a lot guys,

[m_d_marian]

So,
there are only one segment for an object.
That means than an segment can be spread on multiple files?

[jmodic]

[QUOTE][i]Originally posted by m_d_marian [/i]
[B]So,
there are only one segment for an object.
[/B][/QUOTE]
Not necessarily so. A table (object) can have partitions (subobjects), and partitions are segments. So with partitioned tables and indexes you have more than one segment for table/index.

[QUOTE]
[B]
That means than an segment can be spread on multiple files? [/B][/QUOTE]
Yes.

[kmesser]

Good point J.

I went for the simple solution and ignored an important fact for larger/more complex systems.