SEGMENT vs EXTENT vs BLOCK
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Thread: SEGMENT vs EXTENT vs BLOCK

  1. #1
    Join Date
    Feb 2001
    Posts
    66
    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.

  2. #2
    Join Date
    Nov 2000
    Location
    Israel
    Posts
    268

    Talking

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

  3. #3
    Join Date
    Aug 2000
    Posts
    462
    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.

  4. #4
    Join Date
    Feb 2001
    Posts
    66
    Thanks a lot guys,

  5. #5
    Join Date
    Feb 2001
    Posts
    66
    So,
    there are only one segment for an object.
    That means than an segment can be spread on multiple files?

  6. #6
    Join Date
    Dec 2000
    Location
    Ljubljana, Slovenia
    Posts
    4,439
    [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.
    Jurij Modic
    ASCII a stupid question, get a stupid ANSI
    24 hours in a day .... 24 beer in a case .... coincidence?

  7. #7
    Join Date
    Aug 2000
    Posts
    462
    Good point J.

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

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