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  1. #1
    Join Date
    Sep 2008
    Posts
    5

    Unanswered: what is an extent

    hi

    I am new to db2 and i want to know

    what is an extent, Extent size and Prefetch size in db2

    Thanks in advance

  2. #2
    Join Date
    Jan 2003
    Posts
    4,292
    Provided Answers: 5
    Straight from the Glossary in the DB2 documentation:

    extent
    An allocation of space, within a container of a table space, to a single database object. This allocation consists of multiple pages.

    prefetch
    To read data before, and in anticipation of, its use.

    I will let you figure whit the sizes mean.

    Andy

  3. #3
    Join Date
    May 2003
    Location
    USA
    Posts
    5,737
    Basically, an extent is how much data DB2 writes or reads to disk at one time.

    If prefetching is being used for a particular query (DB2 decides whether to use this based on how much data is needed from disk for that query) then multiple extents are requested at the same time, even before they are actually needed by DB2 (in anticipation that they will be needed very shortly).

    An example of prefetch would be if a query is doing a table scan on a large table, DB2 knows that it will eventually need all the pages in memory, so it puts them into memory before the page is actually requested. All DB2 table and index I/O (except for LOB's) are done into or out of memory (bufferpools).

    I would recommend that you look in the PDF versions of the manuals for more explanation. They are free to download.
    M. A. Feldman
    IBM Certified DBA on DB2 for Linux, UNIX, and Windows
    IBM Certified DBA on DB2 for z/OS and OS/390

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