Physical and Logical Ordering of an index

  • Chandhini

    Ten Centuries

    Points: 1133

    Hi,

    Fragmentation will occur if physical and logical order are different..

    but there is a confusion, what exactly are we referring physical order and logical order?

    My understand of them,

    Physical Order : Contiguous pages

  • Chandhini

    Ten Centuries

    Points: 1133

    Hi,

    Fragmentation will occur if physical and logical order are different..

    but there is a confusion, what exactly are we referring physical order and logical order?

    My understand of them,

    Physical Order : Contiguous pages

    and logical order : data pages are linked like a linked list. and references would be there for physical order.

    Is that correct. please guide me if i m wrong.

    Regards,

    Ramani

  • Grant Fritchey

    SSC Guru

    Points: 395157

    Logical order is the ordering of the index. Let’s assume an index based on last name. The logical order would be:

    Adams

    Johnson

    Smith

    Williams

    But, due to page splits, insertions, deletes and the other things that can affect the order in which pages or stored, the physical order, meaning the order of the actual storage, may look like this:

    Adams

    Wiliam

    Johnson

    Smith

    Defragmenting will move the order of the physical storage closer to that of the logical definition. It may not be exact and it’s unlikely to remain so as the data changes.

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  • jsb12

    SSC-Addicted

    Points: 468

    I’m getting it now. I had doubts regarding this.

    But still, one more clarification – when it is said that when clustered index is traversed to the leaf level, the data is retrieved not just pointed to. Does this retrieving have any bearing on the physical storage, or how data is physically stored? what implications does this have in terms of logical order vs physical order?

  • Matt Miller (4)

    SSC Guru

    Points: 124130

    When there IS a clustered index, the physical data is stored in the clustered index. So the leaf level of a CLUSTERED index = the actual record. Any non-clustered index points to the clustered index for its data, so given your description, in a clustered index the data is retrieved as soon as you access the leaf node, instead of the extra hop (called a “bookmark lookup”) you end up with in a non-clustered index.

    Keep in mind that even the pages themselves are logical constructs. Granted – they’re a fixed size, but they exist within physical files which grow as needed, so you could have multiple kinds of “physical fragementation”. For example, with files that grow with “auto-growth on”, the files themselves will be fragmented on the OS layer; the pages can physically exist anywhere within those OS files, then getting logically assigned to the index based on the usage and maintenance patterns on the table.

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  • Gail Shaw

    SSC Guru

    Points: 1004424

    This series on indexing (and especially the comments on the second one) may be worth reading

    http://www.sqlservercentral.com/articles/Indexing/68439/

    Gail Shaw
    Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
    SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

    We walk in the dark places no others will enter
    We stand on the bridge and no one may pass

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