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Sampled Statistics Question

  • August 15, 2019 at 10:21 am

    #3671504

    Hello all

    I'm researching how the Query Optimizer uses statistics to come up with row count estimates, and have found that the histogram value for AVG_RANGE_ROWS, returned by DBCC SHOW_STATISTICS, is usually described as being calculated from RANGE_ROWS divided by DISTINCT_RANGE_ROWS.

    While this appears to hold true for FULLSCAN statistics, there are slight discrepancies with sampled statistics whereby the AVG_RANGE_ROWS value differs by up to +/- 0.5% from the calculated value.

    For example, in this histogram snippet...

    p

    ... the calculated value for step 2 is (16963.41 / 164) = 103.4354, not the stored AVG_RANGE_ROWS value of 103.477.

    I'm sure these admittedly tiny discrepancies have been noted before.  Have they ever been explained?

  • August 15, 2019 at 3:01 pm

    #3671584

    I don't know of any official reasons but my take on it is... we are talking about the company that can't display a match between the listed MB allocations in the files sizes in database properties and the SSRS space used reports. 😀

    --Jeff Moden

    RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
    First step towards the paradigm shift of writing Set Based code:
    ________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

    Change is inevitable... Change for the better is not.

    Helpful Links:
    How to post code problems
    How to Post Performance Problems
    Create a Tally Function (fnTally)

  • August 19, 2019 at 9:27 am

    #3672217

    Thanks Jeff, you may be right!

    Since those AVG_RANGE_ROWS values are stored to four decimal places of accuracy, you'd hope it might be possible to divine the computations that led to them.

    But I guess STATMAN moves in mysterious ways.

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