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Asked: 2026-05-24T23:30:58+00:00

Possible Duplicate: How does SQL server work out the estimated number of rows? I

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Possible Duplicate:
How does SQL server work out the estimated number of rows?

I was just looking through SQL Server Graphical Execution plan and I encountered the following two information:

  1. actual number of rows-2385
  2. estimated number of rows-180

I was wondering why this difference was coming. Can you explain me what is the difference between them. It will be helpful if you can explain with some example.

Thanks.

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1 Answer

  1. Editorial Team

    Row estimates are based on statistics. Inaccurate estimates can come from a number of factors:

    • Low cardinality in your data or stats
    • Out of date or incomplete stats
    • Inefficient execution plan
    • Concurrency issues (changes in data from creation of execution plan to execution time)

    As a rule, don’t worry about it until you have issues.

    I my experience, it’s most often based on the cardinality in your stats.

    If you are selecting based on two fields, both of which have indexes, the row estimate will be based on a product of the likelihood of the values in the respective indexes, times the total number of rows.

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