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The SQL Server documentation for sp_updatestats states that:

If 'resample' is not specified, sp_updatestats updates statistics by using the default sampling.

How exactly does SQL Server calculate the default sample size?

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up vote 1 down vote accepted

That's a good question. If you're interested in statistics, you might want to give Elisabeth Redei's excellent blog post a read:

Lies, Damned Lies and Statistics

It explains that, as the table size increases, the default sampling rate becomes insufficient to accurately represent the table's contents. It also provides a formula that she states is used in calculating the default sample set size (Rowmodctr 500 + 20% of total rows).

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that's wrong, is the formula to determine if a statistics is out dated and then fires un update. the default sample size must be calculated with some other formula that I'm searching too. – user208693 Feb 9 '14 at 12:05

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