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SSC-Enthusiastic
      
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| that means that you have a lot "bad" de dates in table. in reality difference probably will be more significant
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Ten Centuries
      
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No, actually, only 10,000 PersonIds in my test table of 5 million rows (1 million unique PersonIDs) have records that are out of order; that's about 1% of the ids. That also explains the nice lift, because the subselect has far fewer rows for the distinct to operate on.
Note: the subselect finds roughly 20,000 rows, distinct return 10,000 rows.
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