BigQuery で COUNT(DISTINCT(X)) を使わない方が良い理由

BigQuery で COUNT(DISTINCT(X)) を使わない方が良い理由:

Returns the total number of non-NULL values in the scope of the function. If you use the DISTINCT keyword, the function returns the number of distinct values for the specified field. Note that the returned value for DISTINCT is a statistical approximation and is not guaranteed to be exact.
October 19, 2014 at 08:44AM

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