APPROX_COUNT_DISTINCT Function in Oracle Database 11g and 12c

This video demonstrates how to quickly improve the performance of calculating the number of distinct values using the APPROX_COUNT_DISTINCT function. For more information see:

Cameo by Kris Rice

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[SunilKumarNoothi]

By name, I understood that, the value what we are getting is Approximate Value, but what is the reason for difference in the value of APPROX_COUNT_DISTINCT and COUNT(DISTINCT..) ??? Any idea. This will helpful to know when we can use the function and when cannot.