With the release of the Oracle Database 184.108.40.206 there was a number of new features and options. Most of the publicity has been around the in-Memory option. But there was lots of other features for the DBA and a few for the developer.
One of the new SQL functions is the APPROX_COUNT_DISTINCT(). This function is different to the tradition count distinct, COUNT(DISTINCT expression), in that is performs an approximate count distinct. The theory is that this approximate count is a lot more efficient than performing the full count distinct.
The APPROX_COUNT_DISTINCT() function is really only suitable when you are processing very large volumes of data and when the data set contains a large number of distinct values.
The general syntax of the function is:
... APPROX_COUNT_DISTINCT(expression) ...
and returns a Number.
The function returns the approximate number of records that contain distinct value for the expression.
The APPROX_COUNT_DISTINCT() function ignores records that contain a null value for the expression. Plus is performs less work on the sorting and aggregations. Just run and Explain Plan and you can see the differences.
In some of the material from Oracle the APPROX_COUNT_DISTINCT() function can be 5x to 50x++ times faster. But it depends on the number of distinct values and the complexity of the SQL query.
As the result / returned value from the function may not be 100% accurate, Oracle says that the functions has an accuracy of >97% (with 95% confidence).
The function cannot be used on the following data types: BFILE, BLOB, CLOB, LONG, LONG RAW and NCLOB