The APPROX_COUNT_DISTINCT function counts the approximate number of unique items in a field.




  • value - a field or expression that contains the items to be counted.

How the APPROX_COUNT_DISTINCT function works

The APPROX_COUNT_DISTINCT function takes 1 parameter, which can be the name of a metric, dimension, or expression of any type. APPROX_COUNT_DISTINCT returns the approximate number of unique items in that field or expression.

APPROX_COUNT_DISTINCT is more efficient in terms of query processing than COUNT_DISTINCT, but returns less exact results. If your data set is very large, or if the performance of your report is more important than exact counts, consider using APPROX_COUNT_DISTINCT. Using APPROX_COUNT_DISTINCT instead of COUNT_DISTINCT can also help reduce query costs when using BigQuery data sources.

For an in-depth explanation of how approximate aggregation works, see the BigQuery documentation.


APPROX_COUNT_DISTINCT(Page) - counts the approximate number of unique values in the Page dimension.


  • The APPROX_COUNT_DISTINCT function is only available when used with BigQuery and Cloud Spanner data sources.
  • For data sources which do not support APPROX_COUNT_DISTINCT, APPROX_COUNT_DISTINCT will act like COUNT_DISTINCT.
  • You can't apply this function to a pre-aggregated metric (Aggregation type of Auto), or to an expression which is the result of another aggregation function. For example, a formula such as APPROX_COUNT_DISTINCT(Sessions)in a Google Analytics data source will produce an error.
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