diff --git a/extensions/functions_aggregate_approx.yaml b/extensions/functions_aggregate_approx.yaml
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+%YAML 1.2
+---
+aggregate_functions:
+  - name: "approx_count_distinct"
+    description:  >-
+      Calculates the approximate number of rows that contain distinct values of the expression argument using
+      HyperLogLog. This function provides an alternative to the COUNT (DISTINCT expression) function, which
+      returns the exact number of rows that contain distinct values of an expression. APPROX_COUNT_DISTINCT
+      processes large amounts of data significantly faster than COUNT, with negligible deviation from the exact
+      result.
+    impls:
+      - args:
+          - value: any
+        nullability: DECLARED_OUTPUT
+        decomposable: MANY
+        intermediate: i64
+        return: i64