add test for accumulator merge_digests

datafusion/physical-expr/src/aggregate/approx_percentile_cont.rs

@@ -441,3 +441,41 @@ impl Accumulator for ApproxPercentileAccumulator {
             - std::mem::size_of_val(&self.return_type)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::aggregate::approx_percentile_cont::ApproxPercentileAccumulator;
+    use crate::aggregate::tdigest::TDigest;
+    use arrow_schema::DataType;
+    use datafusion_common::ScalarValue;
+    use datafusion_expr::Accumulator;
+
+    #[test]
+    fn test_combine_approx_percentile_accumulator() {
+        let mut digests: Vec<TDigest> = Vec::new();
+
+        // one TDigest with 50_000 values from 1 to 1_000
+        for _ in 1..=50 {
+            let t = TDigest::new(100);
+            let values: Vec<_> = (1..=1_000).map(f64::from).collect();
+            let t = t.merge_unsorted_f64(values);
+            digests.push(t)
+        }
+
+        let t1 = TDigest::merge_digests(&digests);
+        let t2 = TDigest::merge_digests(&digests);
+
+        let mut accumulator =
+            ApproxPercentileAccumulator::new_with_max_size(0.5, DataType::Float64, 100);
+
+        accumulator.merge_digests(&[t1]);
+        assert_eq!(accumulator.digest.count(), 50_000.0);
+        accumulator.merge_digests(&[t2]);
+        assert_eq!(accumulator.digest.count(), 100_000.0);
+
+        assert_eq!(
+            accumulator.evaluate().unwrap(),
+            ScalarValue::Float64(Some(500.0))
+        );
+    }
+}