feat: implement stdvar_over_time function (GreptimeTeam#1291)

* feat: implement stdvar_over_time function

* feat: add more test for stdvar_over_time

* feat: add stdvar_over_time to functions.rs

src/promql/src/functions.rs

@@ -24,7 +24,7 @@ mod test_util;
 
 pub use aggr_over_time::{
     AbsentOverTime, AvgOverTime, CountOverTime, LastOverTime, MaxOverTime, MinOverTime,
-    PresentOverTime, StddevOverTime, SumOverTime,
+    PresentOverTime, StddevOverTime, StdvarOverTime, SumOverTime,
 };
 use datafusion::arrow::array::ArrayRef;
 use datafusion::error::DataFusionError;

src/promql/src/functions/aggr_over_time.rs

@@ -117,7 +117,36 @@ pub fn present_over_time(_: &TimestampMillisecondArray, values: &Float64Array) -
     }
 }
 
-// TODO(ruihang): support quantile_over_time, and stdvar_over_time
+/// the population standard variance of the values in the specified interval.
+/// DataFusion's implementation:
+/// https://github.com/apache/arrow-datafusion/blob/292eb954fc0bad3a1febc597233ba26cb60bda3e/datafusion/physical-expr/src/aggregate/variance.rs#L224-#L241
+#[range_fn(
+    name = "StdvarOverTime",
+    ret = "Float64Array",
+    display_name = "prom_stdvar_over_time"
+)]
+pub fn stdvar_over_time(_: &TimestampMillisecondArray, values: &Float64Array) -> Option<f64> {
+    if values.is_empty() {
+        None
+    } else {
+        let mut count = 0;
+        let mut mean: f64 = 0.0;
+        let mut result: f64 = 0.0;
+        for value in values {
+            let value = value.unwrap();
+            let new_count = count + 1;
+            let delta1 = value - mean;
+            let new_mean = delta1 / new_count as f64 + mean;
+            let delta2 = value - new_mean;
+            let new_result = result + delta1 * delta2;
+
+            count += 1;
+            mean = new_mean;
+            result = new_result;
+        }
+        Some(result / count as f64)
+    }
+}
 
 /// the population standard deviation of the values in the specified interval.
 /// Prometheus's implementation: https://github.com/prometheus/prometheus/blob/f55ab2217984770aa1eecd0f2d5f54580029b1c0/promql/functions.go#L556-L569
@@ -154,6 +183,8 @@ pub fn stddev_over_time(_: &TimestampMillisecondArray, values: &Float64Array) ->
     }
 }
 
+// TODO(ruihang): support quantile_over_time
+
 #[cfg(test)]
 mod test {
     use super::*;
@@ -368,6 +399,52 @@ mod test {
         );
     }
 
+    #[test]
+    fn calculate_stdvar_over_time() {
+        let (ts_array, value_array) = build_test_range_arrays();
+        simple_range_udf_runner(
+            StdvarOverTime::scalar_udf(),
+            ts_array,
+            value_array,
+            vec![
+                Some(1417.8479276253622),
+                Some(808.999919713209),
+                Some(0.0),
+                None,
+                None,
+                Some(328.3638826418587),
+                Some(143.5964181766362),
+                Some(130.91830542386285),
+                Some(0.0),
+                None,
+            ],
+        );
+
+        // add more assertions
+        let ts_array = Arc::new(TimestampMillisecondArray::from_iter(
+            [1000i64, 3000, 5000, 7000, 9000, 11000, 13000, 15000]
+                .into_iter()
+                .map(Some),
+        ));
+        let values_array = Arc::new(Float64Array::from_iter([
+            1.5990505637277868,
+            1.5990505637277868,
+            1.5990505637277868,
+            0.0,
+            8.0,
+            8.0,
+            2.0,
+            3.0,
+        ]));
+        let ranges = [(0, 3), (3, 5)];
+        simple_range_udf_runner(
+            StdvarOverTime::scalar_udf(),
+            RangeArray::from_ranges(ts_array, ranges).unwrap(),
+            RangeArray::from_ranges(values_array, ranges).unwrap(),
+            vec![Some(0.0), Some(10.559999999999999)],
+        );
+    }
+
     #[test]
     fn calculate_std_dev_over_time() {
         let (ts_array, value_array) = build_test_range_arrays();