Transformations: Add standard deviation and variance reducers (#52769)

Co-authored-by: Selva <selvavm@hotmail.com>
Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>

docs/sources/panels/calculation-types.md

@@ -10,7 +10,7 @@ weight: 1100
 
 # Reference: Calculation types
 
-Refer to the following list of calculations you can perform in Grafana. You can find these calculations in the **Transform** tab and in the bar gauge, gauge, and stat visualizations.
+The following table contains a list of calculations you can perform in Grafana. You can find these calculations in the **Transform** tab and in the bar gauge, gauge, and stat visualizations.
 
 | Calculation        | Description                                               |
 | :----------------- | :-------------------------------------------------------- |
@@ -25,6 +25,8 @@ Refer to the following list of calculations you can perform in Grafana. You can
 | First (not null)   | First, not null value in a field                          |
 | Max                | Maximum value of a field                                  |
 | Mean               | Mean value of all values in a field                       |
+| Variance           | Variance of all values in a field                         |
+| StdDev             | Standard deviation of all values in a field               |
 | Min                | Minimum value of a field                                  |
 | Min (above zero)   | Minimum, positive value of a field                        |
 | Range              | Difference between maximum and minimum values of a field  |

packages/grafana-data/src/transformations/fieldReducer.test.ts

@@ -80,11 +80,13 @@ describe('Stats Calculators', () => {
   it('should get non standard stats', () => {
     const stats = reduceField({
       field: basicTable.fields[0],
-      reducers: [ReducerID.distinctCount, ReducerID.changeCount],
+      reducers: [ReducerID.distinctCount, ReducerID.changeCount, ReducerID.variance, ReducerID.stdDev],
     });
 
     expect(stats.distinctCount).toEqual(2);
     expect(stats.changeCount).toEqual(1);
+    expect(stats.variance).toEqual(25);
+    expect(stats.stdDev).toEqual(5);
   });
 
   it('should calculate step', () => {

packages/grafana-data/src/transformations/fieldReducer.ts

@@ -10,6 +10,8 @@ export enum ReducerID {
   min = 'min',
   logmin = 'logmin',
   mean = 'mean',
+  variance = 'variance',
+  stdDev = 'stdDev',
   last = 'last',
   first = 'first',
   count = 'count',
@@ -152,6 +154,20 @@ export const fieldReducers = new Registry<FieldReducerInfo>(() => [
   { id: ReducerID.min, name: 'Min', description: 'Minimum Value', standard: true },
   { id: ReducerID.max, name: 'Max', description: 'Maximum Value', standard: true },
   { id: ReducerID.mean, name: 'Mean', description: 'Average Value', standard: true, aliasIds: ['avg'] },
+  {
+    id: ReducerID.variance,
+    name: 'Variance',
+    description: 'Variance of all values in a field',
+    standard: false,
+    reduce: calculateStdDev,
+  },
+  {
+    id: ReducerID.stdDev,
+    name: 'StdDev',
+    description: 'Standard deviation of all values in a field',
+    standard: false,
+    reduce: calculateStdDev,
+  },
   {
     id: ReducerID.sum,
     name: 'Total',
@@ -419,6 +435,33 @@ function calculateLastNotNull(field: Field, ignoreNulls: boolean, nullAsZero: bo
   return { lastNotNull: null };
 }
 
+/** Calculates standard deviation and variance */
+function calculateStdDev(field: Field, ignoreNulls: boolean, nullAsZero: boolean): FieldCalcs {
+  // Only support number fields
+  if (!(field.type === FieldType.number || field.type === FieldType.time)) {
+    return { variance: 0, stdDev: 0 };
+  }
+
+  let squareSum = 0;
+  let runningMean = 0;
+  let runningNonNullCount = 0;
+  const data = field.values;
+  for (let i = 0; i < data.length; i++) {
+    const currentValue = data.get(i);
+    if (currentValue != null) {
+      runningNonNullCount++;
+      let _oldMean = runningMean;
+      runningMean += (currentValue - _oldMean) / runningNonNullCount;
+      squareSum += (currentValue - _oldMean) * (currentValue - runningMean);
+    }
+  }
+  if (runningNonNullCount > 0) {
+    const variance = squareSum / runningNonNullCount;
+    return { variance, stdDev: Math.sqrt(variance) };
+  }
+  return { variance: 0, stdDev: 0 };
+}
+
 function calculateChangeCount(field: Field, ignoreNulls: boolean, nullAsZero: boolean): FieldCalcs {
   const data = field.values;
   let count = 0;