Calculations: Update First * and Last * reducers to exclude NaNs (#77323)

docs/sources/panels-visualizations/calculation-types/index.md

@@ -29,9 +29,9 @@ The following table contains a list of calculations you can perform in Grafana.
 | Difference percent | Percentage change between first and last value of a field |
 | Distinct count     | Number of unique values in a field                        |
 | First              | First value in a field                                    |
-| First\* (not null) | First, not null value in a field                          |
+| First\* (not null) | First, not null value in a field (also excludes NaNs)     |
 | Last               | Last value in a field                                     |
-| Last\* (not null)  | Last, not null value in a field                           |
+| Last\* (not null)  | Last, not null value in a field (also excludes NaNs)      |
 | Max                | Maximum value of a field                                  |
 | Mean               | Mean value of all values in a field                       |
 | Variance           | Variance of all values in a field                         |

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

@@ -54,7 +54,7 @@ describe('Stats Calculators', () => {
   it('should calculate basic stats', () => {
     const stats = reduceField({
       field: basicTable.fields[0],
-      reducers: ['first', 'last', 'mean', 'count'],
+      reducers: [ReducerID.first, ReducerID.last, ReducerID.mean, ReducerID.count],
     });
 
     expect(stats.first).toEqual(10);
@@ -67,7 +67,7 @@ describe('Stats Calculators', () => {
     basicTable.fields[0].state = undefined; // clear the cache
     const stats = reduceField({
       field: basicTable.fields[0],
-      reducers: ['first'],
+      reducers: [ReducerID.first],
     });
 
     // Should do the simple version that just looks up value
@@ -172,6 +172,58 @@ describe('Stats Calculators', () => {
     }
   });
 
+  it('consistent results for first/last value with NaN', () => {
+    const info = [
+      {
+        data: [NaN, 200, NaN], // first/last value is NaN
+        result: 200,
+      },
+      {
+        data: [NaN, NaN, NaN], // All NaN
+        result: null,
+      },
+      {
+        data: [undefined, undefined, undefined], // Empty row
+        result: null,
+      },
+    ];
+
+    const stats = reduceField({
+      field: createField('x', info[0].data),
+      reducers: [ReducerID.first, ReducerID.last, ReducerID.firstNotNull, ReducerID.lastNotNull, ReducerID.diffperc],
+    });
+
+    expect(stats[ReducerID.first]).toEqual(NaN);
+    expect(stats[ReducerID.last]).toEqual(NaN);
+    expect(stats[ReducerID.firstNotNull]).toEqual(200);
+    expect(stats[ReducerID.lastNotNull]).toEqual(200);
+    expect(stats[ReducerID.diffperc]).toEqual(0);
+
+    const reducers = [ReducerID.lastNotNull, ReducerID.firstNotNull];
+    for (const input of info) {
+      for (const reducer of reducers) {
+        const v1 = reduceField({
+          field: createField('x', input.data),
+          reducers: [reducer, ReducerID.mean], // uses standard path
+        })[reducer];
+
+        const v2 = reduceField({
+          field: createField('x', input.data),
+          reducers: [reducer], // uses optimized path
+        })[reducer];
+
+        if (v1 !== v2 || v1 !== input.result) {
+          const msg =
+            `Invalid ${reducer} result for: ` +
+            input.data.join(', ') +
+            ` Expected: ${input.result}` + // configured
+            ` Received: Multiple: ${v1}, Single: ${v2}`;
+          expect(msg).toEqual(null);
+        }
+      }
+    }
+  });
+
   it('count should ignoreNulls by default', () => {
     const someNulls = createField('x', [1, null, null, 1]);
     expect(reduce(someNulls, ReducerID.count)).toEqual(2);

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

@@ -137,7 +137,7 @@ export const fieldReducers = new Registry<FieldReducerInfo>(() => [
   {
     id: ReducerID.lastNotNull,
     name: 'Last *',
-    description: 'Last non-null value',
+    description: 'Last non-null value (also excludes NaNs)',
     standard: true,
     aliasIds: ['current'],
     reduce: calculateLastNotNull,
@@ -152,7 +152,7 @@ export const fieldReducers = new Registry<FieldReducerInfo>(() => [
   {
     id: ReducerID.firstNotNull,
     name: 'First *',
-    description: 'First non-null value',
+    description: 'First non-null value (also excludes NaNs)',
     standard: true,
     reduce: calculateFirstNotNull,
   },
@@ -320,8 +320,8 @@ export function doStandardCalcs(field: Field, ignoreNulls: boolean, nullAsZero:
 
     calcs.count++;
 
-    if (currentValue != null) {
-      // null || undefined
+    if (currentValue != null && !Number.isNaN(currentValue)) {
+      // null || undefined || NaN
       const isFirst = calcs.firstNotNull === null;
       if (isFirst) {
         calcs.firstNotNull = currentValue;
@@ -418,7 +418,7 @@ function calculateFirstNotNull(field: Field, ignoreNulls: boolean, nullAsZero: b
   const data = field.values;
   for (let idx = 0; idx < data.length; idx++) {
     const v = data[idx];
-    if (v != null && v !== undefined) {
+    if (v != null && !Number.isNaN(v)) {
       return { firstNotNull: v };
     }
   }
@@ -435,7 +435,7 @@ function calculateLastNotNull(field: Field, ignoreNulls: boolean, nullAsZero: bo
   let idx = data.length - 1;
   while (idx >= 0) {
     const v = data[idx--];
-    if (v != null && v !== undefined) {
+    if (v != null && !Number.isNaN(v)) {
       return { lastNotNull: v };
     }
   }