Merge pull request #2354 from neonstalwart/stddev

make stddev work

cmd/influxd/server_integration_test.go

@@ -581,14 +581,41 @@ func runTestsData(t *testing.T, testName string, nodes Cluster, database, retent
 		// Aggregations
 		{
 			reset: true,
-			name:  "large mean",
+			name:  "stddev with just one point",
+			write: `{"database" : "%DB%", "retentionPolicy" : "%RP%", "points": [
+				{"name": "cpu", "timestamp": "2015-04-20T14:27:41Z", "fields": {"value": 45}}
+			]}`,
+			query:    `SELECT stddev(value) FROM cpu`,
+			queryDb:  "%DB%",
+			expected: `{"results":[{"series":[{"name":"cpu","columns":["time","stddev"],"values":[["1970-01-01T00:00:00Z",null]]}]}]}`,
+		},
+		{
+			reset: true,
+			name:  "large mean and stddev",
 			write: `{"database" : "%DB%", "retentionPolicy" : "%RP%", "points": [
 				{"name": "cpu", "timestamp": "2015-04-20T14:27:40Z", "fields": {"value": ` + string(maxFloat64) + `}},
 				{"name": "cpu", "timestamp": "2015-04-20T14:27:41Z", "fields": {"value": ` + string(maxFloat64) + `}}
 			]}`,
-			query:    `SELECT mean(value) FROM cpu`,
+			query:    `SELECT mean(value), stddev(value) FROM cpu`,
 			queryDb:  "%DB%",
-			expected: `{"results":[{"series":[{"name":"cpu","columns":["time","mean"],"values":[["1970-01-01T00:00:00Z",` + string(maxFloat64) + `]]}]}]}`,
+			expected: `{"results":[{"series":[{"name":"cpu","columns":["time","mean","stddev"],"values":[["1970-01-01T00:00:00Z",` + string(maxFloat64) + `,0]]}]}]}`,
+		},
+		{
+			reset: true,
+			name:  "mean and stddev",
+			write: `{"database" : "%DB%", "retentionPolicy" : "%RP%", "points": [
+				{"name": "cpu", "timestamp": "2000-01-01T00:00:00Z", "fields": {"value": 2}},
+				{"name": "cpu", "timestamp": "2000-01-01T00:00:10Z", "fields": {"value": 4}},
+				{"name": "cpu", "timestamp": "2000-01-01T00:00:20Z", "fields": {"value": 4}},
+				{"name": "cpu", "timestamp": "2000-01-01T00:00:30Z", "fields": {"value": 4}},
+				{"name": "cpu", "timestamp": "2000-01-01T00:00:40Z", "fields": {"value": 5}},
+				{"name": "cpu", "timestamp": "2000-01-01T00:00:50Z", "fields": {"value": 5}},
+				{"name": "cpu", "timestamp": "2000-01-01T00:01:00Z", "fields": {"value": 7}},
+				{"name": "cpu", "timestamp": "2000-01-01T00:01:10Z", "fields": {"value": 9}}
+			]}`,
+			query:    `SELECT mean(value), stddev(value) FROM cpu WHERE time >= '2000-01-01' AND time < '2000-01-01T00:02:00Z' GROUP BY time(10m)`,
+			queryDb:  "%DB%",
+			expected: `{"results":[{"series":[{"name":"cpu","columns":["time","mean","stddev"],"values":[["2000-01-01T00:00:00Z",5,2.138089935299395]]}]}]}`,
 		},
 		{
 			reset: true,

influxql/functions.go

@@ -157,6 +157,12 @@ func InitializeUnmarshaller(c *Call) (UnmarshalFunc, error) {
 			err := json.Unmarshal(b, &o)
 			return &o, err
 		}, nil
+	case "stddev":
+		return func(b []byte) (interface{}, error) {
+			val := make([]float64, 0)
+			err := json.Unmarshal(b, &val)
+			return val, err
+		}, nil
 	default:
 		return func(b []byte) (interface{}, error) {
 			var val interface{}
@@ -395,7 +401,7 @@ func MapStddev(itr Iterator) interface{} {
 		values = append(values, v.(float64))
 	}
 
-	return nil
+	return values
 }
 
 // ReduceStddev computes the stddev of values.
@@ -414,13 +420,13 @@ func ReduceStddev(values []interface{}) interface{} {
 		return nil
 	}
 
-	// Get the sum
-	var sum float64
-	for _, v := range data {
-		sum += v
-	}
 	// Get the mean
-	mean := sum / float64(len(data))
+	var mean float64
+	var count int
+	for _, v := range data {
+		count++
+		mean += (v - mean) / float64(count)
+	}
 	// Get the variance
 	var variance float64
 	for _, v := range data {
@@ -428,7 +434,7 @@ func ReduceStddev(values []interface{}) interface{} {
 		sq := math.Pow(dif, 2)
 		variance += sq
 	}
-	variance = variance / float64(len(data)-1)
+	variance = variance / float64(count-1)
 	stddev := math.Sqrt(variance)
 
 	return stddev