get stddev working

this includes avoiding overflow for large values

cmd/influxd/server_integration_test.go

@@ -581,14 +581,14 @@ func runTestsData(t *testing.T, testName string, nodes Cluster, database, retent
 		// Aggregations
 		{
 			reset: true,
-			name:  "large mean",
+			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,

influxql/functions.go

@@ -397,29 +397,31 @@ func ReduceStddev(values []interface{}) interface{} {
 		if value == nil {
 			continue
 		}
-		data = append(data, value.([]float64)...)
+		for _, val := range value.([]float64) {
+			data = append(data, val)
+		}
 	}
 
 	// If no data or we only have one point, it's nil or undefined
 	if len(data) < 2 {
 		return nil
 	}
 
-	// Get the sum
-	var sum float64
+	// Get the mean
+	var mean float64
+	var count int
 	for _, v := range data {
-		sum += v
+		count++
+		mean += (v - mean) / float64(count)
 	}
-	// Get the mean
-	mean := sum / float64(len(data))
 	// Get the variance
 	var variance float64
 	for _, v := range data {
 		dif := v - mean
 		sq := math.Pow(dif, 2)
 		variance += sq
 	}
-	variance = variance / float64(len(data)-1)
+	variance = variance / float64(count-1)
 	stddev := math.Sqrt(variance)
 
 	return stddev