fix #106. Don't panic if we only see one point and can't calculate de…

…rivative

CHANGELOG.md

@@ -154,6 +154,7 @@
 
 - [Issue #90](https://github.com/influxdb/influxdb/issues/90). Group by multiple columns panic
 - [Issue #89](https://github.com/influxdb/influxdb/issues/89). 'Group by' combined with 'where' not working
+- [Issue #106](https://github.com/influxdb/influxdb/issues/106). Don't panic if we only see one point and can't calculate derivative
 
 ### Deprecated
 

src/engine/aggregator.go

@@ -225,15 +225,18 @@ func (self *DerivativeAggregator) GetValues(series string, group interface{}) []
 	oldValue := self.firstValues[series][group]
 	newValue := self.lastValues[series][group]
 
-	// if an old value exist, then compute the derivative and insert it in the points slice
-	deltaT := float64(*newValue.Timestamp-*oldValue.Timestamp) / float64(time.Second/time.Microsecond)
-	deltaV := *newValue.Values[self.fieldIndex].DoubleValue - *oldValue.Values[self.fieldIndex].DoubleValue
-	derivative := deltaV / deltaT
-	return [][]*protocol.FieldValue{
-		[]*protocol.FieldValue{
-			&protocol.FieldValue{DoubleValue: &derivative},
-		},
+	if newValue != nil {
+		// if an old value exist, then compute the derivative and insert it in the points slice
+		deltaT := float64(*newValue.Timestamp-*oldValue.Timestamp) / float64(time.Second/time.Microsecond)
+		deltaV := *newValue.Values[self.fieldIndex].DoubleValue - *oldValue.Values[self.fieldIndex].DoubleValue
+		derivative := deltaV / deltaT
+		return [][]*protocol.FieldValue{
+			[]*protocol.FieldValue{
+				&protocol.FieldValue{DoubleValue: &derivative},
+			},
+		}
 	}
+	return [][]*protocol.FieldValue{}
 }
 
 func NewDerivativeAggregator(q *parser.Query, v *parser.Value) (Aggregator, error) {

src/engine/engine_test.go

@@ -1116,6 +1116,26 @@ func (self *EngineSuite) TestDerivativeQuery(c *C) {
   ]`)
 }
 
+func (self *EngineSuite) TestDerivativeQueryWithOnePoint(c *C) {
+	engine := createEngine(c, `[
+    {
+      "points": [
+        { "values": [{ "int64_value": 1 }], "timestamp": 1381347700000000, "sequence_number": 1 }
+      ],
+      "name": "foo",
+      "fields": ["column_one"]
+    }
+  ]`)
+
+	runQuery(engine, "select derivative(column_one) from foo", c, `[
+    {
+      "points": [],
+      "name": "foo",
+      "fields": ["derivative"]
+    }
+  ]`)
+}
+
 func (self *EngineSuite) TestDistinctQuery(c *C) {
 	engine := createEngine(c, `[
     {

src/integration/benchmark_test.go

@@ -592,6 +592,28 @@ func (self *IntegrationSuite) TestHttpPostWithTime(c *C) {
 	c.Assert(values["val2"], Equals, 2.0)
 }
 
+// test for issue #106
+func (self *IntegrationSuite) TestIssue106(c *C) {
+	err := self.server.WriteData(`
+[
+  {
+    "name": "test_issue_106",
+    "columns": ["time", "a"],
+    "points":[
+		  [1386262529794, 2]
+	  ]
+  }
+]`, "time_precision=m")
+	c.Assert(err, IsNil)
+	bs, err := self.server.RunQuery("select derivative(a) from test_issue_106")
+	c.Assert(err, IsNil)
+	data := []*h.SerializedSeries{}
+	err = json.Unmarshal(bs, &data)
+	c.Assert(data, HasLen, 1)
+	c.Assert(data[0].Columns, HasLen, 3)
+	c.Assert(data[0].Points, HasLen, 0)
+}
+
 // test for issue #41
 func (self *IntegrationSuite) TestDbDelete(c *C) {
 	err := self.server.WriteData(`