fix #405. Percentile shouldn't crash for small number of values

CHANGELOG.md

@@ -364,4 +364,5 @@
 - [Issue #398](https://github.com/influxdb/influxdb/issues/398). Support now() and NOW() in the query lang
 - [Issue #403](https://github.com/influxdb/influxdb/issues/403). Filtering should work with join queries
 - [Issue #404](https://github.com/influxdb/influxdb/issues/404). Filtering with invalid condition shouldn't crash the server
+- [Issue #405](https://github.com/influxdb/influxdb/issues/405). Percentile shouldn't crash for small number of values
 - Close leveldb databases properly if we couldn't create a new Shard. See leveldb\_shard\_datastore\_test:131

src/engine/aggregator.go

@@ -806,22 +806,22 @@ func (self *PercentileAggregator) ColumnNames() []string {
 }
 
 func (self *PercentileAggregator) GetValues(series string, group interface{}) [][]*protocol.FieldValue {
-	returnValues := [][]*protocol.FieldValue{}
 	values := self.float_values[series][group]
 
-	if len(values) == 0 {
-		returnValues = append(returnValues, []*protocol.FieldValue{self.defaultValue})
-	}
-
 	sort.Float64s(values)
 	length := len(values)
 	index := int(math.Floor(float64(length)*self.percentile/100.0+0.5)) - 1
-	point := values[index]
-	returnValues = append(returnValues, []*protocol.FieldValue{
-		&protocol.FieldValue{DoubleValue: &point},
-	})
 
-	return returnValues
+	if index < 0 || index >= len(values) {
+		return [][]*protocol.FieldValue{
+			[]*protocol.FieldValue{self.defaultValue},
+		}
+	}
+
+	point := values[index]
+	return [][]*protocol.FieldValue{
+		[]*protocol.FieldValue{&protocol.FieldValue{DoubleValue: &point}},
+	}
 }
 
 func NewPercentileAggregator(_ *parser.SelectQuery, value *parser.Value, defaultValue *parser.Value) (Aggregator, error) {

src/integration/engine_test.go

@@ -868,6 +868,30 @@ func (self *EngineSuite) TestPercentileQueryWithGroupByTime(c *C) {
   ]`)
 }
 
+// issue #405
+func (self *EngineSuite) TestLowPercentileForOnePointShouldNotCrash(c *C) {
+	// make the mock coordinator return some data
+	self.createEngine(c, `[
+    {
+      "points": [
+        { "values": [{ "int64_value": 1 }], "timestamp": 1381346701000000 }
+      ],
+      "name": "foo",
+      "fields": ["column_one"]
+    }
+  ]`)
+
+	self.runQuery("select percentile(column_one, 40) from foo group by time(1m)", c, `[
+    {
+      "points": [
+        { "values": [{ "double_value": 0 }], "timestamp": 1381346700000000}
+      ],
+      "name": "foo",
+      "fields": ["percentile"]
+    }
+  ]`)
+}
+
 func (self *EngineSuite) TestCountDistinct(c *C) {
 	// make the mock coordinator return some data
 	self.createEngine(c, `[