make fill previous for count() queries work

CHANGELOG.md

@@ -11,6 +11,7 @@
 - [#4872](https://github.com/influxdb/influxdb/pull/4872): Add option to disable logging for meta service.
 
 ### Bugfixes
+- [#4849](https://github.com/influxdb/influxdb/issues/4849): Derivative works with count, mean, median, sum, first, last, max, min, and percentile.
 - [#4984](https://github.com/influxdb/influxdb/pull/4984): Allow math on fields, fixes regression. Thanks @mengjinglei
 - [#4666](https://github.com/influxdb/influxdb/issues/4666): Fix panic in derivative with invalid values.
 - [#4404](https://github.com/influxdb/influxdb/issues/4404): Return better error for currently unsupported DELETE queries.

cmd/influxd/run/server_test.go

@@ -1738,6 +1738,11 @@ cpu value=20 1278010021000000000
 			command: `SELECT derivative(count(value), 4s) from db0.rp0.cpu where time >= '2010-07-01 18:47:00' and time <= '2010-07-01 18:47:03' group by time(2s) fill(previous)`,
 			exp:     `{"results":[{"series":[{"name":"cpu","columns":["time","derivative"],"values":[["2010-07-01T18:47:02Z",0]]}]}]}`,
 		},
+		&Query{
+			name:    "calculate derivative of count of distinct with unit default (4s) group by time with fill previous",
+			command: `SELECT derivative(count(distinct(value))) from db0.rp0.position where time >= '2010-07-01 18:47:00' and time <= '2010-07-01 18:47:07' group by time(4s) fill(previous)`,
+			exp:     `{"results":[{"error":"aggregate call didn't contain a field derivative(count(distinct(value)))"}]}`,
+		},
 		&Query{
 			name:    "calculate derivative of mean with unit default (2s) group by time with fill 0",
 			command: `SELECT derivative(mean(value)) from db0.rp0.cpu where time >= '2010-07-01 18:47:00' and time <= '2010-07-01 18:47:03' group by time(2s) fill(0)`,

influxql/ast.go

@@ -793,6 +793,34 @@ func (s *SelectStatement) IsSimpleDerivative() bool {
 	return false
 }
 
+// HasSimpleCount return true if one of the function calls is a count function with a
+// variable ref as the first arg
+func (s *SelectStatement) HasSimpleCount() bool {
+	// recursively check for a simple count(varref) function
+	var hasCount func(f *Call) bool
+	hasCount = func(f *Call) bool {
+		if strings.HasSuffix(f.Name, "count") {
+			// it's nested if the first argument is an aggregate function
+			if _, ok := f.Args[0].(*VarRef); ok {
+				return true
+			}
+		} else {
+			for _, arg := range f.Args {
+				if child, ok := arg.(*Call); ok {
+					return hasCount(child)
+				}
+			}
+		}
+		return false
+	}
+	for _, f := range s.FunctionCalls() {
+		if hasCount(f) {
+			return true
+		}
+	}
+	return false
+}
+
 // TimeAscending returns true if the time field is sorted in chronological order.
 func (s *SelectStatement) TimeAscending() bool {
 	return len(s.SortFields) == 0 || s.SortFields[0].Ascending

tsdb/aggregate.go

@@ -342,11 +342,12 @@ func (e *AggregateExecutor) processFill(results [][]interface{}) [][]interface{}
 		return newResults
 	}
 
+	isCount := e.stmt.HasSimpleCount()
 	// They're either filling with previous values or a specific number
 	for i, vals := range results {
 		// start at 1 because the first value is always time
 		for j := 1; j < len(vals); j++ {
-			if vals[j] == nil {
+			if vals[j] == nil || (isCount && isZero(vals[j])) {
 				switch e.stmt.Fill {
 				case influxql.PreviousFill:
 					if i != 0 {
@@ -361,6 +362,18 @@ func (e *AggregateExecutor) processFill(results [][]interface{}) [][]interface{}
 	return results
 }
 
+// Returns true if the given interface is a zero valued int64 or float64.
+func isZero(i interface{}) bool {
+	switch v := i.(type) {
+	case int64:
+		return v == 0
+	case float64:
+		return v == 0
+	default:
+		return false
+	}
+}
+
 // processDerivative returns the derivatives of the results
 func (e *AggregateExecutor) processDerivative(results [][]interface{}) [][]interface{} {
 	// Return early if we're not supposed to process the derivatives
@@ -694,7 +707,7 @@ func (m *AggregateMapper) initializeMapFunctions() error {
 		}
 		m.mapFuncs[i] = mfn
 
-		// Check for calls like `derivative(lmean(value), 1d)`
+		// Check for calls like `derivative(mean(value), 1d)`
 		var nested *influxql.Call = c
 		if fn, ok := c.Args[0].(*influxql.Call); ok {
 			nested = fn