diff --git a/tsdb/mapper.go b/tsdb/mapper.go
index e6d78673b5..85bb07977a 100644
--- a/tsdb/mapper.go
+++ b/tsdb/mapper.go
@@ -89,32 +89,8 @@ func (lm *LocalMapper) Open() error {
 	lm.queryTMin, lm.queryTMax = influxql.TimeRangeAsEpochNano(lm.stmt.Condition)
 
 	if !lm.rawMode {
-		// Set up each mapping function for this statement.
-		aggregates := lm.stmt.FunctionCalls()
-		lm.mapFuncs = make([]influxql.MapFunc, len(aggregates))
-		lm.fieldNames = make([]string, len(lm.mapFuncs))
-		for i, c := range aggregates {
-			lm.mapFuncs[i], err = influxql.InitializeMapFunc(c)
-			if err != nil {
-				return err
-			}
-
-			// Check for calls like `derivative(lmean(value), 1d)`
-			var nested *influxql.Call = c
-			if fn, ok := c.Args[0].(*influxql.Call); ok {
-				nested = fn
-			}
-			switch lit := nested.Args[0].(type) {
-			case *influxql.VarRef:
-				lm.fieldNames[i] = lit.Val
-			case *influxql.Distinct:
-				if c.Name != "count" {
-					return fmt.Errorf("aggregate call didn't contain a field %s", c.String())
-				}
-				lm.fieldNames[i] = lit.Val
-			default:
-				return fmt.Errorf("aggregate call didn't contain a field %s", c.String())
-			}
+		if err := lm.initializeMapFunctions(); err != nil {
+			return err
 		}
 
 		// For GROUP BY time queries, limit the number of data points returned by the limit and offset
@@ -360,6 +336,41 @@ func (lm *LocalMapper) nextInterval() (start, end int64) {
 	return
 }
 
+// initializeMapFunctions initialize the mapping functions for the mapper. This only applies
+// to aggregate queries.
+func (lm *LocalMapper) initializeMapFunctions() error {
+	var err error
+	// Set up each mapping function for this statement.
+	aggregates := lm.stmt.FunctionCalls()
+	lm.mapFuncs = make([]influxql.MapFunc, len(aggregates))
+	lm.fieldNames = make([]string, len(lm.mapFuncs))
+	for i, c := range aggregates {
+		lm.mapFuncs[i], err = influxql.InitializeMapFunc(c)
+		if err != nil {
+			return err
+		}
+
+		// Check for calls like `derivative(lmean(value), 1d)`
+		var nested *influxql.Call = c
+		if fn, ok := c.Args[0].(*influxql.Call); ok {
+			nested = fn
+		}
+		switch lit := nested.Args[0].(type) {
+		case *influxql.VarRef:
+			lm.fieldNames[i] = lit.Val
+		case *influxql.Distinct:
+			if c.Name != "count" {
+				return fmt.Errorf("aggregate call didn't contain a field %s", c.String())
+			}
+			lm.fieldNames[i] = lit.Val
+		default:
+			return fmt.Errorf("aggregate call didn't contain a field %s", c.String())
+		}
+	}
+
+	return nil
+}
+
 // TagSets returns the list of TagSets for which this mapper has data.
 func (lm *LocalMapper) TagSets() []string {
 	return tagSetCursors(lm.cursors).Keys()