Add some docs

engine/join_engine.go

@@ -6,20 +6,18 @@ import (
 	"github.com/influxdb/influxdb/protocol"
 )
 
-type JoinEngineState struct {
-	lastFields []string
-	lastPoint  *protocol.Point
-}
-
+// TODO: Explain how JoinEngine work
 type JoinEngine struct {
 	query       *parser.SelectQuery
 	next        Processor
 	name        string // the output table name
 	tableIdx    map[string]int
-	tableState []JoinEngineState
+	tablesState []joinEngineState
 	pts         int
 }
 
+// Create and return a new JoinEngine given the shards that will be
+// processed and the query
 func NewJoinEngine(shards []uint32, query *parser.SelectQuery, next Processor) Processor {
 	tableNames := query.GetFromClause().Names
 	name := query.GetFromClause().GetString()
@@ -29,7 +27,7 @@ func NewJoinEngine(shards []uint32, query *parser.SelectQuery, next Processor) P
 	joinEngine := &JoinEngine{
 		next:        next,
 		name:        name,
-		tableState: make([]JoinEngineState, len(tableNames)),
+		tablesState: make([]joinEngineState, len(tableNames)),
 		tableIdx:    make(map[string]int, len(tableNames)),
 		query:       query,
 		pts:         0,
@@ -37,7 +35,7 @@ func NewJoinEngine(shards []uint32, query *parser.SelectQuery, next Processor) P
 
 	for i, tn := range tableNames {
 		alias := tn.GetAlias()
-		joinEngine.tableState[i] = JoinEngineState{}
+		joinEngine.tablesState[i] = joinEngineState{}
 		joinEngine.tableIdx[alias] = i
 	}
 
@@ -56,11 +54,16 @@ func (je *JoinEngine) Close() error {
 func (je *JoinEngine) Yield(s *protocol.Series) (bool, error) {
 	log4go.Fine("JoinEngine.Yield(): %s", s)
 	idx := je.tableIdx[s.GetName()]
-	state := &je.tableState[idx]
+	state := &je.tablesState[idx]
+	// If the state for this table didn't contain a point already,
+	// increment the number of tables ready to emit a point by
+	// incrementing `pts`
 	if state.lastPoint == nil {
 		je.pts++
 	}
 	state.lastPoint = s.Points[len(s.Points)-1]
+	// update the fields for this table. the fields shouldn't change
+	// after the first point, so we only need to set them once
 	if state.lastFields == nil {
 		for _, f := range s.Fields {
 			state.lastFields = append(state.lastFields, s.GetName()+"."+f)
@@ -68,11 +71,16 @@ func (je *JoinEngine) Yield(s *protocol.Series) (bool, error) {
 	}
 
 	log4go.Fine("JoinEngine: pts = %d", je.pts)
-	if je.pts != len(je.tableState) {
+	// if the number of tables ready to emit a point isn't equal to the
+	// total number of tables being joined, then return
+	if je.pts != len(je.tablesState) {
 		return true, nil
 	}
 
-	ts := je.tableState[0].lastPoint.Timestamp
+	// we arbitrarily use the timestamp of the first table's point as
+	// the timestamp of the resulting point. may be we should use the
+	// smalles (or largest) timestamp.
+	ts := je.tablesState[0].lastPoint.Timestamp
 	newSeries := &protocol.Series{
 		Name:   &je.name,
 		Fields: je.fields(),
@@ -84,7 +92,9 @@ func (je *JoinEngine) Yield(s *protocol.Series) (bool, error) {
 		},
 	}
 
-	// reset the number of points available for
+	// filter the point. the user may have a where clause with the join,
+	// e.g. `select * from join(foo1, foo2) where foo1.val > 10`. we
+	// can't evaluate the where clause until after join happens
 	filteredSeries, err := Filter(je.query, newSeries)
 	if err != nil {
 		return false, err
@@ -96,25 +106,36 @@ func (je *JoinEngine) Yield(s *protocol.Series) (bool, error) {
 	return true, nil
 }
 
+func (self *JoinEngine) Next() Processor {
+	return self.next
+}
+
+// private
+
+type joinEngineState struct {
+	lastFields []string
+	lastPoint  *protocol.Point
+}
+
+// Returns the field names from all tables appended together
 func (je *JoinEngine) fields() []string {
 	fs := []string{}
-	for _, s := range je.tableState {
+	for _, s := range je.tablesState {
 		fs = append(fs, s.lastFields...)
 	}
 	return fs
 }
 
+// Returns the field values from all tables appended together
 func (je *JoinEngine) values() []*protocol.FieldValue {
 	vs := make([]*protocol.FieldValue, 0)
-	for i := range je.tableState {
-		s := &je.tableState[i]
+	for i := range je.tablesState {
+		// Take the address of the slice element, since we set lastPoint
+		// to nil
+		s := &je.tablesState[i]
 		vs = append(vs, s.lastPoint.Values...)
 		s.lastPoint = nil
 	}
 	je.pts = 0
 	return vs
 }
-
-func (self *JoinEngine) Next() Processor {
-	return self.next
-}