Simple change so "SHOW TAG VALUES" is as expected

This change only covers the case where a values for a single tag key is
requested, since there is a question whether anything else should be
supported.

database.go

@@ -1367,15 +1367,16 @@ func (m *Measurement) tagKeys() []string {
 	return keys
 }
 
-func (m *Measurement) tagValuesByKeyAndSeriesID(tagKeys []string, ids seriesIDs) stringSet {
+func (m *Measurement) tagValuesByKeyAndSeriesID(tagKeys []string, ids seriesIDs) (stringSet, stringSet) {
 	// If no tag keys were passed, get all tag keys for the measurement.
 	if len(tagKeys) == 0 {
 		for k := range m.seriesByTagKeyValue {
 			tagKeys = append(tagKeys, k)
 		}
 	}
 
-	// Make a set to hold all tag values found.
+	// Make a set to hold all tag keys and values found.
+	tKeys := newStringSet()
 	tagValues := newStringSet()
 
 	// Iterate all series to collect tag values.
@@ -1389,12 +1390,13 @@ func (m *Measurement) tagValuesByKeyAndSeriesID(tagKeys []string, ids seriesIDs)
 		// from this series, if they exist.
 		for _, tagKey := range tagKeys {
 			if tagVal, ok := s.Tags[tagKey]; ok {
+				tKeys.add(tagKey)
 				tagValues.add(tagVal)
 			}
 		}
 	}
 
-	return tagValues
+	return tKeys, tagValues
 }
 
 type stringSet map[string]struct{}

httpd/handler_test.go

@@ -2036,15 +2036,15 @@ func TestHandler_serveShowTagValues(t *testing.T) {
 						Series: []*influxql.Row{
 							{
 								Name:    "cpu",
-								Columns: []string{"tagValue"},
+								Columns: []string{"host"},
 								Values: [][]interface{}{
 									str2iface([]string{"server01"}),
 									str2iface([]string{"server02"}),
 								},
 							},
 							{
 								Name:    "gpu",
-								Columns: []string{"tagValue"},
+								Columns: []string{"host"},
 								Values: [][]interface{}{
 									str2iface([]string{"server02"}),
 									str2iface([]string{"server03"}),
@@ -2064,7 +2064,7 @@ func TestHandler_serveShowTagValues(t *testing.T) {
 						Series: []*influxql.Row{
 							{
 								Name:    "cpu",
-								Columns: []string{"tagValue"},
+								Columns: []string{"host"},
 								Values: [][]interface{}{
 									str2iface([]string{"server01"}),
 									str2iface([]string{"server02"}),
@@ -2084,7 +2084,7 @@ func TestHandler_serveShowTagValues(t *testing.T) {
 						Series: []*influxql.Row{
 							{
 								Name:    "cpu",
-								Columns: []string{"tagValue"},
+								Columns: []string{"host"},
 								Values: [][]interface{}{
 									str2iface([]string{"server01"}),
 								},
@@ -2103,7 +2103,7 @@ func TestHandler_serveShowTagValues(t *testing.T) {
 						Series: []*influxql.Row{
 							{
 								Name:    "gpu",
-								Columns: []string{"tagValue"},
+								Columns: []string{"host"},
 								Values: [][]interface{}{
 									str2iface([]string{"server03"}),
 								},
@@ -2122,7 +2122,7 @@ func TestHandler_serveShowTagValues(t *testing.T) {
 						Series: []*influxql.Row{
 							{
 								Name:    "gpu",
-								Columns: []string{"tagValue"},
+								Columns: []string{"region"},
 								Values: [][]interface{}{
 									str2iface([]string{"caeast"}),
 								},
@@ -2141,7 +2141,7 @@ func TestHandler_serveShowTagValues(t *testing.T) {
 						Series: []*influxql.Row{
 							{
 								Name:    "cpu",
-								Columns: []string{"tagValue"},
+								Columns: []string{"host", "region"},
 								Values: [][]interface{}{
 									str2iface([]string{"server01"}),
 									str2iface([]string{"uswest"}),

server.go

@@ -2147,11 +2147,14 @@ func (s *Server) executeShowTagValuesStatement(stmt *influxql.ShowTagValuesState
 			ids = m.seriesIDs
 		}
 
-		tagValues := m.tagValuesByKeyAndSeriesID(stmt.TagKeys, ids)
+		tagKeys, tagValues := m.tagValuesByKeyAndSeriesID(stmt.TagKeys, ids)
+
+		keys := tagKeys.list()
+		sort.Strings(keys)
 
 		r := &influxql.Row{
 			Name:    m.Name,
-			Columns: []string{"tagValue"},
+			Columns: keys,
 		}
 
 		vals := tagValues.list()