Order result-set columns by query projection ordering

Closes #14

pkg/plugin/query.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/SnellerInc/sneller/ion"
 	"github.com/grafana/grafana-plugin-sdk-go/data"
+	"golang.org/x/exp/slices"
 )
 
 // frameFromSnellerResult builds a Grafana data frame from a raw Sneller query result.
@@ -362,10 +363,11 @@ type snellerColumn struct {
 }
 
 type snellerFinalStatus struct {
-	Hits    int64  `ion:"hits"`
-	Misses  int64  `ion:"misses"`
-	Scanned int64  `ion:"scanned"`
-	Error   string `ion:"error"`
+	Hits      int64     `ion:"hits"`
+	Misses    int64     `ion:"misses"`
+	Scanned   int64     `ion:"scanned"`
+	Error     string    `ion:"error"`
+	ResultSet ion.Datum `ion:"result_set"`
 }
 
 type snellerQueryError struct {
@@ -374,20 +376,21 @@ type snellerQueryError struct {
 
 // snellerSchema represents the derived schema of a Sneller query result-set.
 type snellerSchema struct {
-	RowCount    int                       // The total number of rows returned by the query
-	Columns     map[string]*snellerColumn // The individual columns indexed by their field names
-	FinalStatus *snellerFinalStatus       // The final query status
+	RowCount    int                 // The total number of rows returned by the query
+	Columns     []*snellerColumn    // The individual columns
+	FinalStatus *snellerFinalStatus // The final query status
 }
 
 func deriveSchema(buf []byte) (*snellerSchema, error) {
 	schema := snellerSchema{
 		RowCount: 0,
-		Columns:  map[string]*snellerColumn{},
+		Columns:  []*snellerColumn{},
 	}
+	lookup := map[string]*snellerColumn{}
 
 	status, err := iterateRows(buf, func(reader *IonReader, index int) error {
 		schema.RowCount += 1
-		return analyzeRow(reader, &schema)
+		return analyzeRow(reader, &schema, lookup)
 	})
 	if err != nil {
 		return nil, err
@@ -402,11 +405,30 @@ func deriveSchema(buf []byte) (*snellerSchema, error) {
 		}
 	}
 
+	// Restore column order
+	index := 0
+	err = status.ResultSet.UnpackStruct(func(field ion.Field) error {
+		for _, col := range schema.Columns {
+			if col.Name == field.Label {
+				col.Index = index
+				break
+			}
+		}
+		index++
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	slices.SortFunc(schema.Columns, func(a, b *snellerColumn) bool {
+		return a.Index < b.Index
+	})
+
 	return &schema, nil
 }
 
-func analyzeRow(reader *IonReader, schema *snellerSchema) error {
-	index := 0
+func analyzeRow(reader *IonReader, schema *snellerSchema, lookup map[string]*snellerColumn) error {
 	for reader.Next() {
 		name, err := reader.FieldName()
 		if err != nil {
@@ -416,23 +438,20 @@ func analyzeRow(reader *IonReader, schema *snellerSchema) error {
 		ionType := reader.Type()
 		snellerType := snellerType(ionType)
 
-		col, ok := schema.Columns[name]
+		col, ok := lookup[name]
 		if !ok {
 			col = &snellerColumn{
-				Index:    index,
 				Name:     name,
 				Typ:      snellerType,
 				Nullable: snellerType == snellerTypeNull,
 				Signed:   ionType == ion.IntType || ionType == ion.FloatType,
 				Optional: schema.RowCount != 1,
 				Count:    0,
 			}
-			schema.Columns[name] = col
+			lookup[name] = col
+			schema.Columns = append(schema.Columns, col)
 		}
 
-		if index != col.Index {
-			col.Index = -1
-		}
 		col.Count++
 
 		// Adjust column type if required
@@ -461,9 +480,8 @@ func analyzeRow(reader *IonReader, schema *snellerSchema) error {
 			}
 			// TODO: Required bits
 		}
-
-		index++
 	}
+
 	return reader.Error()
 }