fix(storage): Ensure _field tag key has special handling

Fixes #19120

v1/services/storage/store.go

@@ -11,6 +11,7 @@ import (
 	"github.com/influxdata/influxdb/v2"
 	"github.com/influxdata/influxdb/v2/influxql/query"
 	"github.com/influxdata/influxdb/v2/models"
+	"github.com/influxdata/influxdb/v2/pkg/slices"
 	"github.com/influxdata/influxdb/v2/storage/reads"
 	"github.com/influxdata/influxdb/v2/storage/reads/datatypes"
 	"github.com/influxdata/influxdb/v2/v1/services/meta"
@@ -261,6 +262,9 @@ func (s *Store) TagValues(ctx context.Context, req *datatypes.TagValuesRequest)
 				MeasurementsSource: req.TagsSource,
 				Predicate:          req.Predicate,
 			})
+
+		case "_field":
+			return s.measurementFields(ctx, req)
 		}
 	}
 
@@ -409,3 +413,70 @@ func (s *Store) GetSource(orgID, bucketID uint64) proto.Message {
 		OrganizationID: orgID,
 	}
 }
+
+func (s *Store) measurementFields(ctx context.Context, req *datatypes.TagValuesRequest) (cursors.StringIterator, error) {
+	source, err := getReadSource(*req.TagsSource)
+	if err != nil {
+		return nil, err
+	}
+
+	database, rp, start, end, err := s.validateArgs(source.OrganizationID, source.BucketID, req.Range.Start, req.Range.End)
+	if err != nil {
+		return nil, err
+	}
+
+	shardIDs, err := s.findShardIDs(database, rp, false, start, end)
+	if err != nil {
+		return nil, err
+	}
+	if len(shardIDs) == 0 {
+		return cursors.EmptyStringIterator, nil
+	}
+
+	var expr influxql.Expr
+	if root := req.Predicate.GetRoot(); root != nil {
+		var err error
+		expr, err = reads.NodeToExpr(root, measurementRemap)
+		if err != nil {
+			return nil, err
+		}
+
+		if found := reads.HasFieldValueKey(expr); found {
+			return nil, errors.New("field values unsupported")
+		}
+		expr = influxql.Reduce(influxql.CloneExpr(expr), nil)
+		if reads.IsTrueBooleanLiteral(expr) {
+			expr = nil
+		}
+	}
+
+	sg := s.TSDBStore.ShardGroup(shardIDs)
+	ms := &influxql.Measurement{
+		Database:        database,
+		RetentionPolicy: rp,
+		SystemIterator:  "_fieldKeys",
+	}
+	opts := query.IteratorOptions{
+		OrgID:      influxdb.ID(source.OrganizationID),
+		Condition:  expr,
+		Authorizer: query.OpenAuthorizer,
+	}
+	iter, err := sg.CreateIterator(ctx, ms, opts)
+	if err != nil {
+		return nil, err
+	}
+	defer func() { _ = iter.Close() }()
+
+	var fieldNames []string
+	fitr := iter.(query.FloatIterator)
+	for p, _ := fitr.Next(); p != nil; p, _ = fitr.Next() {
+		if len(p.Aux) >= 1 {
+			fieldNames = append(fieldNames, p.Aux[0].(string))
+		}
+	}
+
+	sort.Strings(fieldNames)
+	fieldNames = slices.MergeSortedStrings(fieldNames)
+
+	return cursors.NewStringSliceIterator(fieldNames), nil
+}