refactor: allow newIndexSeriesCursor() to accept an influxql.Expr

In order to let TagKeys and TagValues get the right answer,
sometimes they will need to examine their predicate and
see if using the index is possible, or if a block scan is needed.
For those cases we want to examine the predicate before creating
the index series cursor. This change allows us to create an
index series cursor with an already-deserialized influxql.Expr.

v1/services/storage/series_cursor.go

@@ -38,6 +38,18 @@ type indexSeriesCursor struct {
 }
 
 func newIndexSeriesCursor(ctx context.Context, predicate *datatypes.Predicate, shards []*tsdb.Shard) (*indexSeriesCursor, error) {
+	var expr influxql.Expr
+	if root := predicate.GetRoot(); root != nil {
+		var err error
+		if expr, err = reads.NodeToExpr(root, measurementRemap); err != nil {
+			return nil, err
+		}
+	}
+
+	return newIndexSeriesCursorInfluxQL(ctx, expr, shards)
+}
+
+func newIndexSeriesCursorInfluxQL(ctx context.Context, predicate influxql.Expr, shards []*tsdb.Shard) (*indexSeriesCursor, error) {
 	queries, err := tsdb.CreateCursorIterators(ctx, shards)
 	if err != nil {
 		return nil, err
@@ -61,10 +73,8 @@ func newIndexSeriesCursor(ctx context.Context, predicate *datatypes.Predicate, s
 	}
 	p := &indexSeriesCursor{row: reads.SeriesRow{Query: queries}}
 
-	if root := predicate.GetRoot(); root != nil {
-		if p.cond, err = reads.NodeToExpr(root, measurementRemap); err != nil {
-			return nil, err
-		}
+	if predicate != nil {
+		p.cond = predicate
 
 		p.hasFieldExpr, p.hasValueExpr = HasFieldKeyOrValue(p.cond)
 		if !(p.hasFieldExpr || p.hasValueExpr) {