SSE: Update execution order to move DS queries first (#105686)

This is part of getting SSE more consistent when run from the query service vs part of grafana-server. 
fixes #105680

pkg/expr/graph.go

@@ -211,19 +211,29 @@ func (s *Service) buildDependencyGraph(req *Request) (*simple.DirectedGraph, err
 
 // buildExecutionOrder returns a sequence of nodes ordered by dependency.
 // Note: During execution, Datasource query nodes for the same datasource will
-// be grouped into one request and executed first as phase after this call.
+// be grouped into one request and executed first as phase after this call
+// If the groupByDSFlag is enabled.
 func buildExecutionOrder(graph *simple.DirectedGraph) ([]Node, error) {
 	sortedNodes, err := topo.SortStabilized(graph, nil)
 	if err != nil {
 		return nil, err
 	}
 
-	nodes := make([]Node, len(sortedNodes))
-	for i, v := range sortedNodes {
-		nodes[i] = v.(Node)
+	var dsNodes []Node
+	var otherNodes []Node
+
+	for _, v := range sortedNodes {
+		n := v.(Node)
+		switch n.NodeType() {
+		case TypeDatasourceNode, TypeMLNode:
+			dsNodes = append(dsNodes, n)
+		default:
+			otherNodes = append(otherNodes, n)
+		}
 	}
 
-	return nodes, nil
+	// Datasource/ML nodes come first, followed by all others, in original topo order
+	return append(dsNodes, otherNodes...), nil
 }
 
 // buildNodeRegistry returns a lookup table for reference IDs to respective node.