planner: document dead-end nil signal in makeBushyTree and fix test edges

Author: yy782

pkg/planner/core/joinorder/join_order.go

@@ -945,6 +945,9 @@ func makeJoinWithDetector(detector *ConflictDetector, left, right *Node, vertexH
 	if !checkResult.Connected() {
 		checkResult = detector.TryCreateCartesianCheckResult(left, right)
 		if checkResult == nil {
+			// Dead end: no edge connects these fragments and cartesian
+			// fallback is unavailable (see #69986 for the nil-deref panic
+			// this would cause if we returned an error instead).
 			return nil, nil
 		}
 	}
@@ -989,6 +992,9 @@ func makeBushyTree(ctx base.PlanContext, detector *ConflictDetector, cartesianNo
 				return nil, err
 			}
 			if newJoin == nil {
+				// Dead end: no real edge spans these fragments and cartesian
+				// fallback is unavailable. Propagate nil to let the caller
+				// fall through to the partial plan already built.
 				return nil, nil
 			}
 			iterNodes = append(iterNodes, newJoin)

pkg/planner/core/joinorder/join_order_test.go

@@ -127,18 +127,18 @@ func TestMakeBushyTreeDeadEnd(t *testing.T) {
 	detector := &ConflictDetector{
 		allInnerJoin: false,
 		innerEdges: []*edge{
-			{tes: intset.NewFastIntSet(0, 1), idx: 0, joinType: base.InnerJoin, skipRules: true},
+			{tes: intset.NewFastIntSet(0, 1), idx: 0, joinType: base.InnerJoin, skipRules: false},
 			{
 				tes:           intset.NewFastIntSet(0, 1, 2, 3),
 				idx:           2,
 				joinType:      base.InnerJoin,
-				skipRules:     true,
+				skipRules:     false,
 				leftVertexes:  intset.NewFastIntSet(0, 1),
 				rightVertexes: intset.NewFastIntSet(2, 3),
 			},
 		},
 		nonInnerEdges: []*edge{
-			{tes: intset.NewFastIntSet(2, 3), idx: 1, joinType: base.LeftOuterJoin, skipRules: true,
+			{tes: intset.NewFastIntSet(2, 3), idx: 1, joinType: base.LeftOuterJoin, skipRules: false,
 				leftVertexes: intset.NewFastIntSet(2), rightVertexes: intset.NewFastIntSet(3)},
 		},
 	}