planner: return an error when meeting unexpected operator under Union…

…Scan (#53956) (#53971)

ref #53951

pkg/executor/builder.go

@@ -1353,9 +1353,14 @@ func (b *executorBuilder) buildUnionScanFromReader(reader exec.Executor, v *plan
 		us.columns = x.columns
 		us.table = x.table
 		us.virtualColumnIndex = buildVirtualColumnIndex(us.Schema(), us.columns)
-	default:
-		// The mem table will not be written by sql directly, so we can omit the union scan to avoid err reporting.
+	case *PointGetExecutor, *BatchPointGetExec, // PointGet and BatchPoint can handle virtual columns and dirty txn data themselves.
+		*TableDualExec,       // If TableDual, the result must be empty, so we can skip UnionScan and use TableDual directly here.
+		*TableSampleExecutor: // TableSample only supports sampling from disk, don't need to consider in-memory txn data for simplicity.
 		return originReader
+	default:
+		// TODO: consider more operators like Projection.
+		b.err = errors.NewNoStackErrorf("unexpected operator %T under UnionScan", reader)
+		return nil
 	}
 	return us
 }
@@ -4080,6 +4085,9 @@ func (builder *dataReaderBuilder) buildUnionScanForIndexJoin(ctx context.Context
 	}
 
 	ret := builder.buildUnionScanFromReader(reader, v)
+	if builder.err != nil {
+		return nil, builder.err
+	}
 	if us, ok := ret.(*UnionScanExec); ok {
 		err = us.open(ctx)
 	}