diff --git a/go/vt/vttablet/tabletmanager/vdiff/table_differ.go b/go/vt/vttablet/tabletmanager/vdiff/table_differ.go
index a98af2d54fa..ebe2901af05 100644
--- a/go/vt/vttablet/tabletmanager/vdiff/table_differ.go
+++ b/go/vt/vttablet/tabletmanager/vdiff/table_differ.go
@@ -927,19 +927,23 @@ func (td *tableDiffer) getSourcePKCols() error {
 		// We use the columns from a PKE if there is one.
 		executeFetch := func(query string, maxrows int, wantfields bool) (*sqltypes.Result, error) {
 			res, err := td.wd.ct.tmc.ExecuteFetchAsApp(ctx, sourceTablet.Tablet, false, &tabletmanagerdatapb.ExecuteFetchAsAppRequest{
-				Query:   []byte(query),
-				MaxRows: 1,
+				Query: []byte(query),
 			})
 			if err != nil {
 				return nil, err
 			}
 			return sqltypes.Proto3ToResult(res), nil
 		}
-		pkeCols, _, err := mysqlctl.GetPrimaryKeyEquivalentColumns(ctx, executeFetch, td.wd.ct.sourceKeyspace, td.table.Name)
+		pkeCols, _, err := mysqlctl.GetPrimaryKeyEquivalentColumns(ctx, executeFetch, sourceTablet.DbName(), td.table.Name)
 		if err != nil {
 			return err
 		}
-		sourceTable.PrimaryKeyColumns = pkeCols
+		if len(pkeCols) > 0 {
+			sourceTable.PrimaryKeyColumns = pkeCols
+		} else {
+			// We use every column together as a substitute PK.
+			sourceTable.PrimaryKeyColumns = append(sourceTable.PrimaryKeyColumns, td.table.Columns...)
+		}
 	}
 	sourcePKColumns := make(map[string]struct{}, len(sourceTable.PrimaryKeyColumns))
 	td.tablePlan.sourcePkCols = make([]int, 0, len(sourceTable.PrimaryKeyColumns))