schemadiff: fix scenario where no tables exist in schema and with jus…

…t views reading from DUAL (#12189)

go/vt/schemadiff/schema.go

@@ -263,6 +263,15 @@ func (s *Schema) normalize() error {
 	// - then we only want views that depend on 1st level views or on tables. These are 2nd level views.
 	// - etc.
 	// we stop when we have been unable to find a view in an iteration.
+
+	// It's possible that there's never been any tables in this schema. Which means
+	// iterationLevel remains zero.
+	// To deal with views, we must have iterationLevel at least 1. This is because any view reads
+	// from _something_: at the very least it reads from DUAL (inplicitly or explicitly). Which
+	// puts the view at a higher level.
+	if iterationLevel < 1 {
+		iterationLevel = 1
+	}
 	for {
 		handledAnyViewsInIteration := false
 		for _, v := range s.views {

go/vt/schemadiff/schema_test.go

@@ -126,6 +126,24 @@ func TestNewSchemaFromQueriesUnresolvedAlias(t *testing.T) {
 	assert.EqualError(t, err, (&ViewDependencyUnresolvedError{View: "v7"}).Error())
 }
 
+func TestNewSchemaFromQueriesViewFromDual(t *testing.T) {
+	// Schema will not contain any tables, just a view selecting from DUAL
+	queries := []string{
+		"create view v20 as select 1 from dual",
+	}
+	_, err := NewSchemaFromQueries(queries)
+	assert.NoError(t, err)
+}
+
+func TestNewSchemaFromQueriesViewFromDualImplicit(t *testing.T) {
+	// Schema will not contain any tables, just a view implicitly selecting from DUAL
+	queries := []string{
+		"create view v20 as select 1",
+	}
+	_, err := NewSchemaFromQueries(queries)
+	assert.NoError(t, err)
+}
+
 func TestNewSchemaFromQueriesLoop(t *testing.T) {
 	// v7 and v8 depend on each other
 	queries := append(createQueries,