dialect/sql/schema: fix postgres index migration (#1351)

Fix PostgreSQL index migration when table_name = type_name.
Closed #1344

dialect/sql/schema/migrate.go

@@ -390,9 +390,7 @@ func (m *Migrate) changeSet(curr, new *Table) (*changes, error) {
 
 	// Drop indexes.
 	for _, idx := range curr.Indexes {
-		_, ok1 := new.fk(idx.Name)
-		_, ok2 := new.index(idx.Name)
-		if !ok1 && !ok2 {
+		if _, isFK := new.fk(idx.Name); !isFK && !new.hasIndex(idx.Name, idx.realname) {
 			change.index.drop.append(idx)
 		}
 	}

dialect/sql/schema/schema.go

@@ -139,6 +139,19 @@ func (t *Table) index(name string) (*Index, bool) {
 	return nil, false
 }
 
+// hasIndex reports if the table has at least one index that matches the given names.
+func (t *Table) hasIndex(names ...string) bool {
+	for i := range names {
+		if names[i] == "" {
+			continue
+		}
+		if _, ok := t.index(names[i]); ok {
+			return true
+		}
+	}
+	return false
+}
+
 // fk returns a table foreign-key by its symbol.
 // faster than map lookup for most cases.
 func (t *Table) fk(symbol string) (*ForeignKey, bool) {