dialect/sql/schema: support for USING method in create index builder (#1730)

* Support for USING 'method' in create index

* Add USING 'method' in create index support for MySQL

* Fix USING 'method' for MySQL to be a raw string

dialect/sql/builder.go

@@ -526,6 +526,7 @@ type IndexBuilder struct {
 	unique  bool
 	exists  bool
 	table   string
+	method  string
 	columns []string
 }
 
@@ -565,6 +566,12 @@ func (i *IndexBuilder) Table(table string) *IndexBuilder {
 	return i
 }
 
+// Using sets the method to create the index with.
+func (i *IndexBuilder) Using(method string) *IndexBuilder {
+	i.method = method
+	return i
+}
+
 // Column appends a column to the column list for the index.
 func (i *IndexBuilder) Column(column string) *IndexBuilder {
 	i.columns = append(i.columns, column)
@@ -589,9 +596,27 @@ func (i *IndexBuilder) Query() (string, []interface{}) {
 	}
 	i.Ident(i.name)
 	i.WriteString(" ON ")
-	i.Ident(i.table).Nested(func(b *Builder) {
-		b.IdentComma(i.columns...)
-	})
+	i.Ident(i.table)
+	switch i.dialect {
+	case dialect.Postgres:
+		if i.method != "" {
+			i.WriteString(" USING ").Ident(i.method)
+		}
+		i.Nested(func(b *Builder) {
+			b.IdentComma(i.columns...)
+		})
+	case dialect.MySQL:
+		i.Nested(func(b *Builder) {
+			b.IdentComma(i.columns...)
+		})
+		if i.method != "" {
+			i.WriteString(" USING " + i.method)
+		}
+	default:
+		i.Nested(func(b *Builder) {
+			b.IdentComma(i.columns...)
+		})
+	}
 	return i.String(), nil
 }