fix now frac in column option

parser.y

@@ -1625,8 +1625,7 @@ ColumnOption:
 	}
 |	"ON" "UPDATE" NowSymOptionFraction
 	{
-		nowFunc := &ast.FuncCallExpr{FnName: model.NewCIStr("CURRENT_TIMESTAMP")}
-		$$ = &ast.ColumnOption{Tp: ast.ColumnOptionOnUpdate, Expr: nowFunc}
+		$$ = &ast.ColumnOption{Tp: ast.ColumnOptionOnUpdate, Expr: $3}
 	}
 |	"COMMENT" stringLit
 	{

parser_test.go

@@ -1822,7 +1822,7 @@ func (s *testParserSuite) TestDDL(c *C) {
 		{"CREATE TABLE t (c TEXT) shard_row_id_bits = 1;", true, "CREATE TABLE `t` (`c` TEXT) SHARD_ROW_ID_BITS = 1"},
 		{"CREATE TABLE t (c TEXT) shard_row_id_bits = 1, PRE_SPLIT_REGIONS = 1;", true, "CREATE TABLE `t` (`c` TEXT) SHARD_ROW_ID_BITS = 1 PRE_SPLIT_REGIONS = 1"},
 		// Create table with ON UPDATE CURRENT_TIMESTAMP(6), specify fraction part.
-		{"CREATE TABLE IF NOT EXISTS `general_log` (`event_time` timestamp(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6),`user_host` mediumtext NOT NULL,`thread_id` bigint(20) unsigned NOT NULL,`server_id` int(10) unsigned NOT NULL,`command_type` varchar(64) NOT NULL,`argument` mediumblob NOT NULL) ENGINE=CSV DEFAULT CHARSET=utf8 COMMENT='General log'", true, "CREATE TABLE IF NOT EXISTS `general_log` (`event_time` TIMESTAMP(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(),`user_host` MEDIUMTEXT NOT NULL,`thread_id` BIGINT(20) UNSIGNED NOT NULL,`server_id` INT(10) UNSIGNED NOT NULL,`command_type` VARCHAR(64) NOT NULL,`argument` MEDIUMBLOB NOT NULL) ENGINE = CSV DEFAULT CHARACTER SET = UTF8 COMMENT = 'General log'"}, //TODO: The number yacc in parentheses has not been implemented yet.
+		{"CREATE TABLE IF NOT EXISTS `general_log` (`event_time` timestamp(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6),`user_host` mediumtext NOT NULL,`thread_id` bigint(20) unsigned NOT NULL,`server_id` int(10) unsigned NOT NULL,`command_type` varchar(64) NOT NULL,`argument` mediumblob NOT NULL) ENGINE=CSV DEFAULT CHARSET=utf8 COMMENT='General log'", true, "CREATE TABLE IF NOT EXISTS `general_log` (`event_time` TIMESTAMP(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6),`user_host` MEDIUMTEXT NOT NULL,`thread_id` BIGINT(20) UNSIGNED NOT NULL,`server_id` INT(10) UNSIGNED NOT NULL,`command_type` VARCHAR(64) NOT NULL,`argument` MEDIUMBLOB NOT NULL) ENGINE = CSV DEFAULT CHARACTER SET = UTF8 COMMENT = 'General log'"}, //TODO: The number yacc in parentheses has not been implemented yet.
 		// For reference_definition in column_definition.
 		{"CREATE TABLE followers ( f1 int NOT NULL REFERENCES user_profiles (uid) );", true, "CREATE TABLE `followers` (`f1` INT NOT NULL REFERENCES `user_profiles`(`uid`))"},