result.txt

id  select_type   table   partitions  type    possible_keys   key     key_len ref     rows    filtered   Extra
1   SIMPLE        foo                 ref     code            code    34      const   1       100.00     Using index

id  select_type   table   partitions  type    possible_keys   key     key_len ref     rows    filtered   Extra
1   SIMPLE        foo                 index   code            code    34              12      10.00      "Using where; Using index"

usingindex.sql

SET NAMES utf8;
SET time_zone = '+00:00';
SET foreign_key_checks = 0;
SET sql_mode = 'NO_AUTO_VALUE_ON_ZERO';

DROP TABLE IF EXISTS `foo`;
CREATE TABLE `foo` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `code` varchar(32) NOT NULL,
  PRIMARY KEY (`id`) USING BTREE,
  INDEX `code` (`code`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `foo` (`id`, `code`) VALUES
(12,	'123'),
(11,	'546g876870'),
(6,	'7877897wrw'),
(3,	'78r97qr'),
(7,	'7e8w99w'),
(1,	'857545'),
(10,	'b54cv6vc68b7'),
(9,	'v5d5fd5gd'),
(8,	'vv55v55v'),
(4,	'w987w987'),
(2,	'we52wer5'),
(5,	'wrwr78w77');

-- 2017-07-21 11:52:03