MySQL LEFT JOIN – Why Query Runs for a Long Time

I have this query, it works fine and takes over 2.5021 secs:

SELECT c.id AS 'id', count(c.id) as orders, o.created_at as last_order 
FROM customer c, order_2016 o 
WHERE o.customer_relacional_id = c.id 
GROUP BY c.id 
ORDER BY o.created_at DESC

But I want to request the customers with 0 orders too, then I try:

SELECT c.id AS 'id', count(o.customer_relacional_id) orders, o.created_at
FROM customer c
LEFT JOIN order_2016 o
ON (o.customer_relacional_id = c.id)
GROUP BY o.customer_relacional_id  
ORDER BY o.created_at  DESC

But this query never ends :S

EXPLAIN:

EXPLAIN 
SELECT c.id AS 'id', count(c.id) orders, o.created_at 
FROM customer c 
LEFT JOIN order_2016 o ON (o.customer_relacional_id = c.id) 
GROUP BY c.id 
ORDER BY o.created_at DESC

id select_type table type  possible_keys key    key_len ref  rows    Extra
1  SIMPLE      c     index NULL          email  212     NULL 987470  Using index; Using temporary; Using filesort
1  SIMPLE      o     ALL   NULL          NULL   NULL    NULL 240979

I'm trying to create an index in order_2016.customer_relacional_id but is taking too long

CREATE INDEX index_customer ON order_2016 (customer_relacional_id) USING BTREE; 

meanwhile in order_2015 it was fast

CREATE INDEX index_customer ON order_2015 (customer_relacional_id) USING BTREE;
Query OK, 0 rows affected (8.60 sec)
Records: 0  Duplicates: 0  Warnings: 0

Maybe there is something corrupt in order_2016 uhm

Finally I reset mysql and with the index it works slow, but works:

995625 rows in set (15.63 sec)

+--------------------------------+-----------+
| Status                         | Duration  |
+--------------------------------+-----------+
| starting                       |  0.000093 |
| Waiting for query cache lock   |  0.000007 |
| checking query cache for query |  0.000071 |
| checking permissions           |  0.000008 |
| checking permissions           |  0.000007 |
| Opening tables                 |  0.000053 |
| System lock                    |  0.000014 |
| Waiting for query cache lock   |  0.000032 |
| init                           |  0.000032 |
| optimizing                     |  0.000008 |
| statistics                     |  0.000029 |
| preparing                      |  0.000020 |
| Creating tmp table             |  0.000032 |
| Sorting for group              |  0.000020 |
| executing                      |  0.000005 |
| Copying to tmp table           | 11.926916 |
| converting HEAP to MyISAM      |  0.049412 |
| Copying to tmp table on disk   |  2.910412 |
| Sorting result                 |  0.421047 |
| Sending data                   |  0.315151 |
| end                            |  0.000035 |
| removing tmp table             |  0.007676 |
| end                            |  0.000036 |
| query end                      |  0.000018 |
| closing tables                 |  0.000025 |
| freeing items                  |  0.000047 |
| logging slow query             |  0.000006 |
| logging slow query             |  0.000010 |
| cleaning up                    |  0.000010 |
+--------------------------------+-----------+

Thanks for the help.

SHOW CREATE TABLES

SHOW CREATE TABLE customer;

 customer | CREATE TABLE `customer` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `email` varchar(70) COLLATE utf8_unicode_ci NOT NULL,
  `orders` int(11) DEFAULT NULL,
  `country` text COLLATE utf8_unicode_ci,
  `last_date_order` datetime DEFAULT NULL,
  `name` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
  `lastname` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
  `male` tinyint(1) DEFAULT NULL,
  `gender_probability` decimal(10,2) DEFAULT NULL,
  `phone` varchar(20) COLLATE utf8_unicode_ci DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `email` (`email`)
) ENGINE=InnoDB AUTO_INCREMENT=45672 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

SHOW CREATE TABLE order_2016;

order_2016 | CREATE TABLE `order_2016` (
  `id` bigint(12) NOT NULL,
  `name` varchar(20) COLLATE utf8_unicode_ci DEFAULT NULL,
  `created_at` datetime DEFAULT NULL,
  `created_at_day` date DEFAULT NULL,
  `paid_at` datetime DEFAULT NULL,
  `financial_status` enum('pending','authorized','paid','partially_paid','refunded','partially_refunded','voided') COLLATE utf8_unicode_ci DEFAULT NULL,
  `accepts_marketing` enum('yes','no') COLLATE utf8_unicode_ci DEFAULT NULL,
  `subtotal` decimal(6,2) DEFAULT NULL,
  `shipping` decimal(3,2) DEFAULT NULL,
  `total` decimal(6,2) DEFAULT NULL,
  `discount_code` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
  `discount_amount` decimal(6,2) DEFAULT NULL,
  `billing_country` varchar(2) COLLATE utf8_unicode_ci DEFAULT NULL,
  `shipping_country` varchar(2) COLLATE utf8_unicode_ci DEFAULT NULL,
  `shipping_province` varchar(20) COLLATE utf8_unicode_ci DEFAULT NULL,
  `shipping_phone` varchar(20) COLLATE utf8_unicode_ci DEFAULT NULL,
  `shipping_city` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
  `shipping_zip` varchar(10) COLLATE utf8_unicode_ci DEFAULT NULL,
  `cancelled_at` datetime DEFAULT NULL,
  `cancel_reason` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
  `customer_relacional_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `paid_at` (`paid_at`),
  KEY `created_at_day` (`created_at_day`),
  KEY `index_customer` (`customer_relacional_id`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

Updated EXPLAIN

id select_type table type  possible_keys  key            key_len ref          rows    Extra
1  SIMPLE      c     index NULL           email          212     NULL         1108494 Using index; Using temporary; Using filesort
1  SIMPLE      o     ref   index_customer index_customer 4       hawkers.c.id 1