Slow query after upgrade mysql from 5.5 to 5.6

Question

We're upgrading mysql from 5.5 to 5.6 and some queries are deadly slow now.

Queries that took 0.005 seconds before are now taking 49 seconds.

Queries on 5.6 are skipping the indexes, it seems:

+----+-------------+-------+-------+----------------------------------------------------+---------+---------+------+--------+-------------+
| id | select_type | table | type  | possible_keys                                      | key     | key_len | ref  | rows   | Extra       |
+----+-------------+-------+-------+----------------------------------------------------+---------+---------+------+--------+-------------+
|  1 | SIMPLE      | pens  | index | index_contents_on_slug,index_contents_on_slug_hash | PRIMARY | 4       | NULL | 471440 | Using where |
+----+-------------+-------+-------+----------------------------------------------------+---------+---------+------+--------+-------------+
1 row in set (0.00 sec)

But are not being skipped on 5.5:

+----+-------------+-------+-------------+----------------------------------------------------+----------------------------------------------------+---------+------+------+----------------------------------------------------------------------------------------------+
| id | select_type | table | type        | possible_keys                                      | key                                                | key_len | ref  | rows | Extra                                                                                        |
+----+-------------+-------+-------------+----------------------------------------------------+----------------------------------------------------+---------+------+------+----------------------------------------------------------------------------------------------+
|  1 | SIMPLE      | pens  | index_merge | index_contents_on_slug,index_contents_on_slug_hash | index_contents_on_slug_hash,index_contents_on_slug | 768,768 | NULL |    2 | Using union(index_contents_on_slug_hash,index_contents_on_slug); Using where; Using filesort |
+----+-------------+-------+-------------+----------------------------------------------------+----------------------------------------------------+---------+------+------+----------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

Both DBs were created from the same mysql dump.

Are these indexes not being constructed when I do the import on 5.6? How do I force the index creation?

The query:

SELECT  `pens`.* FROM `pens`  WHERE (slug_hash = 'style' OR slug = 'style') ORDER BY `pens`.`id` DESC LIMIT 1

Edit: Removed the schema

Answer 1

Ultimately the accepted answer above is correct.

The help from @RandomSeed got me thinking in the right direction. Basically the optimization plans created in 5.6 are significantly different from those in 5.5, so you'll probably have to rework your query, much like I did.

I did not end up using the FORCE INDEX, but instead removed portions of the query until I determined what was causing 5.6 to miss the index. Then I reworked the application logic to deal with that.