indexing - Slow query after upgrade mysql from 5.5 to 5.6 -

we're upgrading mysql 5.5 5.6 , queries deadly slow now.

queries took 0.005 seconds before taking 49 seconds.

queries on 5.6 skipping indexes, seems:

+----+-------------+-------+-------+----------------------------------------------------+---------+---------+------+--------+-------------+ | id | select_type | table | type  | possible_keys                                      | key     | key_len | ref  | rows   |       | +----+-------------+-------+-------+----------------------------------------------------+---------+---------+------+--------+-------------+ |  1 | simple      | pens  | index | index_contents_on_slug,index_contents_on_slug_hash | primary | 4       | null | 471440 | using | +----+-------------+-------+-------+----------------------------------------------------+---------+---------+------+--------+-------------+ 1 row in set (0.00 sec) 

but not being skipped on 5.5:

+----+-------------+-------+-------------+----------------------------------------------------+----------------------------------------------------+---------+------+------+----------------------------------------------------------------------------------------------+ | id | select_type | table | type        | possible_keys                                      | key                                                | key_len | ref  | rows |                                                                                        | +----+-------------+-------+-------------+----------------------------------------------------+----------------------------------------------------+---------+------+------+----------------------------------------------------------------------------------------------+ |  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 created same mysql dump.

are these indexes not being constructed when import on 5.6? how force index creation?

the query:

select  `pens`.* `pens`  (slug_hash = 'style' or slug = 'style') order `pens`.`id` desc limit 1 

edit: removed schema

ultimately accepted answer above correct.

the @randomseed got me thinking in right direction. optimization plans created in 5.6 different in 5.5, you'll have rework query, did.

i did not end using force index, instead removed portions of query until determined causing 5.6 miss index. reworked application logic deal that.