Subquery not optimized when join is between Aria and InnoDB tables

See my stack exchange post and answer here for reference:

http://dba.stackexchange.com/questions/134813/materialized-subquery-not-optimized

Primary query table uses Aria engine. Subquery uses InnoDB. Query was not optimizing properly and was joining on "index" type. Expecting "eq_ref" type because primary table constraint was using primary key. Changing primary table to InnoDB fixed the issue and used eq_ref as expected.