The function that checks whether we can use keys for aggregates,
find_key_for_maxmin(), assumes that keys disabled by ALTER TABLE
... DISABLE KEYS are not in the set table->keys_in_use_for_query.
I.E., if a key is in this set, the optimizer assumes it is free to 
use it.
  
The bug is that keys disabled with ALTER TABLE ... DISABLE KEYS still
appear in table->keys_in_use_for_query When the TABLE object has been 
initialized with setup_tables(). Before setup_tables is called, however, 
keys that are disabled in the aforementioned way are not included in
TABLE::keys_in_use_for_query. 
  
The provided patch changes the code that updates keys_is_use_for_query so 
that it assumes that keys_is_use_for_query already takes into account all 
disabled keys, and generally all keys that should be used by the query.