Comment 1 for bug 992942

The problem is actually inherited from 5.2. There, one can also observe that
- the first invocation of find_all_keys() is done with quick_select!=NULL, and the quick select is used to read records
- the second invocation of find_all_keys() is done with quick_select==NULL, and full scan is used.

this is clearly not what was intended. However, the effect is only performance, the query produces correct result. MariaDB 5.3/ MySQL 5.6 add IndexConditionPushdown, which is not cleaned up correctly and causes wrong query result.