- The problem was that
  = we've picked a LooseScan that used full index scan (tab->type==JT_ALL) on certain index.
  = there was also a quick select (tab->quick!=NULL), that used other indexes.
  = some old code assumes that (tab->type==JT_ALL && tab->quick) -> means that the
    quick select should be used, which is not true.
Fixed by discarding the quick select as soon as we know we're using LooseScan
without using the quick select.