storage/maria/ma_write.c:
  select ... group by ... - take temporary tables into account, they have TRN->pins == 0
storage/maria/trnman.c:
  one cannot modify TRN->trid, it's a key in the trid_to_trn
  hash, if trid is modified, TRN won't be found (and deleted in
  trnman_free_trn) from the hash.