Bug#40634 table scan temporary table is 4x slower due to mmap instead instead of caching

    mmap is slower that caching indeed.
    Here the problem is that mmap is used even if --myisam-use-mmap=OFF

    solved by checking the flag in ha_myisam::extra() as it is called in
    init_read_record()

per-file comments:
  storage/myisam/ha_myisam.cc
Bug#40634 table scan temporary table is 4x slower due to mmap instead instead of caching 
    do nothing for HA_EXTRA_MMAP if no opt_myisam_use_mmap

storage/myisam/ha_myisam.cc

@@ -1789,6 +1789,8 @@ int ha_myisam::extra(enum ha_extra_function operation)
 {
   if ((specialflag & SPECIAL_SAFE_MODE) && operation == HA_EXTRA_KEYREAD)
     return 0;
+  if (operation == HA_EXTRA_MMAP && !opt_myisam_use_mmap)
+    return 0;
   return mi_extra(file, operation, 0);
 }