For "count(*) while index_column = value" an index read
is done. It consists of an index scan and retrieval of
each key.

For efficiency reasons the index scan stores the key in
the special buffer 'lastkey2' once only. At the first 
iteration it notes this fact with the flag 
HA_STATE_RNEXT_SAME in 'info->update'.

For efficiency reasons, the key retrieval for blobs
does not allocate a new buffer, but uses 'lastkey2'...

Now I clear the HA_STATE_RNEXT_SAME flag whenever the 
buffer has been polluted. In this case, the index scan
copies the key value again (and sets the flag again).