functions are involved.

When subselect is a join with set functions and no record have been found in
it, end_send_group() sets null_row for all tables in order aggregate functions 
to calculate their values correctly. Normally this null_row flag is cleared for 
each table in sub_select(), but flush_cached_records() doesn't do so.
Due to this all fields from the table processed by flush_cached_records() are 
always evaluated as nulls and whole select produces wrong result.

flush_cached_records() now clears null_row flag at the very beginning.