1. Item_windowfunc must check the nullness of the field if it returns the
value from the result_field.
2. The decimal value must come from the window_func instead of the
result_field when not fetching values from the result_field.
Added a more extensive test case that highlights if these bugs occur.
The 2 select statements check behaviour for decimal and real typed
fields respectively.
Query result had 0 where it should have had NULLs:
- Make Item_window_func::val* functions honor NULL-handling conventions:
1. set null_value to indicate whether we've returned a NULL value
2. val_str and val_decimal should return NULL pointer when they're
returning SQL NULL value.
Fix assertion failure when sending results to network.
- The assert was due to window func returing SQL NULL despite having
maybe_null=false
- Fixed by settting Item_window_func::maybe_null correctly in fix_fields
- Item_sum_count::remove() should check if the argument's value is NULL.
- Window Function item must have its Item_window_func::split_sum_func
called,
- and it must call split_sum_func for aggregate's arguments (see the
comment near Item_window_func::split_sum_func for explanation why)
- Add temporary code: clone_read_record() clones READ_RECORD structure,
as long as it is used for reading filesort() result that fits into
memory.
- Add frame bounds for ROWS-type frames. ROWS n PRECEDING|FOLLOWING,
ROWS UNBOUNDED PRECEDING|FOLLOWING, CURRENT ROW are supported.
- Add Item_sum_count::remove() which allows "streaming" computation
of COUNT() over a moving frame.
