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This is backport of 25ad623d64e for 10.2.
The issue is similar to the one from MDEV-13240. Item::save_in_field()
returns an error during tmp table population in a create table from select query
as we try to save an empty string as a date value when
force_return_blank is set to true for window functions.
MDEV-13240 Wrong warning with MAX(datetime_field) OVER (...)
The problem resided in Item_window_func implementation,
and it was revealed by bb-10.2-ext specific changes:
Item_window_func::save_in_field() works differently in bb-10.2-ext vs 10.2:
- 10.2 goes through val_str()
- bb-10.2-ext goes through get_date(), due to Type_handler related changes.
get_date() tries to convert empty string to DATETIME, hence the warning.
During a discussion with Vicentiu, it was decided to fix
Item_window_func::val_xxx() to return NULL
(instead of an "empty" value, such as 0 for numbers and '' for strings)
when force_return_blank is set.
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