update the history of all items in sql

[den.rai]

I want to update all item`s history setting(from 365 to 90) in one time.
I used sql like "update items set history='90' where history='365'".
It is reflected in the GUI,but the table size is not decreased.
PS:housekeeper is working. [Deleted 7782 records from history and trends]
I also have tried to restart the zabbix server serveral times.
Is there any mistake?
please tell me!!