Use ReturnValues if you want to get the item attributes as they appeared either before or after they were updated. For UpdateItem, the valid values are:
NONE - If ReturnValues is not specified, or if its value is NONE, then
nothing is returned. (This setting is the default for ReturnValues.)
ALL_OLD - If UpdateItem overwrote an attribute name-value pair, then
the content of the old item is returned.
UPDATED_OLD - The old versions of only the updated attributes are returned. ALL_NEW - All of the attributes of the new version of the item are returned. UPDATED_NEW - The new versions of only the updated attributes are returned. There is no additional cost associated with requesting a return value
aside from the small network and processing overhead of receiving a larger response. No Read Capacity Units are consumed.
Values returned are strongly consistent