used.

Sorting by RAND() uses a temporary table in order to get a correct results.
User defined variable was set during filling the temporary table and later
on it is substituted for its value from the temporary table. Due to this
it contains the last value stored in the temporary table.

Now if the result_field is set for the Item_func_set_user_var object it 
updates variable from the result_field value when being sent to a client.

The Item_func_set_user_var::check() now accepts a use_result_field
parameter. Depending on its value the result_field or the args[0] is used
to get current value.