(BUG#1484)
mysql-test/r/variables.result:
test of swaping variables
mysql-test/t/variables.test:
test of swaping variables
sql/item_func.cc:
new method of user variables to pre-fetch value
sql/item_func.h:
new method of user variables to pre-fetch value
sql/set_var.cc:
check() methods fetch & store value
sql/set_var.h:
check() methods fetch & store value
