bugfix: RAND is VCOL_SESSION_FUNC

it's not "non deterministic", it's completely defined
by @@rand_seed1 and @@rand_seed2. And as a session func it needs
to be re-fixed at the beginning of every statement.

mysql-test/suite/vcol/r/vcol_misc.result

@@ -533,3 +533,19 @@ drop table t1;
 #
 # End of 10.2 tests
 #
+#
+# RAND is session func
+#
+create table t1 (a int, b float default rand(1));
+insert into t1 (a) values (1);
+insert into t1 (a) values (2);
+insert into t1 (a) values (3);
+select * from t1;
+a	b
+1	0.405404
+2	0.405404
+3	0.405404
+drop table t1;
+#
+# End of 10.3 tests
+#

mysql-test/suite/vcol/t/vcol_misc.test

@@ -506,3 +506,16 @@ drop table t1;
 --echo # End of 10.2 tests
 --echo #
 
+--echo #
+--echo # RAND is session func
+--echo #
+create table t1 (a int, b float default rand(1));
+insert into t1 (a) values (1);
+insert into t1 (a) values (2);
+insert into t1 (a) values (3);
+select * from t1;
+drop table t1;
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

sql/item_func.h

@@ -1513,7 +1513,7 @@ class Item_func_rand :public Item_real_func
   void cleanup() { first_eval= TRUE; Item_real_func::cleanup(); }
   bool check_vcol_func_processor(void *arg)
   {
-    return mark_unsupported_function(func_name(), "()", arg, VCOL_NON_DETERMINISTIC);
+    return mark_unsupported_function(func_name(), "()", arg, VCOL_SESSION_FUNC);
   }
   Item *get_copy(THD *thd)
   { return get_item_copy<Item_func_rand>(thd, this); }