Fixed bugs 9820, 9799 and 9800.

mysql-test/r/select.result:
  Added test cases for bugs 9799, 9800 and 9820
mysql-test/t/select.test:
  Added test cases for bugs 9799, 9800 and 9820

mysql-test/r/select.result

@@ -2455,3 +2455,24 @@ a
 select x.a, y.a, z.a from ( (t1 x inner join t2 y on x.a=y.a) inner join t2 z on y.a=z.a) WHERE x.a=1;
 a	a	a
 drop table t1,t2;
+create table t1 (s1 varchar(5));
+insert into t1 values ('Wall');
+select min(s1) from t1 group by s1 with rollup;
+min(s1)
+Wall
+Wall
+drop table t1;
+create table t1 (s1 int) engine=myisam;
+insert into t1 values (0);
+select avg(distinct s1) from t1 group by s1 with rollup;
+avg(distinct s1)
+0.0000
+0.0000
+drop table t1;
+create table t1 (s1 int);
+insert into t1 values (null),(1);
+select distinct avg(s1) as x from t1 group by s1 with rollup;
+x
+NULL
+1.0000
+drop table t1;

mysql-test/t/select.test

@@ -2029,3 +2029,30 @@ select t1.a from (t1 inner join t2 on t1.a=t2.a) where t2.a=1;
 select t1.a from ((t1 inner join t2 on t1.a=t2.a)) where t2.a=1;
 select x.a, y.a, z.a from ( (t1 x inner join t2 y on x.a=y.a) inner join t2 z on y.a=z.a) WHERE x.a=1;
 drop table t1,t2;
+
+#
+# Bug#9820
+#
+
+create table t1 (s1 varchar(5));
+insert into t1 values ('Wall');
+select min(s1) from t1 group by s1 with rollup;
+drop table t1;
+
+#
+# Bug#9799
+#
+
+create table t1 (s1 int) engine=myisam;
+insert into t1 values (0);
+select avg(distinct s1) from t1 group by s1 with rollup;
+drop table t1;
+
+#
+# Bug#9800
+#
+
+create table t1 (s1 int);
+insert into t1 values (null),(1);
+select distinct avg(s1) as x from t1 group by s1 with rollup;
+drop table t1;

sql/item_sum.cc

@@ -240,6 +240,8 @@ Item_sum_hybrid::Item_sum_hybrid(THD *thd, Item_sum_hybrid *item)
   case REAL_RESULT:
     sum= item->sum;
     break;
+  case STRING_RESULT: // This can happen with ROLLUP. Note that the value is already
+    break;            // copied at function call.
   case ROW_RESULT:
   default:
     DBUG_ASSERT(0);
@@ -585,7 +587,10 @@ bool Item_sum_distinct::setup(THD *thd)
 
   DBUG_ENTER("Item_sum_distinct::setup");
 
-  DBUG_ASSERT(tree == 0);                 /* setup can not be called twice */
+  /*
+    Setup can be called twice for ROLLUP items. This is a bug.
+    Please add DBUG_ASSERT(tree == 0) here when it's fixed.
+  */
 
   /*
     Virtual table and the tree are created anew on each re-execution of