Bug#48920: COUNT DISTINCT returns 1 for NULL values when in a

           subquery in the select list
      
When a dependent subquery with count(distinct <col>) was 
evaluated multiple times, the Distinct_Aggregator was reused. 
However, the Aggregator was not reset, so when the subquery was
evaluated for the next record in the outer select, old dependent
info was used.
      
The fix is to clear() the existing aggregator in 
Item_sum::set_aggregator(). This ensures that the aggregator is
reevaluated with the new dependent information.

mysql-test/r/subselect3.result:
  Added test case for BUG#48920
mysql-test/t/subselect3.test:
  Added test case for BUG#48920
sql/item_sum.cc:
  If an aggregator exists when Item_sum::set_aggregator() is
  called (i.e., set_aggregator is called in a dependent
  subquery), the aggregator is reset so that the aggregator is
  reevaluated with the dependent information from the outer
  record being evaluated.

mysql-test/r/subselect3.result

@@ -964,3 +964,53 @@ Variable_name	Value
 Handler_read_rnd_next	18
 DROP TABLE t1,t2;
 End of 5.1 tests
+#
+# BUG#48920: COUNT DISTINCT returns 1 for NULL values when in a subquery 
+#            in the select list
+#
+
+CREATE TABLE t1 (
+i int(11) DEFAULT NULL,
+v varchar(1) DEFAULT NULL
+);
+
+INSERT INTO t1 VALUES (8,'v');
+INSERT INTO t1 VALUES (9,'r');
+INSERT INTO t1 VALUES (NULL,'y');
+
+CREATE TABLE t2 (
+i int(11) DEFAULT NULL,
+v varchar(1) DEFAULT NULL,
+KEY i_key (i)
+);
+
+INSERT INTO t2 VALUES (NULL,'r');
+INSERT INTO t2 VALUES (0,'c');
+INSERT INTO t2 VALUES (0,'o');
+INSERT INTO t2 VALUES (2,'v');
+INSERT INTO t2 VALUES (7,'c');
+
+SELECT i, v, (SELECT COUNT(DISTINCT i)
+FROM t1
+WHERE v  = t2.v) as subsel
+FROM t2;
+i	v	subsel
+NULL	r	1
+0	c	0
+0	o	0
+2	v	1
+7	c	0
+
+EXPLAIN EXTENDED
+SELECT i, v, (SELECT COUNT(DISTINCT i)
+FROM t1
+WHERE v  = t2.v) as subsel
+FROM t2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	5	100.00	
+2	DEPENDENT SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where
+Warnings:
+Note	1276	Field or reference 'test.t2.v' of SELECT #2 was resolved in SELECT #1
+Note	1003	select `test`.`t2`.`i` AS `i`,`test`.`t2`.`v` AS `v`,(select count(distinct `test`.`t1`.`i`) AS `COUNT(DISTINCT i)` from `test`.`t1` where (`test`.`t1`.`v` = `test`.`t2`.`v`)) AS `subsel` from `test`.`t2`
+DROP TABLE t1,t2;
+End of 5.6 tests

mysql-test/t/subselect3.test

@@ -794,3 +794,50 @@ SHOW STATUS LIKE '%Handler_read_rnd_next';
 DROP TABLE t1,t2;
 
 --echo End of 5.1 tests
+
+--echo #
+--echo # BUG#48920: COUNT DISTINCT returns 1 for NULL values when in a subquery 
+--echo #            in the select list
+--echo #
+
+--echo 
+CREATE TABLE t1 (
+  i int(11) DEFAULT NULL,
+  v varchar(1) DEFAULT NULL
+);
+
+--echo 
+INSERT INTO t1 VALUES (8,'v');
+INSERT INTO t1 VALUES (9,'r');
+INSERT INTO t1 VALUES (NULL,'y');
+
+--echo 
+CREATE TABLE t2 (
+  i int(11) DEFAULT NULL,
+  v varchar(1) DEFAULT NULL,
+  KEY i_key (i)
+);
+
+--echo 
+INSERT INTO t2 VALUES (NULL,'r');
+INSERT INTO t2 VALUES (0,'c');
+INSERT INTO t2 VALUES (0,'o');
+INSERT INTO t2 VALUES (2,'v');
+INSERT INTO t2 VALUES (7,'c');
+
+--echo 
+SELECT i, v, (SELECT COUNT(DISTINCT i)
+              FROM t1
+              WHERE v  = t2.v) as subsel
+FROM t2;
+
+--echo 
+EXPLAIN EXTENDED
+SELECT i, v, (SELECT COUNT(DISTINCT i)
+              FROM t1
+              WHERE v  = t2.v) as subsel
+FROM t2;
+
+DROP TABLE t1,t2;
+
+--echo End of 5.6 tests

sql/item_sum.cc

@@ -578,7 +578,14 @@ int Item_sum::set_aggregator(Aggregator::Aggregator_type aggregator)
 {
   if (aggr)
   {
+    /* 
+      Dependent subselects may be executed multiple times, making
+      set_aggregator to be called multiple times. The aggregator type
+      will be the same, but it needs to be reset so that it is
+      reevaluated with the new dependent data.
+    */
     DBUG_ASSERT(aggregator == aggr->Aggrtype());
+    aggr->clear();
     return FALSE;
   }
   switch (aggregator)