From 3dab08f157b33a248dee7037b7a42eb9c06798f6 Mon Sep 17 00:00:00 2001
From: Jorgen Loland <jorgen.loland@sun.com>
Date: Mon, 4 Jan 2010 10:39:42 +0100
Subject: [PATCH] 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 | 50 ++++++++++++++++++++++++++++++++++
 mysql-test/t/subselect3.test   | 47 ++++++++++++++++++++++++++++++++
 sql/item_sum.cc                |  7 +++++
 3 files changed, 104 insertions(+)

diff --git a/mysql-test/r/subselect3.result b/mysql-test/r/subselect3.result
index 3a0576768b2..a51cef6833d 100644
--- a/mysql-test/r/subselect3.result
+++ b/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
diff --git a/mysql-test/t/subselect3.test b/mysql-test/t/subselect3.test
index fab0a462157..39c97941031 100644
--- a/mysql-test/t/subselect3.test
+++ b/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
diff --git a/sql/item_sum.cc b/sql/item_sum.cc
index c33088e0276..a61c5d59d67 100644
--- a/sql/item_sum.cc
+++ b/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)