Bug#47669 Query showed by EXPLAIN EXTENDED gives different result from original query

Item_field::print method does not take into
account fields whose values may be null.
The fix is to print 'NULL' if field value is null.

mysql-test/r/explain.result

@@ -210,4 +210,19 @@ Error	1140	Mixing of GROUP columns (MIN(),MAX(),COUNT(),...) with no GROUP colum
 Note	1003	select 1 AS `1` from `test`.`t1` where <not>(<exists>(...))
 SET SESSION sql_mode=@old_sql_mode;
 DROP TABLE t1;
+#
+# Bug#47669: Query showed by EXPLAIN EXTENDED gives different result from original query
+#
+CREATE TABLE t1 (c int);
+INSERT INTO t1 VALUES (NULL);
+CREATE TABLE t2 (d int);
+INSERT INTO t2 VALUES (NULL), (0);
+EXPLAIN EXTENDED SELECT (SELECT 1 FROM t2 WHERE d = c) FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	1	100.00	
+2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+Warnings:
+Note	1276	Field or reference 'test.t1.c' of SELECT #2 was resolved in SELECT #1
+Note	1003	select (select 1 AS `1` from `test`.`t2` where (`test`.`t2`.`d` = NULL)) AS `(SELECT 1 FROM t2 WHERE d = c)` from `test`.`t1`
+DROP TABLE t1, t2;
 End of 5.1 tests.

mysql-test/r/func_str.result

@@ -1307,12 +1307,12 @@ explain extended select encode(f1,'zxcv') as 'enc' from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 Warnings:
-Note	1003	select encode('','zxcv') AS `enc` from `test`.`t1`
+Note	1003	select encode(NULL,'zxcv') AS `enc` from `test`.`t1`
 explain extended select decode(f1,'zxcv') as 'enc' from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 Warnings:
-Note	1003	select decode('','zxcv') AS `enc` from `test`.`t1`
+Note	1003	select decode(NULL,'zxcv') AS `enc` from `test`.`t1`
 drop table t1;
 create table t1 (a bigint not null)engine=myisam;
 insert into t1 set a = 1024*1024*1024*4;

mysql-test/r/having.result

@@ -12,7 +12,7 @@ explain extended select count(a) as b from t1 where a=0 having b >=0;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 Warnings:
-Note	1003	select count('0') AS `b` from `test`.`t1` where 0 having (`b` >= 0)
+Note	1003	select count(NULL) AS `b` from `test`.`t1` where 0 having (`b` >= 0)
 drop table t1;
 CREATE TABLE t1 (
 raw_id int(10) NOT NULL default '0',

mysql-test/r/select.result

@@ -4050,10 +4050,10 @@ join_0.c1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 Warnings:
-Note	1003	select '0' AS `c1` from `test`.`t1` `join_0` join `test`.`t1` `join_1` join `test`.`t1` `join_2` join `test`.`t1` `join_3` join `test`.`t1` `join_4` join `test`.`t1` `join_5` join `test`.`t1` `join_6` join `test`.`t1` `join_7` where 0 group by '0','0','0','0','0'
+Note	1003	select NULL AS `c1` from `test`.`t1` `join_0` join `test`.`t1` `join_1` join `test`.`t1` `join_2` join `test`.`t1` `join_3` join `test`.`t1` `join_4` join `test`.`t1` `join_5` join `test`.`t1` `join_6` join `test`.`t1` `join_7` where 0 group by NULL,NULL,NULL,NULL,NULL
 SHOW WARNINGS;
 Level	Code	Message
-Note	1003	select '0' AS `c1` from `test`.`t1` `join_0` join `test`.`t1` `join_1` join `test`.`t1` `join_2` join `test`.`t1` `join_3` join `test`.`t1` `join_4` join `test`.`t1` `join_5` join `test`.`t1` `join_6` join `test`.`t1` `join_7` where 0 group by '0','0','0','0','0'
+Note	1003	select NULL AS `c1` from `test`.`t1` `join_0` join `test`.`t1` `join_1` join `test`.`t1` `join_2` join `test`.`t1` `join_3` join `test`.`t1` `join_4` join `test`.`t1` `join_5` join `test`.`t1` `join_6` join `test`.`t1` `join_7` where 0 group by NULL,NULL,NULL,NULL,NULL
 DROP TABLE t1;
 SELECT 1 AS ` `;
 
@@ -4648,17 +4648,17 @@ EXPLAIN EXTENDED SELECT * FROM t1 WHERE (a=a AND b=b AND c=c) OR b > 20;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 Warnings:
-Note	1003	select '0' AS `a`,'0' AS `b`,'0' AS `c` from `test`.`t1` where 1
+Note	1003	select NULL AS `a`,NULL AS `b`,NULL AS `c` from `test`.`t1` where 1
 EXPLAIN EXTENDED SELECT * FROM t1 WHERE (a=a AND a=a AND b=b) OR b > 20;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 Warnings:
-Note	1003	select '0' AS `a`,'0' AS `b`,'0' AS `c` from `test`.`t1` where 1
+Note	1003	select NULL AS `a`,NULL AS `b`,NULL AS `c` from `test`.`t1` where 1
 EXPLAIN EXTENDED SELECT * FROM t1 WHERE (a=a AND b=b AND a=a) OR b > 20;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 Warnings:
-Note	1003	select '0' AS `a`,'0' AS `b`,'0' AS `c` from `test`.`t1` where 1
+Note	1003	select NULL AS `a`,NULL AS `b`,NULL AS `c` from `test`.`t1` where 1
 DROP TABLE t1;
 #
 # Bug#45266: Uninitialized variable lead to an empty result.

mysql-test/r/subselect.result

@@ -1496,7 +1496,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where
 2	SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 Warnings:
-Note	1003	select `test`.`t3`.`a` AS `a` from `test`.`t3` where <not>((`test`.`t3`.`a` < (select max('0') from `test`.`t2`)))
+Note	1003	select `test`.`t3`.`a` AS `a` from `test`.`t3` where <not>((`test`.`t3`.`a` < (select max(NULL) from `test`.`t2`)))
 select * from t3 where a >= some (select b from t2);
 a
 explain extended select * from t3 where a >= some (select b from t2);
@@ -1504,7 +1504,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where
 2	SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 Warnings:
-Note	1003	select `test`.`t3`.`a` AS `a` from `test`.`t3` where <nop>((`test`.`t3`.`a` >= (select min('0') from `test`.`t2`)))
+Note	1003	select `test`.`t3`.`a` AS `a` from `test`.`t3` where <nop>((`test`.`t3`.`a` >= (select min(NULL) from `test`.`t2`)))
 select * from t3 where a >= all (select b from t2 group by 1);
 a
 6
@@ -1515,7 +1515,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where
 2	SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 Warnings:
-Note	1003	select `test`.`t3`.`a` AS `a` from `test`.`t3` where <not>((`test`.`t3`.`a` < <max>(select '0' AS `b` from `test`.`t2` group by 1)))
+Note	1003	select `test`.`t3`.`a` AS `a` from `test`.`t3` where <not>((`test`.`t3`.`a` < <max>(select NULL AS `b` from `test`.`t2` group by 1)))
 select * from t3 where a >= some (select b from t2 group by 1);
 a
 explain extended select * from t3 where a >= some (select b from t2 group by 1);
@@ -1523,7 +1523,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where
 2	SUBQUERY	t2	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 Warnings:
-Note	1003	select `test`.`t3`.`a` AS `a` from `test`.`t3` where <nop>((`test`.`t3`.`a` >= <min>(select '0' AS `b` from `test`.`t2` group by 1)))
+Note	1003	select `test`.`t3`.`a` AS `a` from `test`.`t3` where <nop>((`test`.`t3`.`a` >= <min>(select NULL AS `b` from `test`.`t2` group by 1)))
 select * from t3 where NULL >= any (select b from t2);
 a
 explain extended select * from t3 where NULL >= any (select b from t2);

mysql-test/r/union.result

@@ -1585,7 +1585,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 2	UNION	t1	system	NULL	NULL	NULL	NULL	0	0.00	const row not found
 NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	NULL	Using filesort
 Warnings:
-Note	1003	select '0' AS `a` from `test`.`t1` union select '0' AS `a` from `test`.`t1` order by `a`
+Note	1003	select NULL AS `a` from `test`.`t1` union select NULL AS `a` from `test`.`t1` order by `a`
 DROP TABLE t1;
 End of 5.0 tests
 # 

mysql-test/t/explain.test

@@ -187,4 +187,16 @@ SET SESSION sql_mode=@old_sql_mode;
 
 DROP TABLE t1;
 
+--echo #
+--echo # Bug#47669: Query showed by EXPLAIN EXTENDED gives different result from original query
+--echo #
+
+CREATE TABLE t1 (c int);
+INSERT INTO t1 VALUES (NULL);
+CREATE TABLE t2 (d int);
+INSERT INTO t2 VALUES (NULL), (0);
+EXPLAIN EXTENDED SELECT (SELECT 1 FROM t2 WHERE d = c) FROM t1;
+DROP TABLE t1, t2;
+
+
 --echo End of 5.1 tests.

sql/item.cc

@@ -5690,9 +5690,14 @@ void Item_field::print(String *str, enum_query_type query_type)
     char buff[MAX_FIELD_WIDTH];
     String tmp(buff,sizeof(buff),str->charset());
     field->val_str(&tmp);
+    if (field->is_null())
+      str->append("NULL");
+    else
+    {
       str->append('\'');
       str->append(tmp);
       str->append('\'');
+    }
     return;
   }
   Item_ident::print(str, query_type);