From 53d97a7d721463fd8004c1472b3e10ac352daec1 Mon Sep 17 00:00:00 2001
From: "igor@olga.mysql.com" <>
Date: Mon, 11 Dec 2006 18:57:23 -0800
Subject: [PATCH] Fixed bug #24670: optimizations that are legal only for
 subqueries without tables and no WHERE condition were applied for any
 subquery without tables.

---
 mysql-test/r/subselect.result | 13 +++++++++++++
 mysql-test/t/subselect.test   | 12 ++++++++++++
 sql/item_subselect.cc         | 19 +++++++++++++++++--
 sql/item_subselect.h          |  3 ++-
 4 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/mysql-test/r/subselect.result b/mysql-test/r/subselect.result
index 89315342f5e..a4ea8e21d61 100644
--- a/mysql-test/r/subselect.result
+++ b/mysql-test/r/subselect.result
@@ -3013,3 +3013,16 @@ t3	CREATE TABLE `t3` (
   `a` datetime default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1,t2,t3;
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (1), (2);
+SELECT a FROM t1 WHERE (SELECT 1 FROM DUAL WHERE 1=0) > 0;
+a
+SELECT a FROM t1 WHERE (SELECT 1 FROM DUAL WHERE 1=0) IS NULL;
+a
+1
+2
+EXPLAIN SELECT a FROM t1 WHERE (SELECT 1 FROM DUAL WHERE 1=0) IS NULL;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
+DROP TABLE t1;
diff --git a/mysql-test/t/subselect.test b/mysql-test/t/subselect.test
index 804cc2274c9..f816551e51f 100644
--- a/mysql-test/t/subselect.test
+++ b/mysql-test/t/subselect.test
@@ -1981,4 +1981,16 @@ SHOW CREATE TABLE t3;
 
 DROP TABLE t1,t2,t3;
 
+#
+# Bug 24670: subquery witout tables but with a WHERE clause
+#
+
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (1), (2);
+
+SELECT a FROM t1 WHERE (SELECT 1 FROM DUAL WHERE 1=0) > 0;
+SELECT a FROM t1 WHERE (SELECT 1 FROM DUAL WHERE 1=0) IS NULL;
+EXPLAIN SELECT a FROM t1 WHERE (SELECT 1 FROM DUAL WHERE 1=0) IS NULL;
+
+DROP TABLE t1;
 # End of 4.1 tests
diff --git a/sql/item_subselect.cc b/sql/item_subselect.cc
index cd1f8f83821..20a092d7607 100644
--- a/sql/item_subselect.cc
+++ b/sql/item_subselect.cc
@@ -350,6 +350,7 @@ Item_singlerow_subselect::select_transformer(JOIN *join)
       */
       !(select_lex->item_list.head()->type() == FIELD_ITEM ||
 	select_lex->item_list.head()->type() == REF_ITEM) &&
+      !join->conds && !join->having &&
       /*
         switch off this optimisation for prepare statement,
         because we do not rollback this changes
@@ -374,8 +375,6 @@ Item_singlerow_subselect::select_transformer(JOIN *join)
     */
     substitution->walk(&Item::remove_dependence_processor,
 		       (byte *) select_lex->outer_select());
-    /* SELECT without FROM clause can't have WHERE or HAVING clause */
-    DBUG_ASSERT(join->conds == 0 && join->having == 0);
     return RES_REDUCE;
   }
   return RES_OK;
@@ -1795,6 +1794,22 @@ bool subselect_single_select_engine::no_tables()
 }
 
 
+/*
+  Check statically whether the subquery can return NULL
+
+  SINOPSYS
+    subselect_single_select_engine::may_be_null()
+
+  RETURN
+    FALSE  can guarantee that the subquery never return NULL
+    TRUE   otherwise
+*/
+bool subselect_single_select_engine::may_be_null()
+{
+  return ((no_tables() && !join->conds && !join->having) ? maybe_null : 1);
+}
+
+
 /*
   Report about presence of tables in subquery
 
diff --git a/sql/item_subselect.h b/sql/item_subselect.h
index 7b064bfe92c..539dcc5676a 100644
--- a/sql/item_subselect.h
+++ b/sql/item_subselect.h
@@ -301,7 +301,7 @@ public:
   enum Item_result type() { return res_type; }
   enum_field_types field_type() { return res_field_type; }
   virtual void exclude()= 0;
-  bool may_be_null() { return maybe_null; };
+  virtual bool may_be_null() { return maybe_null; };
   virtual table_map upper_select_const_tables()= 0;
   static table_map calc_const_tables(TABLE_LIST *);
   virtual void print(String *str)= 0;
@@ -335,6 +335,7 @@ public:
   void print (String *str);
   int change_item(Item_subselect *si, select_subselect *result);
   bool no_tables();
+  bool may_be_null();
 };