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:
  Added a test case for bug #24670.
mysql-test/t/subselect.test:
  Added a test case for bug #24670.
sql/item_subselect.cc:
  Fixed bug #24670: optimizations that are legal only for subqueries without tables
  and no WHERE condition were applied for any subquery without tables.
  
  Removed an assertion that caused an abort for subqueries without tables and no 
  WHERE condition. 
  Blocked substitution of a single-row subquery without tables for the constant 
  row from its select list when the subquery contained a WHERE condition.
  This optimization is valid only for subquries without tables with no conditions.
  Any subquery without tables with WHERE clause returns NULL if the WHERE condition
  is FALSE. Erroneously it was always considered as non-nullable that could trigger 
  another optimization concerning IS NULL predicates which is applicable only for 
  non-nullable expressions and ultimately led to a wrong result returned by the outer
  query.
  Added a proper implementation of the virtual method may_be_null for class 
  subselect_single_select_engine.
sql/item_subselect.h:
  Fixed bug #24670: optimizations that are legal only for subqueries without tables
  and no WHERE condition were applied for any subquery without tables.
  Made method may_by_null for class subselect_engine vvirtual.

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;
@@ -1796,6 +1795,22 @@ 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
 
   SINOPSYS
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();
 };