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

@@ -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;

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

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
 

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();
 };