Fix for #1992

This bug happens when a query, having subselects in the fields list,
needs temporary table.
Item_subselect::get_tmp_table_item isn't implemented and just returns
*this. So the tmp_table_item takes value not from temporary but
from original table.

mysql-test/r/subselect.result

@@ -1569,3 +1569,13 @@ INSERT INTO t2 VALUES (100, 200, 'C');
 SELECT DISTINCT COLC FROM t1 WHERE COLA = (SELECT COLA FROM t2 WHERE COLB = 200 AND COLC ='C' LIMIT 1);
 COLC
 DROP TABLE t1, t2;
+CREATE TABLE t1 (a int(1));
+INSERT INTO t1 VALUES (1),(1),(1),(1),(1),(2),(3),(4),(5);
+SELECT DISTINCT (SELECT a) FROM t1 LIMIT 100;
+(SELECT a)
+1
+2
+3
+4
+5
+DROP TABLE t1;

mysql-test/t/subselect.test

@@ -1009,3 +1009,8 @@ INSERT INTO t1 VALUES (1,1,'1A3240'), (1,2,'4W2365');
 INSERT INTO t2 VALUES (100, 200, 'C');
 SELECT DISTINCT COLC FROM t1 WHERE COLA = (SELECT COLA FROM t2 WHERE COLB = 200 AND COLC ='C' LIMIT 1);
 DROP TABLE t1, t2;
+
+CREATE TABLE t1 (a int(1));
+INSERT INTO t1 VALUES (1),(1),(1),(1),(1),(2),(3),(4),(5);
+SELECT DISTINCT (SELECT a) FROM t1 LIMIT 100;
+DROP TABLE t1;

sql/item_subselect.cc

@@ -166,6 +166,12 @@ bool Item_subselect::const_item() const
   return const_item_cache;
 }
 
+Item *Item_subselect::get_tmp_table_item(THD *thd)
+{
+  if (!with_sum_func && !const_item())
+    return new Item_field(result_field);
+  return copy_or_same(thd);
+}
 
 void Item_subselect::update_used_tables()
 {

sql/item_subselect.h

@@ -89,6 +89,7 @@ class Item_subselect :public Item_result_field
   bool const_item() const;
   inline table_map get_used_tables_cache() { return used_tables_cache; }
   inline bool get_const_item_cache() { return const_item_cache; }
+  Item *get_tmp_table_item(THD *thd);
   void update_used_tables();
   void print(String *str);
   bool change_engine(subselect_engine *eng)