Fix for bug#42009: SELECT into variable gives different results to direct SELECT

Problem: storing "SELECT ... INTO @var ..." results in variables we used val_xxx()
methods which returned results of the current row. 
So, in some cases (e.g. SELECT DISTINCT, GROUP BY or HAVING) we got data
from the first row of a new group (where we evaluate a clause) instead of
data from the last row of the previous group.

Fix: use val_xxx_result() counterparts to get proper results.

mysql-test/r/distinct.result

@@ -629,21 +629,21 @@ SELECT DISTINCT @v5:= fruit_id, @v6:= fruit_name INTO @v7, @v8 FROM t1 WHERE
 fruit_name = 'APPLE';
 SELECT @v5, @v6, @v7, @v8;
 @v5	@v6	@v7	@v8
-3	PEAR	3	PEAR
+2	APPLE	2	APPLE
 SELECT DISTINCT @v5 + fruit_id, CONCAT(@v6, fruit_name) INTO @v9, @v10 FROM t1 
 WHERE fruit_name = 'APPLE';
 SELECT @v5, @v6, @v7, @v8, @v9, @v10;
 @v5	@v6	@v7	@v8	@v9	@v10
-3	PEAR	3	PEAR	5	PEARAPPLE
+2	APPLE	2	APPLE	4	APPLEAPPLE
 SELECT DISTINCT @v11:= @v5 + fruit_id, @v12:= CONCAT(@v6, fruit_name) INTO 
 @v13, @v14 FROM t1 WHERE fruit_name = 'APPLE';
 SELECT @v11, @v12, @v13, @v14;
 @v11	@v12	@v13	@v14
-6	PEARPEAR	6	PEARPEAR
+4	APPLEAPPLE	4	APPLEAPPLE
 SELECT DISTINCT @v13, @v14 INTO @v15, @v16 FROM t1 WHERE fruit_name = 'APPLE';
 SELECT @v15, @v16;
 @v15	@v16
-6	PEARPEAR
+4	APPLEAPPLE
 SELECT DISTINCT 2 + 2, 'Bob' INTO @v17, @v18 FROM t1 WHERE fruit_name = 
 'APPLE';
 SELECT @v17, @v18;

mysql-test/r/user_var.result

@@ -353,3 +353,14 @@ select @a:=f4, count(f4) from t1 group by 1 desc;
 2.6	1
 1.6	4
 drop table t1;
+CREATE TABLE t1(a INT, b INT);
+INSERT INTO t1 VALUES (0, 0), (2, 1), (2, 3), (1, 1), (30, 20);
+SELECT a, b INTO @a, @b FROM t1 WHERE a=2 AND b=3 GROUP BY a, b;
+SELECT @a, @b;
+@a	@b
+2	3
+SELECT a, b FROM t1 WHERE a=2 AND b=3 GROUP BY a, b;
+a	b
+2	3
+DROP TABLE t1;
+End of 5.0 tests

mysql-test/t/user_var.test

@@ -237,3 +237,15 @@ select @a:=f2, count(f2) from t1 group by 1 desc;
 select @a:=f3, count(f3) from t1 group by 1 desc;
 select @a:=f4, count(f4) from t1 group by 1 desc;
 drop table t1;
+
+#
+# Bug#42009: SELECT into variable gives different results to direct SELECT
+#
+CREATE TABLE t1(a INT, b INT);
+INSERT INTO t1 VALUES (0, 0), (2, 1), (2, 3), (1, 1), (30, 20);
+SELECT a, b INTO @a, @b FROM t1 WHERE a=2 AND b=3 GROUP BY a, b;
+SELECT @a, @b;
+SELECT a, b FROM t1 WHERE a=2 AND b=3 GROUP BY a, b;
+DROP TABLE t1;
+
+--echo End of 5.0 tests

sql/item_func.cc

@@ -4151,6 +4151,41 @@ Item_func_set_user_var::check(bool use_result_field)
 }
 
 
+/**
+  @brief Evaluate and store item's result.
+  This function is invoked on "SELECT ... INTO @var ...".
+  
+  @param    item    An item to get value from.
+*/
+
+void Item_func_set_user_var::save_item_result(Item *item)
+{
+  DBUG_ENTER("Item_func_set_user_var::save_item_result");
+
+  switch (cached_result_type) {
+  case REAL_RESULT:
+    save_result.vreal= item->val_result();
+    break;
+  case INT_RESULT:
+    save_result.vint= item->val_int_result();
+    unsigned_flag= item->unsigned_flag;
+    break;
+  case STRING_RESULT:
+    save_result.vstr= item->str_result(&value);
+    break;
+  case DECIMAL_RESULT:
+    save_result.vdec= item->val_decimal_result(&decimal_buff);
+    break;
+  case ROW_RESULT:
+  default:
+    // Should never happen
+    DBUG_ASSERT(0);
+    break;
+  }
+  DBUG_VOID_RETURN;
+}
+
+
 /*
   This functions is invoked on SET @variable or @variable:= expression.
 

sql/item_func.h

@@ -1308,6 +1308,7 @@ public:
   bool send(Protocol *protocol, String *str_arg);
   void make_field(Send_field *tmp_field);
   bool check(bool use_result_field);
+  void save_item_result(Item *item);
   bool update();
   enum Item_result result_type () const { return cached_result_type; }
   bool fix_fields(THD *thd, Item **ref);

sql/sql_class.cc

@@ -2052,7 +2052,7 @@ bool select_dumpvar::send_data(List<Item> &items)
     {
       Item_func_set_user_var *suv= new Item_func_set_user_var(mv->s, item);
       suv->fix_fields(thd, 0);
-      suv->check(0);
+      suv->save_item_result(item);
       suv->update();
     }
   }