MWL#89: Cost-based choice between Materialization and IN->EXISTS transformation

- Corrected a wrong result that was recorded by the MySQL fix for BUG#39069.

- Removed Item_func_isnull::cached_value and all the logic around this custom-made
  caching of the NULL result because MWL#89 optimizes subqueries before the outer
  query is being executed, and this cache cannot be made easily to work for all
  kinds of Items (specifically Item_sum_sum, but others too).

mysql-test/r/subselect3.result

@@ -843,8 +843,8 @@ x	ROW(11, 12) = (SELECT MAX(x), 22)	ROW(11, 12) IN (SELECT MAX(x), 22)
 # 2nd and 3rd columns should be same for x == 11 only
 SELECT a AS x, ROW(11, 12) = (SELECT MAX(x), 12), ROW(11, 12) IN (SELECT MAX(x), 12) FROM t1;
 x	ROW(11, 12) = (SELECT MAX(x), 12)	ROW(11, 12) IN (SELECT MAX(x), 12)
-1	0	1
-2	0	1
+1	0	0
+2	0	0
 11	1	1
 DROP TABLE t1;
 # both columns should be same

sql/item_cmpfunc.cc

@@ -4631,12 +4631,6 @@ Item *and_expressions(Item *a, Item *b, Item **org_item)
 longlong Item_func_isnull::val_int()
 {
   DBUG_ASSERT(fixed == 1);
-  /*
-    Handle optimization if the argument can't be null
-    This has to be here because of the test in update_used_tables().
-  */
-  if (!used_tables_cache && !with_subselect)
-    return cached_value;
   return args[0]->is_null() ? 1: 0;
 }
 
@@ -4644,12 +4638,6 @@ longlong Item_is_not_null_test::val_int()
 {
   DBUG_ASSERT(fixed == 1);
   DBUG_ENTER("Item_is_not_null_test::val_int");
-  if (!used_tables_cache && !with_subselect)
-  {
-    owner->was_null|= (!cached_value);
-    DBUG_PRINT("info", ("cached: %ld", (long) cached_value));
-    DBUG_RETURN(cached_value);
-  }
   if (args[0]->is_null())
   {
     DBUG_PRINT("info", ("null"));
@@ -4666,19 +4654,9 @@ longlong Item_is_not_null_test::val_int()
 void Item_is_not_null_test::update_used_tables()
 {
   if (!args[0]->maybe_null)
-  {
     used_tables_cache= 0;			/* is always true */
-    cached_value= (longlong) 1;
-  }
   else
-  {
     args[0]->update_used_tables();
-    if (!(used_tables_cache=args[0]->used_tables()) && !with_subselect)
-    {
-      /* Remember if the value is always NULL or never NULL */
-      cached_value= (longlong) !args[0]->is_null();
-    }
-  }
 }
 
 

sql/item_cmpfunc.h

@@ -1304,8 +1304,6 @@ public:
 
 class Item_func_isnull :public Item_bool_func
 {
-protected:
-  longlong cached_value;
 public:
   Item_func_isnull(Item *a) :Item_bool_func(a) {}
   longlong val_int();
@@ -1323,18 +1321,9 @@ public:
     {
       used_tables_cache= 0;			/* is always false */
       const_item_cache= 1;
-      cached_value= (longlong) 0;
     }
     else
-    {
       args[0]->update_used_tables();
-      if ((const_item_cache= !(used_tables_cache= args[0]->used_tables())) &&
-          !with_subselect)
-      {
-	/* Remember if the value is always NULL or never NULL */
-	cached_value= (longlong) args[0]->is_null();
-      }
-    }
   }
   table_map not_null_tables() const { return 0; }
   optimize_type select_optimize() const { return OPTIMIZE_NULL; }

sql/opt_subselect.cc

@@ -3543,12 +3543,17 @@ bool JOIN::choose_subquery_plan()
                               select_lex->master_unit()->item;
 
   /* Always revert to IN->EXISTS. */
-  mat_strategy_cost= 1;
-  in_exists_strategy_cost= 0;
+  mat_strategy_cost= 0;
+  in_exists_strategy_cost= 1;
 
-  if (mat_strategy_cost < in_exists_strategy_cost)
+  /*
+    If (1) materialization is a possible strategy based on static analysis, and
+    (2) it is cheaper strategy than the IN->EXISTS transformation, then compute
+    in_subs via the materialization trategy.
+  */
+  if (in_subs->exec_method == Item_in_subselect::MATERIALIZATION && // 1
+      mat_strategy_cost < in_exists_strategy_cost)                  // 2
   {
-    in_subs->exec_method = Item_in_subselect::MATERIALIZATION;
     if (in_subs->setup_mat_engine())
     {
       /*