Fix for bug lp:944706, task MDEV-193

The patch enables back constant subquery execution during
query optimization after it was disabled during the development
of MWL#89 (cost-based choice of IN-TO-EXISTS vs MATERIALIZATION).

The main idea is that constant subqueries are allowed to be executed
during optimization if their execution is not expensive.

The approach is as follows:
- Constant subqueries are recursively optimized in the beginning of
  JOIN::optimize of the outer query. This is done by the new method
  JOIN::optimize_constant_subqueries(). This is done so that the cost
  of executing these queries can be estimated.
- Optimization of the outer query proceeds normally. During this phase
  the optimizer may request execution of non-expensive constant subqueries.
  Each place where the optimizer may potentially execute an expensive
  expression is guarded with the predicate Item::is_expensive().
- The implementation of Item_subselect::is_expensive has been extended
  to use the number of examined rows (estimated by the optimizer) as a
  way to determine whether the subquery is expensive or not.
- The new system variable "expensive_subquery_limit" controls how many
  examined rows are considered to be not expensive. The default is 100.

In addition, multiple changes were needed to make this solution work
in the light of the changes made by MWL#89. These changes were needed
to fix various crashes and wrong results, and legacy bugs discovered
during development.

1 files changed, 6 insertions, 2 deletions

diff --git a/sql/item_subselect.h b/sql/item_subselect.h
index 1a4c8a25a01..746813407c2 100644
--- a/sql/item_subselect.h
+++ b/sql/item_subselect.h
@@ -209,7 +209,7 @@ public:
   */
   bool is_evaluated() const;
   bool is_uncacheable() const;
-  bool is_expensive() { return TRUE; }
+  bool is_expensive();
 
   /*
     Used by max/min subquery to initialize value presence registration
@@ -235,7 +235,7 @@ public:
     @retval TRUE  if the predicate is expensive
     @retval FALSE otherwise
   */
-  bool is_expensive_processor(uchar *arg) { return TRUE; }
+  bool is_expensive_processor(uchar *arg) { return is_expensive(); }
 
   /**
     Get the SELECT_LEX structure associated with this Item.
@@ -581,6 +581,10 @@ public:
   bool fix_fields(THD *thd, Item **ref);
   void fix_length_and_dec();
   void fix_after_pullout(st_select_lex *new_parent, Item **ref);
+  bool const_item() const
+  {
+    return Item_subselect::const_item() && left_expr->const_item();
+  }
   void update_used_tables();
   bool setup_mat_engine();
   bool init_left_expr_cache();