MDEV-37713/MDEV-37714 Fold boolean literals in SELECT-list

The order of evaluation of the expressions that appear in a SELECT-list
is undefined. This change exploits this fact by recursively folding
TRUE/FALSE literals in OR/AND expressions which may allow for skipping
evaluation of some parts of the expression or even the whole expression.

Author: jaeheonshim

sql/item_cmpfunc.cc

@@ -5736,6 +5736,45 @@ bool Item_cond::excl_dep_on_grouping_fields(st_select_lex *sel)
   return true;
 }
 
+Item *Item_cond::simplify_cond(THD *thd)
+{
+  List_iterator<Item> li(list);
+  Item *child;
+  while ((child= li++))
+  {
+    if (child->type() == Item::COND_ITEM)
+    {
+      Item *new_child= static_cast<Item_cond *>(child)->simplify_cond(thd);
+      if (new_child != child)
+        li.replace(new_child);
+    }
+  }
+  bool is_and= functype() == Item_func::COND_AND_FUNC;
+  bool is_or= functype() == Item_func::COND_OR_FUNC;
+  if (is_and || is_or)
+  {
+    List_iterator<Item> li2(list);
+    while ((child= li2++))
+    {
+      Item *real= child->real_item();
+      if (real->is_bool_literal())
+      {
+        bool v= real->val_bool();
+        if (is_and && v)
+          li2.remove();
+        if (is_and && !v)
+          return new (thd->mem_root) Item_bool(thd, false);
+        if (is_or && v)
+          return new (thd->mem_root) Item_bool(thd, true);
+        if (is_or && !v)
+          li2.remove();
+      }
+    }
+    if (list.is_empty())
+      return new (thd->mem_root) Item_bool(thd, false);
+  }
+  return this;
+}
 
 void Item_cond_and::mark_as_condition_AND_part(TABLE_LIST *embedding)
 {

sql/item_cmpfunc.h

@@ -3450,6 +3450,7 @@ class Item_cond :public Item_bool_func
   Item *deep_copy(THD *thd) const override;
   bool excl_dep_on_table(table_map tab_map) override;
   bool excl_dep_on_grouping_fields(st_select_lex *sel) override;
+  Item *simplify_cond(THD *thd);
 
 private:
   void merge_sub_condition(List_iterator<Item>& li);

sql/sql_base.cc

@@ -8223,6 +8223,24 @@ bool setup_fields(THD *thd, Ref_ptr_array ref_pointer_array,
       DBUG_RETURN(TRUE); /* purecov: inspected */
     }
     item= *(it.ref()); // Item might have changed in fix_fields()
+
+    /*
+      If item is a SELECT-list COND_ITEM, rewrite it on the first time this
+      query is optimized to fold boolean expressions
+    */
+    if (thd->lex->current_select->context_analysis_place == SELECT_LIST &&
+        item->type() == Item::COND_ITEM &&
+        thd->lex->current_select->first_cond_optimization)
+    {
+      Query_arena_stmt on_stmt_arena(thd);
+      Item *new_item= static_cast<Item_cond *>(item)->simplify_cond(thd);
+      if (new_item != item)
+      {
+        new_item->share_name_with(item);
+        it.replace(new_item);
+        item= new_item;
+      }
+    }
     if (!ref.is_null())
     {
       ref[0]= item;