MWL#17: Table elimination

- Make elimination check to be able detect cases like  t.primary_key_col1=othertbl.col AND t.primary_key_col2=func(t.primary_key_col1).
  These are needed to handle e.g. the case of func() being a correlated subquery that selects the latest value.
- If we've removed a condition with subquery predicate, EXPLAIN [EXTENDED] won't show the subquery anymore

sql/item.cc:
  MWL#17: Table elimination
  - Add tem_field::check_column_usage_processor(). it allows to check which key parts a condition depends on.
sql/item.h:
  MWL#17: Table elimination
  - Add tem_field::check_column_usage_processor(). it allows to check which key parts a condition depends on.
sql/item_subselect.cc:
  MWL#17: Table elimination
  - Item_subselect got 'eliminated' attribute. It is used only to determine if the subselect should be printed by EXPLAIN.
  - Item_subselect got List<Item> refers_to - a list of item in the current select that are referred to from within the subselect.
  - Added Item_*::check_column_usage_processor(). it allows to check which key parts a condition depends on.
  - Added a comment about possible problem in Item_subselect::walk
sql/item_subselect.h:
  MWL#17: Table elimination
  - Item_subselect got 'eliminated' attribute. It is used only to determine if the subselect should be printed by EXPLAIN.
  - Item_subselect got List<Item> refers_to - a list of item in the current select that are referred to from within the subselect.
  - Added Item_*::check_column_usage_processor(). it allows to check which key parts a condition depends on.
sql/item_sum.cc:
  MWL#17: Table elimination
sql/sql_lex.cc:
  MWL#17: Table elimination
sql/sql_lex.h:
  MWL#17: Table elimination
sql/sql_select.h:
  MWL#17: Table elimination

sql/item.cc

@@ -1915,6 +1915,30 @@ void Item_field::reset_field(Field *f)
   name= (char*) f->field_name;
 }
 
+bool Item_field::check_column_usage_processor(uchar *arg)
+{
+  Field_processor_info* info=(Field_processor_info*)arg;
+  if (used_tables() & ~info->allowed_tables)
+    return FALSE;
+
+  if (field->table == info->table)
+  {
+    if (!(field->part_of_key.is_set(info->keyno)))
+       return TRUE;
+
+    KEY *key= &field->table->key_info[info->keyno];
+    for (uint part= 0; part < key->key_parts; part++)
+    {
+      if (field->field_index == key->key_part[part].field->field_index)
+      {
+        info->needed_key_parts |= key_part_map(1) << part;
+        break;
+      }
+    }
+  }
+  return FALSE;
+}
+
 const char *Item_ident::full_name() const
 {
   char *tmp;
@@ -3380,7 +3404,7 @@ static void mark_as_dependent(THD *thd, SELECT_LEX *last, SELECT_LEX *current,
   /* store pointer on SELECT_LEX from which item is dependent */
   if (mark_item)
     mark_item->depended_from= last;
-  current->mark_as_dependent(last);
+  current->mark_as_dependent(last, resolved_item);
   if (thd->lex->describe & DESCRIBE_EXTENDED)
   {
     char warn_buff[MYSQL_ERRMSG_SIZE];

sql/item.h

@@ -888,6 +888,8 @@ public:
   virtual bool reset_query_id_processor(uchar *query_id_arg) { return 0; }
   virtual bool is_expensive_processor(uchar *arg) { return 0; }
   virtual bool register_field_in_read_map(uchar *arg) { return 0; }
+  virtual bool check_column_usage_processor(uchar *arg) { return 0; }
+  virtual bool mark_as_eliminated_processor(uchar *arg) { return 0; }
   /*
     Check if a partition function is allowed
     SYNOPSIS
@@ -1012,6 +1014,14 @@ public:
 };
 
 
+typedef struct  
+{
+  table_map allowed_tables;
+  TABLE *table;
+  uint keyno;
+  uint needed_key_parts;
+} Field_processor_info;
+
 class sp_head;
 
 
@@ -1477,6 +1487,7 @@ public:
   bool find_item_in_field_list_processor(uchar *arg);
   bool register_field_in_read_map(uchar *arg);
   bool check_partition_func_processor(uchar *int_arg) {return FALSE;}
+  bool check_column_usage_processor(uchar *arg);
   void cleanup();
   bool result_as_longlong()
   {

sql/item_subselect.cc

@@ -39,7 +39,7 @@ inline Item * and_items(Item* cond, Item *item)
 Item_subselect::Item_subselect():
   Item_result_field(), value_assigned(0), thd(0), substitution(0),
   engine(0), old_engine(0), used_tables_cache(0), have_to_be_excluded(0),
-  const_item_cache(1), engine_changed(0), changed(0), is_correlated(FALSE)
+  const_item_cache(1), in_fix_fields(0), engine_changed(0), changed(0), is_correlated(FALSE)
 {
   with_subselect= 1;
   reset();
@@ -151,10 +151,14 @@ bool Item_subselect::fix_fields(THD *thd_param, Item **ref)
 
   DBUG_ASSERT(fixed == 0);
   engine->set_thd((thd= thd_param));
+  if (!in_fix_fields)
+    refers_to.empty();
+  eliminated= FALSE;
 
   if (check_stack_overrun(thd, STACK_MIN_SIZE, (uchar*)&res))
     return TRUE;
   
+  in_fix_fields++;
   res= engine->prepare();
 
   // all transformation is done (used by prepared statements)
@@ -181,12 +185,14 @@ bool Item_subselect::fix_fields(THD *thd_param, Item **ref)
       if (!(*ref)->fixed)
 	ret= (*ref)->fix_fields(thd, ref);
       thd->where= save_where;
+      in_fix_fields--;
       return ret;
     }
     // Is it one field subselect?
     if (engine->cols() > max_columns)
     {
       my_error(ER_OPERAND_COLUMNS, MYF(0), 1);
+      in_fix_fields--;
       return TRUE;
     }
     fix_length_and_dec();
@@ -203,11 +209,30 @@ bool Item_subselect::fix_fields(THD *thd_param, Item **ref)
   fixed= 1;
 
 err:
+  in_fix_fields--;
   thd->where= save_where;
   return res;
 }
 
 
+bool Item_subselect::check_column_usage_processor(uchar *arg)
+{
+  List_iterator<Item> it(refers_to);
+  Item *item;
+  while ((item= it++))
+  {
+    if (item->walk(&Item::check_column_usage_processor,FALSE, arg))
+      return TRUE;
+  }
+  return FALSE;
+}
+
+bool Item_subselect::mark_as_eliminated_processor(uchar *arg)
+{
+  eliminated= TRUE;
+  return FALSE;
+}
+
 bool Item_subselect::walk(Item_processor processor, bool walk_subquery,
                           uchar *argument)
 {
@@ -225,6 +250,7 @@ bool Item_subselect::walk(Item_processor processor, bool walk_subquery,
       if (lex->having && (lex->having)->walk(processor, walk_subquery,
                                              argument))
         return 1;
+      /* TODO: why doesn't this walk the OUTER JOINs' ON expressions */
 
       while ((item=li++))
       {

sql/item_subselect.h

@@ -54,6 +54,14 @@ protected:
   bool const_item_cache;
   
 public:
+  /* 
+    References from inside the subquery to the select that this predicate is
+    in.  References to parent selects not included.
+  */
+  List<Item> refers_to;
+  int in_fix_fields;
+  bool eliminated;
+  
   /* changed engine indicator */
   bool engine_changed;
   /* subquery is transformed */
@@ -126,6 +134,8 @@ public:
   virtual void reset_value_registration() {}
   enum_parsing_place place() { return parsing_place; }
   bool walk(Item_processor processor, bool walk_subquery, uchar *arg);
+  bool mark_as_eliminated_processor(uchar *arg);
+  bool check_column_usage_processor(uchar *arg);
 
   /**
     Get the SELECT_LEX structure associated with this Item.

sql/item_sum.cc

@@ -350,7 +350,7 @@ bool Item_sum::register_sum_func(THD *thd, Item **ref)
          sl= sl->master_unit()->outer_select() )
       sl->master_unit()->item->with_sum_func= 1;
   }
-  thd->lex->current_select->mark_as_dependent(aggr_sel);
+  thd->lex->current_select->mark_as_dependent(aggr_sel, NULL);
   return FALSE;
 }
 

sql/sql_lex.cc

@@ -1778,7 +1778,7 @@ void st_select_lex_unit::exclude_tree()
     'last' should be reachable from this st_select_lex_node
 */
 
-void st_select_lex::mark_as_dependent(st_select_lex *last)
+void st_select_lex::mark_as_dependent(st_select_lex *last, Item *dependency)
 {
   /*
     Mark all selects from resolved to 1 before select where was
@@ -1804,6 +1804,8 @@ void st_select_lex::mark_as_dependent(st_select_lex *last)
     }
   is_correlated= TRUE;
   this->master_unit()->item->is_correlated= TRUE;
+  if (dependency)
+    this->master_unit()->item->refers_to.push_back(dependency);
 }
 
 bool st_select_lex_node::set_braces(bool value)      { return 1; }

sql/sql_lex.h

@@ -743,7 +743,7 @@ public:
     return master_unit()->return_after_parsing();
   }
 
-  void mark_as_dependent(st_select_lex *last);
+  void mark_as_dependent(st_select_lex *last, Item *dependency);
 
   bool set_braces(bool value);
   bool inc_in_sum_expr();

sql/sql_select.h

@@ -51,6 +51,7 @@ typedef struct keyuse_t {
     NULL  - Otherwise (the source equality can't be turned off)
   */
   bool *cond_guard;
+  bool usable;
 } KEYUSE;
 
 class store_key;