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

1 files changed, 25 insertions, 1 deletions

diff --git a/sql/item.cc b/sql/item.cc
index 010e4cb441a..d317b16a264 100644
--- a/sql/item.cc
+++ b/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];