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

Added missing logic to handle the case when subquery tables are optimized
away early during optimization.

mysql-test/include/subselect_mat_cost.inc

@@ -123,13 +123,13 @@ select a1 from t1 where a1 in (select b1 from t2 where b1 = b2 and b2 = '1 - 03'
 -- echo /* E.5 opt_sum_query detects no matching min/max row or substitutes MIN/MAX with a const. */
 -- echo TODO this test produces wrong result due to missing logic to handle the case
 -- echo when JOIN::optimize detects an empty subquery result.
-#explain
-#select a1 from t1 where a1 in (select max(b1) from t2);
-#select a1 from t1 where a1 in (select max(b1) from t2);
+explain
+select a1 from t1 where a1 in (select max(b1) from t2);
+select a1 from t1 where a1 in (select max(b1) from t2);
 -- echo
-#explain
-#select a1 from t1 where a1 in (select max(b1) from t2 where b1 = '7 - 02');
-#select a1 from t1 where a1 in (select max(b1) from t2 where b1 = '7 - 02');
+explain
+select a1 from t1 where a1 in (select max(b1) from t2 where b1 = '7 - 02');
+select a1 from t1 where a1 in (select max(b1) from t2 where b1 = '7 - 02');
 
 -- echo /* E.6 make_join_select detects impossible WHERE. *
 

sql/item_subselect.cc

@@ -36,7 +36,7 @@ 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), 
-  inside_first_fix_fields(0), done_first_fix_fields(FALSE),
+  inside_first_fix_fields(0), done_first_fix_fields(FALSE), forced_const(FALSE),
   eliminated(FALSE), 
   engine_changed(0), changed(0), is_correlated(FALSE)
 {
@@ -121,6 +121,7 @@ void Item_subselect::cleanup()
   depends_on.empty();
   reset();
   value_assigned= 0;
+  forced_const= FALSE;
   DBUG_VOID_RETURN;
 }
 
@@ -158,7 +159,6 @@ void Item_in_subselect::cleanup()
     left_expr_cache= NULL;
   }
   first_execution= TRUE;
-  is_constant= FALSE;
   if (in_strategy & SUBS_MATERIALIZATION)
     in_strategy= 0;
   pushed_cond_guards= NULL;
@@ -682,6 +682,8 @@ Item *Item_subselect::get_tmp_table_item(THD *thd_arg)
 
 void Item_subselect::update_used_tables()
 {
+  if (!forced_const)
+  {
     recalc_used_tables(parent_select, FALSE);
     if (!engine->uncacheable())
     {
@@ -689,6 +691,7 @@ void Item_subselect::update_used_tables()
       if (!(used_tables_cache & ~engine->upper_select_const_tables()))
         const_item_cache= 1;
     }
+  }
 }
 
 
@@ -753,7 +756,7 @@ Item_maxmin_subselect::Item_maxmin_subselect(THD *thd_param,
     of Items belonged to subquery, which will be not repeated
   */
   used_tables_cache= parent->get_used_tables_cache();
-  const_item_cache= parent->get_const_item_cache();
+  const_item_cache= parent->const_item();
 
   /*
     this subquery always creates during preparation, so we can assign
@@ -791,8 +794,7 @@ void Item_maxmin_subselect::print(String *str, enum_query_type query_type)
 
 void Item_singlerow_subselect::reset()
 {
-  eliminated= FALSE;
-  null_value= 1;
+  Item_subselect::reset();
   if (value)
     value->null_value= 1;
 }
@@ -1082,8 +1084,7 @@ bool Item_in_subselect::test_limit(st_select_lex_unit *unit_arg)
 Item_in_subselect::Item_in_subselect(Item * left_exp,
 				     st_select_lex *select_lex):
   Item_exists_subselect(), left_expr_cache(0), first_execution(TRUE),
-  is_constant(FALSE), optimizer(0), pushed_cond_guards(NULL),
-  in_strategy(0), upper_item(0)
+  optimizer(0), pushed_cond_guards(NULL), in_strategy(0), upper_item(0)
 {
   DBUG_ENTER("Item_in_subselect::Item_in_subselect");
   left_expr= left_exp;
@@ -1313,7 +1314,7 @@ bool Item_in_subselect::val_bool()
 {
   DBUG_ASSERT(fixed == 1);
   null_value= 0;
-  if (is_constant)
+  if (forced_const)
     return value;
   if (exec())
   {
@@ -4147,11 +4148,10 @@ int subselect_hash_sj_engine::exec()
   tmp_table->file->info(HA_STATUS_VARIABLE);
   if (!tmp_table->file->stats.records)
   {
-    item_in->value= FALSE;
     /* The value of IN will not change during this execution. */
-    item_in->is_constant= TRUE;
+    item_in->reset();
+    item_in->make_const();
     item_in->set_first_execution();
-    /* TIMOUR: check if we need this: item_in->null_value= FALSE; */
     DBUG_RETURN(FALSE);
   }
 

sql/item_subselect.h

@@ -70,6 +70,13 @@ class Item_subselect :public Item_result_field
   
   bool inside_first_fix_fields;
   bool done_first_fix_fields;
+  /*
+    Set to TRUE if at optimization or execution time we determine that this
+    item's value is a constant. We need this member because it is not possible
+    to substitute 'this' with a constant item.
+  */
+  bool forced_const;
+
 public:
   /* A reference from inside subquery predicate to somewhere outside of it */
   class Ref_to_outside : public Sql_alloc
@@ -154,12 +161,21 @@ class Item_subselect :public Item_result_field
   void fix_after_pullout(st_select_lex *new_parent, Item **ref);
   void recalc_used_tables(st_select_lex *new_parent, bool after_pullout);
   virtual bool exec();
+  /*
+    If subquery optimization or execution determines that the subquery has
+    an empty result, mark the subquery predicate as a constant value.
+  */
+  void make_const()
+  { 
+    used_tables_cache= 0;
+    const_item_cache= 0;
+    forced_const= TRUE; 
+  }
   virtual void fix_length_and_dec();
   table_map used_tables() const;
   table_map not_null_tables() const { return 0; }
   bool const_item() const;
   inline table_map get_used_tables_cache() { return used_tables_cache; }
-  inline bool get_const_item_cache() { return const_item_cache; }
   Item *get_tmp_table_item(THD *thd);
   void update_used_tables();
   virtual void print(String *str, enum_query_type query_type);
@@ -353,12 +369,6 @@ class Item_in_subselect :public Item_exists_subselect
   */
   List<Cached_item> *left_expr_cache;
   bool first_execution;
-  /*
-    Set to TRUE if at query execution time we determine that this item's
-    value is a constant during this execution. We need this member because
-    it is not possible to substitute 'this' with a constant item.
-  */
-  bool is_constant;
 
   /*
     expr & optimizer used in subselect rewriting to store Item for
@@ -435,7 +445,7 @@ class Item_in_subselect :public Item_exists_subselect
   Item_in_subselect(Item * left_expr, st_select_lex *select_lex);
   Item_in_subselect()
     :Item_exists_subselect(), left_expr_cache(0), first_execution(TRUE),
-    is_constant(FALSE), optimizer(0), abort_on_null(0),
+    optimizer(0), abort_on_null(0),
     pushed_cond_guards(NULL), func(NULL), in_strategy(0),
     upper_item(0)
     {}

sql/opt_subselect.cc

@@ -3823,3 +3823,65 @@ bool JOIN::choose_subquery_plan(table_map join_tables)
 
   return FALSE;
 }
+
+
+/**
+  Choose a query plan for a table-less subquery.
+
+  @notes
+
+  @retval FALSE     success.
+  @retval TRUE      error occurred.
+*/
+
+bool JOIN::choose_tableless_subquery_plan()
+{
+  DBUG_ASSERT(!tables_list || !tables);
+  if (select_lex->master_unit()->item)
+  {
+    DBUG_ASSERT(select_lex->master_unit()->item->type() ==
+                Item::SUBSELECT_ITEM);
+    Item_subselect *subs_predicate= select_lex->master_unit()->item;
+
+    /*
+      If the optimizer determined that his query has an empty result,
+      in most cases the subquery predicate is a known constant value -
+      either FALSE or NULL. The implementation of Item_subselect::reset()
+      determines which one.
+    */
+    if (zero_result_cause)
+    {
+      if (!implicit_grouping)
+      {
+        /*
+          Both group by queries and non-group by queries without aggregate
+          functions produce empty subquery result.
+        */
+        subs_predicate->reset();
+        subs_predicate->make_const();
+        return FALSE;
+      }
+
+      /* TODO:
+         A further optimization is possible when a non-group query with
+         MIN/MAX/COUNT is optimized by opt_sum_query. Then, if there are
+         only MIN/MAX functions over an empty result set, the subquery
+         result is a NULL value/row, thus the value of subs_predicate is
+         NULL.
+      */
+    }
+
+    if (subs_predicate->is_in_predicate())
+    {
+      Item_in_subselect *in_subs;
+      in_subs= (Item_in_subselect*) subs_predicate;
+      in_subs->in_strategy= SUBS_IN_TO_EXISTS;
+      if (in_subs->create_in_to_exists_cond(this) ||
+          in_subs->inject_in_to_exists_cond(this))
+        return TRUE;
+      tmp_having= having;
+    }
+  }
+  return FALSE;
+}
+

sql/sql_select.cc

@@ -829,6 +829,7 @@ JOIN::optimize()
                            "Impossible HAVING" : "Impossible WHERE";
       tables= 0;
       error= 0;
+      choose_tableless_subquery_plan();
       goto setup_subq_exit;
     }
   }
@@ -873,12 +874,13 @@ JOIN::optimize()
     */
     if ((res=opt_sum_query(select_lex->leaf_tables, all_fields, conds)))
     {
-      if (res == HA_ERR_KEY_NOT_FOUND)
+      if (res == HA_ERR_KEY_NOT_FOUND || res < 0)
       {
         DBUG_PRINT("info",("No matching min/max row"));
 	zero_result_cause= "No matching min/max row";
         tables= 0;
 	error=0;
+        choose_tableless_subquery_plan();
         goto setup_subq_exit;
       }
       if (res > 1)
@@ -887,14 +889,7 @@ JOIN::optimize()
         DBUG_PRINT("error",("Error from opt_sum_query"));
         DBUG_RETURN(1);
       }
-      if (res < 0)
-      {
-        DBUG_PRINT("info",("No matching min/max row"));
-        zero_result_cause= "No matching min/max row";
-        tables= 0;
-        error=0;
-        goto setup_subq_exit;
-      }
+
       DBUG_PRINT("info",("Select tables optimized away"));
       zero_result_cause= "Select tables optimized away";
       tables_list= 0;				// All tables resolved
@@ -919,40 +914,14 @@ JOIN::optimize()
                                  QT_ORDINARY););
         conds= table_independent_conds;
       }
-      goto setup_subq_exit;
     }
   }
   if (!tables_list)
   {
     DBUG_PRINT("info",("No tables"));
     error= 0;
-    if (optimize_unflattened_subqueries())
-      DBUG_RETURN(1);
-    /*
-      TIMOUR: TODO: consider do we need to optimize here at all and refactor
-      this block and JOIN::choose_subquery_plan.
-
-    if (choose_subquery_plan())
-      DBUG_RETURN(1);
-    */
-    Item_in_subselect *in_subs;
-    if (select_lex->master_unit()->item &&
-        select_lex->master_unit()->item->is_in_predicate())
-    {
-      in_subs= (Item_in_subselect*) select_lex->master_unit()->item;
-      if (in_subs->in_strategy & SUBS_MATERIALIZATION &&
-          in_subs->setup_mat_engine())
-        in_subs->in_strategy= SUBS_IN_TO_EXISTS;
-      if (in_subs->in_strategy & SUBS_IN_TO_EXISTS)
-      {
-        if (in_subs->create_in_to_exists_cond(this))
-          DBUG_RETURN(1);
-        if (in_subs->inject_in_to_exists_cond(this))
-          DBUG_RETURN(1);
-        tmp_having= having;
-      }
-    }
-    DBUG_RETURN(0);
+    choose_tableless_subquery_plan();
+    goto setup_subq_exit;
   }
   error= -1;					// Error is sent to client
   sort_by_table= get_sort_by_table(order, group_list, select_lex->leaf_tables);

sql/sql_select.h

@@ -1383,6 +1383,8 @@ class JOIN :public Sql_alloc
   void restore_query_plan(DYNAMIC_ARRAY *save_keyuse, POSITION *save_positions,
                       KEYUSE **save_join_tab_keyuse,
                       key_map *save_join_tab_checked_keys);
+  /* Choose a subquery plan for a table-less subquery. */
+  bool choose_tableless_subquery_plan();
 
 public:
   JOIN_TAB *join_tab,**best_ref;