MWL#17: Table elimination

- Do not show eliminated tables in the output of EXPLAIN EXTENDED

mysql-test/r/table_elim.result

@@ -10,6 +10,11 @@ as select a, a as b from t1 where a in (1,3);
 explain select t1.a from t1 left join t2 on t2.a=t1.a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+explain extended select t1.a from t1 left join t2 on t2.a=t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	100.00	
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where 1
 select t1.a from t1 left join t2 on t2.a=t1.a;
 a
 0
@@ -45,14 +50,16 @@ explain select t1.a from t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
 # Elimination when done within an outer join nest:
-explain
+explain extended
 select t0.*
 from
 t0 left join (t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
 t3.a=t1.a) on t0.a=t1.a;
-id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	4	
-1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
+1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	4	100.00	
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	100.00	
+Warnings:
+Note	1003	select `test`.`t0`.`a` AS `a` from `test`.`t0` left join (`test`.`t1`) on((`test`.`t0`.`a` = `test`.`t1`.`a`)) where 1
 # Elimination with aggregate functions
 explain select count(*) from t1 left join t2 on t2.a=t1.a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

mysql-test/t/table_elim.test

@@ -17,6 +17,7 @@ create table t3 (a int primary key, b int)
 
 --echo # This will be  eliminated:
 explain select t1.a from t1 left join t2 on t2.a=t1.a;
+explain extended select t1.a from t1 left join t2 on t2.a=t1.a;
 
 select t1.a from t1 left join t2 on t2.a=t1.a;
 
@@ -39,7 +40,7 @@ explain select t1.a from t1 left join (t2 join t3) on t2.a=t1.a and t3.a=t1.a;
 explain select t1.a from t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and t3.a=t1.a;
 
 --echo # Elimination when done within an outer join nest:
-explain
+explain extended
 select t0.*
 from
   t0 left join (t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and

sql/sql_select.cc

@@ -2386,6 +2386,10 @@ mysql_select(THD *thd, Item ***rref_pointer_array,
   }
   else
   {
+    // psergey{
+    if (select_options & SELECT_DESCRIBE)
+      free_join= 0;
+    // }psergey
     if (!(join= new JOIN(thd, fields, select_options, result)))
 	DBUG_RETURN(TRUE);
     thd_proc_info(thd, "init");
@@ -2523,7 +2527,7 @@ static void mark_table_as_eliminated(JOIN *join, TABLE *table, uint *const_tbl_c
   {
     DBUG_PRINT("info", ("Eliminated table %s", table->alias));
     tab->type= JT_CONST;
-    tab->eliminated= TRUE;
+    join->eliminated_tables |= table->map;
     *const_tables |= table->map;
     join->const_table_map|= table->map;
     set_position(join, (*const_tbl_count)++, tab, (KEYUSE*)0);
@@ -2726,6 +2730,10 @@ static void eliminate_tables(JOIN *join, uint *const_tbl_count, table_map *const
   Item *item;
   table_map used_tables;
   DBUG_ENTER("eliminate_tables");
+  
+  join->eliminated_tables= 0;
+  
+  /* MWL#17 is only about outer join elimination, so: */
   if (!join->outer_join)
     DBUG_VOID_RETURN;
 
@@ -6060,6 +6068,7 @@ JOIN::make_simple_join(JOIN *parent, TABLE *tmp_table)
   tables= 1;
   const_tables= 0;
   const_table_map= 0;
+  eliminated_tables= 0;
   tmp_table_param.field_count= tmp_table_param.sum_func_count=
     tmp_table_param.func_count= 0;
   tmp_table_param.copy_field= tmp_table_param.copy_field_end=0;
@@ -16509,7 +16518,7 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
       quick_type= -1;
 
       //psergey-todo:
-      if (tab->eliminated)
+      if (table->map & join->eliminated_tables)
       {
         used_tables|=table->map;
         continue;
@@ -16912,6 +16921,7 @@ bool mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
 */
 
 static void print_join(THD *thd,
+                       table_map eliminated_tables,
                        String *str,
                        List<TABLE_LIST> *tables,
                        enum_query_type query_type)
@@ -16927,12 +16937,22 @@ static void print_join(THD *thd,
     *t= ti++;
 
   DBUG_ASSERT(tables->elements >= 1);
-  (*table)->print(thd, str, query_type);
+  //pserey:TODO check!
+  (*table)->print(thd, eliminated_tables, str, query_type);
 
   TABLE_LIST **end= table + tables->elements;
   for (TABLE_LIST **tbl= table + 1; tbl < end; tbl++)
   {
     TABLE_LIST *curr= *tbl;
+    // psergey-todo-todo: 
+    //  base table: check 
+    if (curr->table && (curr->table->map & eliminated_tables) ||
+        curr->nested_join && !(curr->nested_join->used_tables &
+                               ~eliminated_tables))
+    {
+      continue;
+    }
+
     if (curr->outer_join)
     {
       /* MySQL converts right to left joins */
@@ -16942,7 +16962,7 @@ static void print_join(THD *thd,
       str->append(STRING_WITH_LEN(" straight_join "));
     else
       str->append(STRING_WITH_LEN(" join "));
-    curr->print(thd, str, query_type);
+    curr->print(thd, eliminated_tables, str, query_type);
     if (curr->on_expr)
     {
       str->append(STRING_WITH_LEN(" on("));
@@ -16996,12 +17016,13 @@ Index_hint::print(THD *thd, String *str)
   @param str   string where table should be printed
 */
 
-void TABLE_LIST::print(THD *thd, String *str, enum_query_type query_type)
+void TABLE_LIST::print(THD *thd, table_map eliminated_tables, String *str, 
+                       enum_query_type query_type)
 {
   if (nested_join)
   {
     str->append('(');
-    print_join(thd, str, &nested_join->join_list, query_type);
+    print_join(thd, eliminated_tables, str, &nested_join->join_list, query_type);
     str->append(')');
   }
   else
@@ -17143,7 +17164,7 @@ void st_select_lex::print(THD *thd, String *str, enum_query_type query_type)
   {
     str->append(STRING_WITH_LEN(" from "));
     /* go through join tree */
-    print_join(thd, str, &top_join_list, query_type);
+    print_join(thd, join->eliminated_tables, str, &top_join_list, query_type);
   }
   else if (where)
   {

sql/sql_select.h

@@ -211,9 +211,6 @@ typedef struct st_join_table {
   /** Bitmap of nested joins this table is part of */
   nested_join_map embedding_map;
   
-  //psergey-todo: more justified place
-  bool eliminated;
-
   void cleanup();
   inline bool is_using_loose_index_scan()
   {
@@ -289,6 +286,8 @@ public:
   */
   bool     resume_nested_loop;
   table_map const_table_map,found_const_table_map;
+
+  table_map eliminated_tables;
   /*
      Bitmap of all inner tables from outer joins
   */

sql/table.h

@@ -1366,7 +1366,8 @@ struct TABLE_LIST
     return (derived || view || schema_table || (create && !table->db_stat) ||
             !table);
   }
-  void print(THD *thd, String *str, enum_query_type query_type);
+  void print(THD *thd, table_map eliminated_tables, String *str, 
+             enum_query_type query_type);
   bool check_single_table(TABLE_LIST **table, table_map map,
                           TABLE_LIST *view);
   bool set_insert_values(MEM_ROOT *mem_root);