BUG#16393: Let the 'ref' optimizer use ON condition of nested join to construct

'ref' accesses to all tables that are direct children of the nested join and are 
not inner wrt their siblings.

mysql-test/r/join_nested.result

@@ -960,7 +960,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	Using where
 1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	2	Using where
 1	SIMPLE	t4	ref	idx_b	idx_b	5	test.t2.b	2	Using where
-1	SIMPLE	t5	ALL	NULL	NULL	NULL	NULL	3	Using where
+1	SIMPLE	t5	ALL	idx_b	NULL	NULL	NULL	3	Using where
 1	SIMPLE	t7	ALL	NULL	NULL	NULL	NULL	2	Using where
 1	SIMPLE	t6	ALL	NULL	NULL	NULL	NULL	3	Using where
 1	SIMPLE	t8	ALL	NULL	NULL	NULL	NULL	2	Using where
@@ -1009,7 +1009,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	Using where
 1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	2	Using where
 1	SIMPLE	t4	ref	idx_b	idx_b	5	test.t2.b	2	Using where
-1	SIMPLE	t5	ALL	NULL	NULL	NULL	NULL	3	Using where
+1	SIMPLE	t5	ALL	idx_b	NULL	NULL	NULL	3	Using where
 1	SIMPLE	t7	ALL	NULL	NULL	NULL	NULL	2	Using where
 1	SIMPLE	t6	ALL	NULL	NULL	NULL	NULL	3	Using where
 1	SIMPLE	t8	ref	idx_b	idx_b	5	test.t5.b	2	Using where
@@ -1059,7 +1059,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	Using where
 1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	2	Using where
 1	SIMPLE	t4	ref	idx_b	idx_b	5	test.t2.b	2	Using where
-1	SIMPLE	t5	ALL	NULL	NULL	NULL	NULL	3	Using where
+1	SIMPLE	t5	ALL	idx_b	NULL	NULL	NULL	3	Using where
 1	SIMPLE	t7	ALL	NULL	NULL	NULL	NULL	2	Using where
 1	SIMPLE	t6	ALL	NULL	NULL	NULL	NULL	3	Using where
 1	SIMPLE	t8	ref	idx_b	idx_b	5	test.t5.b	2	Using where
@@ -1467,3 +1467,17 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t6	ref	a	a	5	test.t4.b	X	
 1	SIMPLE	t5	ref	a	a	5	test.t3.b	X	
 drop table t0, t1, t2, t3, t4, t5, t6, t7;
+create table t1 (a int);
+insert into t1 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t2 (a int, filler char(100), key(a));
+insert into t2 select A.a + 10*B.a, '' from t1 A, t1 B;
+create table t3 like t2;
+insert into t3 select * from t2;
+explain select * from t1 left join 
+(t2 left join t3 on (t2.a = t3.a)) 
+on (t1.a = t2.a);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	10	
+1	SIMPLE	t2	ref	a	a	5	test.t1.a	1	
+1	SIMPLE	t3	ref	a	a	5	test.t2.a	1	
+drop table t1, t2, t3;

mysql-test/t/join_nested.test

@@ -900,3 +900,17 @@ explain select * from t2 left join
    join t5 on t5.a=t3.b) on t3.a=t2.b;
 
 drop table t0, t1, t2, t3, t4, t5, t6, t7;
+
+# BUG#16393
+create table t1 (a int);
+insert into t1 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t2 (a int, filler char(100), key(a));
+insert into t2 select A.a + 10*B.a, '' from t1 A, t1 B;
+create table t3 like t2;
+insert into t3 select * from t2;
+
+explain select * from t1 left join 
+  (t2 left join t3 on (t2.a = t3.a)) 
+  on (t1.a = t2.a);
+drop table t1, t2, t3;
+

sql/sql_select.cc

@@ -2917,6 +2917,64 @@ sort_keyuse(KEYUSE *a,KEYUSE *b)
 }
 
 
+/*
+  Add to KEY_FIELD array all 'ref' access candidates within nested join
+
+  SYNPOSIS
+    add_key_fields_for_nj()
+      nested_join_table  IN     Nested join pseudo-table to process
+      end                INOUT  End of the key field array
+      and_level          INOUT  And-level
+    
+  DESCRIPTION
+    This function populates KEY_FIELD array with entries generated from the 
+    ON condition of the given nested join, and does the same for nested joins 
+    contained within this nested join.
+    
+  IMPLEMENTATION
+    We can add accesses to the tables that are direct children of this nested 
+    join (1), and are not inner tables w.r.t their neighbours (2).
+    
+    Example for #1 (outer brackets pair denotes nested join this function is 
+    invoked for):
+
+     ... LEFT JOIN (t1 LEFT JOIN (t2 ... ) ) ON cond
+     
+    Example for #2:
+
+     ... LEFT JOIN (t1 LEFT JOIN t2 ) ON cond
+
+    In examples 1-2 for condition cond, we can add 'ref' access candidates to 
+    t1 only.
+    
+    Example #3:
+     
+     ... LEFT JOIN (t1, t2 JOIN t3 ON inner_cond) ON cond
+    
+    Here we can add 'ref' access candidates for t1 and t2, but not for t3.
+    
+*/
+
+static void add_key_fields_for_nj(TABLE_LIST *nested_join_table,
+                                  KEY_FIELD **end, uint *and_level)
+{
+  List_iterator<TABLE_LIST> li(nested_join_table->nested_join->join_list);
+  table_map tables= 0;
+  TABLE_LIST *table;
+  DBUG_ASSERT(nested_join_table->nested_join);
+
+  while ((table= li++))
+  {
+    if (table->nested_join)
+      add_key_fields_for_nj(table, end, and_level);
+    else
+      if (!table->on_expr)
+	tables |= table->table->map;
+  }
+  add_key_fields(end, and_level, nested_join_table->on_expr, tables);
+}
+
+
 /*
   Update keyuse array with all possible keys we can use to fetch rows
   
@@ -2981,23 +3039,21 @@ update_ref_and_keys(THD *thd, DYNAMIC_ARRAY *keyuse,JOIN_TAB *join_tab,
       into account as well.
     */ 
     if (*join_tab[i].on_expr_ref)
-    {
       add_key_fields(&end,&and_level,*join_tab[i].on_expr_ref,
 		     join_tab[i].table->map);
-    }
-    else 
+  }
+
+  /* Process ON conditions for the nested joins */
+  {
+    List_iterator<TABLE_LIST> li(*join_tab->join->join_list);
+    TABLE_LIST *table;
+    while ((table= li++))
     {
-      TABLE_LIST *tab= join_tab[i].table->pos_in_table_list;
-      TABLE_LIST *embedding= tab->embedding;
-      if (embedding)
-      {
-        NESTED_JOIN *nested_join= embedding->nested_join;
-        if (nested_join->join_list.head() == tab)
-          add_key_fields(&end, &and_level, embedding->on_expr,
-                         nested_join->used_tables);
-      }
+      if (table->nested_join)
+	add_key_fields_for_nj(table, &end, &and_level);
     }
   }
+
   /* fill keyuse with found key parts */
   for ( ; field != end ; field++)
     add_key_part(keyuse,field);