Simplified code in generate_derived_keys() and when using pos_in_tables

Added comments that not used keys of derivied tables will be deleted.
Added some comments about checking if pos_in_table_list is 0.

Other things:
- Added a marker (DBTYPE_IN_PREDICATE) in TABLE_LIST->derived_type
  to indicate that the table was generated from IN (list). This is
  useful for debugging and can later be used by explain if needed.
- Removed a not needed test of table->pos_in_table_list as it should
  always be valid at this point in time.

sql/sql_select.cc

@@ -4266,7 +4266,6 @@ JOIN::add_sorting_to_table(JOIN_TAB *tab, ORDER *order)
 
   TABLE *table= tab->table;
   if ((tab == join_tab + const_tables) &&
-       table->pos_in_table_list &&
        table->pos_in_table_list->is_sjm_scan_table())
   {
     tab->filesort->set_all_read_bits= TRUE;
@@ -7391,6 +7390,10 @@ static void remember_if_eq_ref_key(JOIN *join, KEYUSE *use)
   Special treatment for ft-keys.
   Update join->eq_ref_tables with a bitmap of all tables that can possible
   have a EQ_REF key.
+
+  Note that the keys are generated to be used by best_access_path() during
+  the optimization stage. Unused keys will later be deleted by
+  JOIN::drop_unused_derived_keys().
 */
 
 bool sort_and_filter_keyuse(JOIN *join, DYNAMIC_ARRAY *keyuse,
@@ -7440,7 +7443,9 @@ bool sort_and_filter_keyuse(JOIN *join, DYNAMIC_ARRAY *keyuse,
         }
         else
         {
-          /* Key changed, check if previous key was a primary/unique key lookup */
+          /*
+            Key changed, check if previous key was a primary/unique key lookup
+          */
           if (prev != &key_end && !found_unprefixed_key_part)
             remember_if_eq_ref_key(join, prev);
           found_unprefixed_key_part= 0;
@@ -13794,36 +13799,45 @@ bool generate_derived_keys_for_table(KEYUSE *keyuse, uint count, uint keys)
 static
 bool generate_derived_keys(DYNAMIC_ARRAY *keyuse_array)
 {
-  KEYUSE *keyuse= dynamic_element(keyuse_array, 0, KEYUSE*);
+  KEYUSE *keyuse, *end_keyuse;
   size_t elements= keyuse_array->elements;
   TABLE *prev_table= 0;
-  for (size_t i= 0; i < elements; i++, keyuse++)
+
+  DBUG_ASSERT(elements > 0);
+  /* The last element is an end marker */
+  DBUG_ASSERT(dynamic_element(keyuse_array, elements-1,
+                              KEYUSE*)[0].table == 0);
+
+  for (keyuse= dynamic_element(keyuse_array, 0, KEYUSE*),
+         end_keyuse= keyuse + elements - 1;
+       keyuse < end_keyuse;
+       keyuse++)
   {
-    if (!keyuse->table)
-      break;
+    DBUG_ASSERT(keyuse->table);
+
     KEYUSE *first_table_keyuse= NULL;
     table_map last_used_tables= 0;
     uint count= 0;
     uint keys= 0;
     TABLE_LIST *derived= NULL;
+
     if (keyuse->table != prev_table)
       derived= keyuse->table->pos_in_table_list;
-    while (derived && derived->is_materialized_derived())
+
+    if (!derived->is_materialized_derived())
+      continue;
+
+    for (;;)
     {
       if (keyuse->table != prev_table)
       {
         prev_table= keyuse->table;
         while (keyuse->table == prev_table && keyuse->key != MAX_KEY)
-	{
           keyuse++;
-          i++;
-        }
         if (keyuse->table != prev_table)
 	{
           keyuse--;
-          i--;
-          derived= NULL;
-          continue;
+          break;
         }
         first_table_keyuse= keyuse;
         last_used_tables= keyuse->used_tables;
@@ -13837,14 +13851,12 @@ bool generate_derived_keys(DYNAMIC_ARRAY *keyuse_array)
       }
       count++;
       keyuse++;
-      i++;
       if (keyuse->table != prev_table)
       {
         if (generate_derived_keys_for_table(first_table_keyuse, count, ++keys))
           return TRUE;
         keyuse--;
-        i--;
-	derived= NULL;
+        break;
       }
     }
   }

sql/sql_show.cc

@@ -8899,6 +8899,10 @@ bool optimize_schema_tables_reads(JOIN *join)
        tab; 
        tab= next_linear_tab(join, tab, WITH_BUSH_ROOTS))
   {
+    /*
+      The following is true for the temporary table that will hold the
+      final result.
+    */
     if (!tab->table || !tab->table->pos_in_table_list)
       continue;
 
@@ -8970,6 +8974,10 @@ bool get_schema_tables_result(JOIN *join,
        tab; 
        tab= next_linear_tab(join, tab, WITH_BUSH_ROOTS))
   {
+    /*
+      The following is true for the temporary table that will hold the
+      final result.
+    */
     if (!tab->table || !tab->table->pos_in_table_list)
       break;
 

sql/sql_tvc.cc

@@ -1047,7 +1047,9 @@ Item *Item_func_in::in_predicate_to_in_subs_transformer(THD *thd,
   sq_select->add_where_field(derived_unit->first_select());
   sq_select->context.table_list= sq_select->table_list.first;
   sq_select->context.first_name_resolution_table= sq_select->table_list.first;
-  sq_select->table_list.first->derived_type= DTYPE_TABLE | DTYPE_MATERIALIZE;
+  sq_select->table_list.first->derived_type= (DTYPE_TABLE |
+                                              DTYPE_MATERIALIZE |
+                                              DTYPE_IN_PREDICATE);
   lex->derived_tables|= DERIVED_SUBQUERY;
 
   sq_select->where= 0;

sql/table.h

@@ -1993,7 +1993,8 @@ class IS_table_read_plan;
 #define DTYPE_MERGE                  4U
 #define DTYPE_MATERIALIZE            8U
 #define DTYPE_MULTITABLE             16U
-#define DTYPE_MASK                   (DTYPE_VIEW|DTYPE_TABLE|DTYPE_MULTITABLE)
+#define DTYPE_IN_PREDICATE           32U
+#define DTYPE_MASK                   (DTYPE_VIEW|DTYPE_TABLE|DTYPE_MULTITABLE|DTYPE_IN_PREDICATE)
 
 /*
   Phases of derived tables/views handling, see sql_derived.cc