Don't use flags in the group_by_handler class

instead pass the whole query down and let the engine return
unsupported parts back

sql/group_by_handler.h

@@ -30,13 +30,44 @@
   SELECT a, (select sum(*) from t2 where t1.a=t2.a) from t2;
 */
 
+/**
+  The structure describing various parts of the query
+
+  The engine is supposed to take out parts that it can do internally.
+  For example, if the engine can return results sorted according to
+  the specified order_by clause, it sets Query::order_by=NULL before
+  returning.
+
+  At the moment the engine must take group_by (or return an error), and
+  optionally can take distinct, where, order_by, and having.
+
+  The engine should not modify the select list. It is the extended SELECT
+  clause (extended, because it has more items than the original
+  user-specified SELECT clause) and it contains all aggregate functions,
+  used in the query.
+*/
+struct Query
+{
+  List<Item> *select;
+  bool        distinct;
+  TABLE_LIST *from;
+  Item       *where;
+  ORDER      *group_by;
+  ORDER      *order_by;
+  Item       *having;
+  // LIMIT
+};
+
 class group_by_handler
 {
 public:
   THD *thd;
   handlerton *ht;
 
-  /* Temporary table where all results should be stored in record[0] */
+  /*
+    Temporary table where all results should be stored in record[0]
+    The table has a field for every item from the Query::select list.
+  */
   TABLE *table;
 
   group_by_handler(THD *thd_arg, handlerton *ht_arg)
@@ -67,19 +98,6 @@ class group_by_handler
     return init(having_arg, order_by_arg);
   }
 
-  /*
-    Bits of things the storage engine can do for this query.
-    Should be initialized on object creation.
-    Result data is sorted by the storage engine according to order_by (if it
-    exists) else according to the group_by.  If this is not specified,
-    MariaDB will store the result set into the temporary table and sort the
-    result.
-  */
-  #define GROUP_BY_ORDER_BY 1
-  /* The storage engine can handle DISTINCT */
-  #define GROUP_BY_DISTINCT 2
-  virtual uint flags() { return 0; }
-
   /*
     Functions to scan data. All these returns 0 if ok, error code in case
     of error

sql/handler.h

@@ -950,6 +950,7 @@ struct handler_iterator {
 
 class handler;
 class group_by_handler;
+struct Query;
 typedef class st_select_lex SELECT_LEX;
 typedef struct st_order ORDER;
 
@@ -1267,10 +1268,7 @@ struct handlerton
     The server guaranteeds that all tables in the list belong to this
     storage engine.
   */
-  group_by_handler *(*create_group_by)(THD *thd, List<Item> *fields,
-                                       TABLE_LIST *table_list, ORDER *group_by,
-                                       ORDER *order_by, Item *where,
-                                       Item *having);
+  group_by_handler *(*create_group_by)(THD *thd, Query *query);
 
    /*********************************************************************
      Table discovery API.

sql/sql_select.cc

@@ -1916,31 +1916,26 @@ JOIN::optimize_inner()
     if (ht && ht->create_group_by)
     {
       /* Check if the storage engine can intercept the query */
-      group_by_handler *gbh= ht->create_group_by(thd, &all_fields,
-                               tables_list, group_list, order, conds, having);
+      Query query= {&all_fields, select_distinct, tables_list, conds,
+                    group_list, order ? order : group_list, having};
+      group_by_handler *gbh= ht->create_group_by(thd, &query);
       if (gbh)
       {
         pushdown_query= new (thd->mem_root) Pushdown_query(select_lex, gbh);
-        uint handler_flags= gbh->flags();
         int err;
 
         /*
           We must store rows in the tmp table if we need to do an ORDER BY
           or DISTINCT and the storage handler can't handle it.
         */
-        need_tmp= ((!(handler_flags & GROUP_BY_ORDER_BY) &&
-                    (order || group_list)) ||
-                   (!(handler_flags & GROUP_BY_DISTINCT) && select_distinct));
+        need_tmp= query.order_by || query.group_by || query.distinct;
         tmp_table_param.hidden_field_count= (all_fields.elements -
                                              fields_list.elements);
         if (!(exec_tmp_table1=
               create_tmp_table(thd, &tmp_table_param, all_fields, 0,
-                               handler_flags & GROUP_BY_DISTINCT ?
-                               0 : select_distinct, 1,
+                               query.distinct, 1,
                                select_options, HA_POS_ERROR, "",
-                               !need_tmp,
-                               (!order ||
-                                (handler_flags & GROUP_BY_ORDER_BY)))))
+                               !need_tmp, query.order_by || query.group_by)))
           DBUG_RETURN(1);
 
         /*
@@ -1965,25 +1960,19 @@ JOIN::optimize_inner()
         }
         pushdown_query->store_data_in_temp_table= need_tmp;
         pushdown_query->having= having;
-        /*
-          If no ORDER BY clause was specified explicitly, we should sort things
-          according to the group_by
-        */
-        if (!order)
-          order= group_list;
         /*
           Group by and having is calculated by the group_by handler.
           Reset the group by and having
         */
+        DBUG_ASSERT(query.group_by == NULL);
         group= 0; group_list= 0;
         having= tmp_having= 0;
         /*
           Select distinct is handled by handler or by creating an unique index
           over all fields in the temporary table
         */
         select_distinct= 0;
-        if (handler_flags & GROUP_BY_ORDER_BY)
-          order= 0;
+        order= query.order_by;
         tmp_table_param.field_count+= tmp_table_param.sum_func_count;
         tmp_table_param.sum_func_count= 0;
 

storage/sequence/sequence.cc

@@ -375,19 +375,17 @@ class ha_seq_group_by_handler: public group_by_handler
 };
 
 static group_by_handler *
-create_group_by_handler(THD *thd, List<Item> *fields, TABLE_LIST *table_list,
-                        ORDER *group_by, ORDER *order_by, Item *where,
-                        Item *having)
+create_group_by_handler(THD *thd, Query *query)
 {
   ha_seq_group_by_handler *handler;
   Item *item;
-  List_iterator_fast<Item> it(*fields);
+  List_iterator_fast<Item> it(*query->select);
 
   /* check that only one table is used in FROM clause and no sub queries */
-  if (table_list->next_local != 0)
+  if (query->from->next_local != 0)
     return 0;
   /* check that there is no where clause and no group_by */
-  if (where != 0 || group_by != 0)
+  if (query->where != 0 || query->group_by != 0)
     return 0;
 
   /*
@@ -417,15 +415,15 @@ create_group_by_handler(THD *thd, List<Item> *fields, TABLE_LIST *table_list,
       Check that we are using the sequence table (the only table in the FROM
       clause) and not an outer table.
     */
-    if (field->table != table_list->table)
+    if (field->table != query->from->table)
       return 0;
     /* Check that we are using a SUM() on the primary key */
     if (strcmp(field->field_name, "seq"))
       return 0;
   }
 
   /* Create handler and return it */
-  handler= new ha_seq_group_by_handler(thd, fields, table_list);
+  handler= new ha_seq_group_by_handler(thd, query->select, query->from);
   return handler;
 }