MDEV-334: Backport of UNION ALL optimization from mysql-5.7.

Although the original code of mysql-5.7 was adjusted
to the current MariaDB code the main ideas of the optimization
were preserved.

sql/sql_class.h

@@ -3914,6 +3914,23 @@ protected:
 public:
   select_result();
   virtual ~select_result() {};
+  /**
+    Change wrapped select_result.
+
+    Replace the wrapped result object with new_result and call
+    prepare() and prepare2() on new_result.
+
+    This base class implementation doesn't wrap other select_results.
+
+    @param new_result The new result object to wrap around
+
+    @retval false Success
+    @retval true  Error
+  */
+  virtual bool change_result(select_result *new_result)
+  {
+    return false;
+  }
   virtual int prepare(List<Item> &list, SELECT_LEX_UNIT *u)
   {
     unit= u;
@@ -4321,9 +4338,19 @@ public:
 
   select_union() :write_err(0), table(0), records(0) { tmp_table_param.init(); }
   int prepare(List<Item> &list, SELECT_LEX_UNIT *u);
+  /**
+    Do prepare() and prepare2() if they have been postponed until
+    column type information is computed (used by select_union_direct).
+
+    @param types Column types
+
+    @return false on success, true on failure
+  */
+  virtual bool postponed_prepare(List<Item> &types)
+  { return false; }
   int send_data(List<Item> &items);
   bool send_eof();
-  bool flush();
+  virtual bool flush();
   void cleanup();
   virtual bool create_result_table(THD *thd, List<Item> *column_types,
                                    bool is_distinct, ulonglong options,
@@ -4334,6 +4361,101 @@ public:
   TMP_TABLE_PARAM *get_tmp_table_param() { return &tmp_table_param; }
 };
 
+
+/**
+  UNION result that is passed directly to the receiving select_result
+  without filling a temporary table.
+
+  Function calls are forwarded to the wrapped select_result, but some
+  functions are expected to be called only once for each query, so
+  they are only executed for the first SELECT in the union (execept
+  for send_eof(), which is executed only for the last SELECT).
+
+  This select_result is used when a UNION is not DISTINCT and doesn't
+  have a global ORDER BY clause. @see st_select_lex_unit::prepare().
+*/
+
+class select_union_direct :public select_union
+{
+private:
+  /* Result object that receives all rows */
+  select_result *result;
+  /* The last SELECT_LEX of the union */
+  SELECT_LEX *last_select_lex;
+
+  /* Wrapped result has received metadata */
+  bool done_send_result_set_metadata;
+  /* Wrapped result has initialized tables */
+  bool done_initialize_tables;
+
+  /* Accumulated limit_found_rows */
+  ulonglong limit_found_rows;
+
+  /* Number of rows offset */
+  ha_rows offset;
+  /* Number of rows limit + offset, @see select_union_direct::send_data() */
+  ha_rows limit;
+
+public:
+  select_union_direct(select_result *result, SELECT_LEX *last_select_lex)
+    :result(result), last_select_lex(last_select_lex),
+    done_send_result_set_metadata(false), done_initialize_tables(false),
+    limit_found_rows(0)
+  {}
+  bool change_result(select_result *new_result);
+  uint field_count(List<Item> &fields) const
+  {
+    // Only called for top-level select_results, usually select_send
+    DBUG_ASSERT(false); /* purecov: inspected */
+    return 0; /* purecov: inspected */
+  }
+  bool postponed_prepare(List<Item> &types);
+  bool send_result_set_metadata(List<Item> &list, uint flags);
+  int send_data(List<Item> &items);
+  bool initialize_tables (JOIN *join= NULL);
+  bool send_eof();
+  bool flush() { return false; }
+  bool check_simple_select() const
+  {
+    /* Only called for top-level select_results, usually select_send */
+    DBUG_ASSERT(false); /* purecov: inspected */
+    return false; /* purecov: inspected */
+  }
+  void abort_result_set()
+  {
+    result->abort_result_set(); /* purecov: inspected */
+  }
+  void cleanup()
+  {
+    /*
+      Only called for top-level select_results, usually select_send,
+      and for the results of subquery engines
+      (select_<something>_subselect).
+    */
+    DBUG_ASSERT(false); /* purecov: inspected */
+  }
+  void set_thd(THD *thd_arg)
+  {
+    /*
+      Only called for top-level select_results, usually select_send,
+      and for the results of subquery engines
+      (select_<something>_subselect).
+    */
+    DBUG_ASSERT(false); /* purecov: inspected */
+  }
+  void reset_offset_limit_cnt()
+  {
+    // EXPLAIN should never output to a select_union_direct
+    DBUG_ASSERT(false); /* purecov: inspected */
+  }
+  void begin_dataset()
+  {
+    // Only called for sp_cursor::Select_fetch_into_spvars
+    DBUG_ASSERT(false); /* purecov: inspected */
+  }
+};
+
+
 /* Base subselect interface class */
 class select_subselect :public select_result_interceptor
 {