MDEV-17096 Pushdown of simple derived tables to storage engines

MDEV-17631 select_handler for a full query pushdown

Added comments and file headers for files introduced in these tasks.

1 files changed, 27 insertions, 3 deletions

diff --git a/sql/derived_handler.h b/sql/derived_handler.h
index c312a93a1f7..171165bbe6f 100644
--- a/sql/derived_handler.h
+++ b/sql/derived_handler.h
@@ -1,3 +1,19 @@
+/*
+   Copyright (c) 2016, 2017 MariaDB
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
+
 #ifndef DERIVED_HANDLER_INCLUDED
 #define DERIVED_HANDLER_INCLUDED
 
@@ -8,6 +24,13 @@ class TMP_TABLE_PARAM;
 
 typedef class st_select_lex_unit SELECT_LEX_UNIT;
 
+/**
+  @class derived_handler
+
+  This interface class is to be used for execution of queries that specify
+  derived table by foreign engines
+*/
+
 class derived_handler
 {
 public:
@@ -23,11 +46,12 @@ public:
   */
   TABLE *table;
 
+  /* The parameters if the temporary table used at its creation */
   TMP_TABLE_PARAM *tmp_table_param;
 
-  SELECT_LEX_UNIT *unit;
+  SELECT_LEX_UNIT *unit;   // Specifies the derived table
 
-  SELECT_LEX *select;
+  SELECT_LEX *select;      // The first select of the specification
 
   derived_handler(THD *thd_arg, handlerton *ht_arg)
     : thd(thd_arg), ht(ht_arg), derived(0),table(0), tmp_table_param(0),
@@ -53,7 +77,7 @@ public:
   virtual int end_scan()=0;
 
   /* Report errors */
-  virtual void print_error(int error, myf errflag)=0;
+  virtual void print_error(int error, myf errflag);
 
   void set_derived(TABLE_LIST *tbl);
 };