MDEV-6080: Allowing storage engine to shortcut group by queries

This task is to allow storage engines that can execute GROUP BY or
summary queries efficiently to intercept a full query or sub query from
MariaDB and deliver the result either to the client or to a temporary
table for further processing.

- Added code in sql_select.cc to intercept GROUP BY queries.
  Creation of group_by_handler is done after all optimizations to allow
  storage engine to benefit of an optimized WHERE clause and suggested
  indexes to use.
- Added group by handler to sequence engine and a group_by test suite as
  a way to test the new interface.
- Intercept EXPLAIN with a message "Storage engine handles GROUP BY"

libmysqld/CMakeLists.txt:
  Added new group_by_handler files
sql/CMakeLists.txt:
  Added new group_by_handler files
sql/group_by_handler.cc:
  Implementation of group_by_handler functions
sql/group_by_handler.h:
  Definition of group_by_handler class
sql/handler.h:
  Added handlerton function to create a group_by_handler, if the storage
  engine can intercept the query.
sql/item_cmpfunc.cc:
  Allow one to evaluate item_equal any time.
sql/sql_select.cc:
  Added code to intercept GROUP BY queries
  - If all tables are from the same storage engine and the query is
    using sum functions, call create_group_by() to check if the storage
    engine can intercept the query.
    - If yes:
       - create a temporary table to hold a GROUP_BY row or result
       - In do_select() intercept normal query execution by instead
         calling the group_by_handler to get the result
       - Intercept EXPLAIN
sql/sql_select.h:
  Added handling of group_by_handler
  Added caching of the original join tab (needed for cleanup after
  group_by handler)
storage/sequence/mysql-test/sequence/group_by.result:
  Test group_by_handler interface
storage/sequence/mysql-test/sequence/group_by.test:
  Test group_by_handler interface
storage/sequence/sequence.cc:
  Added simple group_by_engine for handling COUNT(*) and
  SUM(primary_key).  This was done as a test of the group_by_handler
  interface

1 files changed, 144 insertions, 0 deletions

diff --git a/sql/group_by_handler.cc b/sql/group_by_handler.cc
new file mode 100644
index 00000000000..db03ce9196b
--- /dev/null
+++ b/sql/group_by_handler.cc
@@ -0,0 +1,144 @@
+/*
+   Copyright (c) 2014, SkySQL Ab & MariaDB Foundation
+
+   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., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+
+/*
+  This file implements the group_by_handler code. This interface
+  can be used by storage handlers that can intercept summary or GROUP
+  BY queries from MariaDB and itself return the result to the user or
+  upper level.
+*/
+
+#ifdef USE_PRAGMA_IMPLEMENTATION
+#pragma implementation				// gcc: Class implementation
+#endif
+
+#include "sql_priv.h"
+#include "sql_select.h"
+
+/*
+  Same return values as do_select();
+
+  @retval
+    0  if ok
+  @retval
+    1  if error is sent
+  @retval
+    -1  if error should be sent
+*/
+
+int group_by_handler::execute(JOIN *join)
+{
+  int err;
+  ha_rows max_limit;
+  ha_rows *reset_limit= 0;
+  Item **reset_item= 0;
+  DBUG_ENTER("group_by_handler");
+
+  if ((err= init_scan()))
+    goto error;
+
+  if (store_data_in_temp_table)
+  {
+    max_limit= join->tmp_table_param.end_write_records;
+    reset_limit= &join->unit->select_limit_cnt;
+  }
+  else
+  {
+    max_limit= join->unit->select_limit_cnt;
+    if (join->unit->fake_select_lex)
+      reset_item= &join->unit->fake_select_lex->select_limit;
+  }
+
+  while (!(err= next_row()))
+  {
+    if (thd->check_killed())
+    {
+      thd->send_kill_message();
+      (void) end_scan();
+      DBUG_RETURN(-1);
+    }
+
+    /* Check if we can accept the row */
+    if (!having || having->val_bool())
+    {
+      if (store_data_in_temp_table)
+      {
+        if ((err= table->file->ha_write_tmp_row(table->record[0])))
+        {
+          bool is_duplicate;
+          if (!table->file->is_fatal_error(err, HA_CHECK_DUP))
+            continue;                           // Distinct elimination
+
+          if (create_internal_tmp_table_from_heap(thd, table,
+                                                  join->tmp_table_param.
+                                                  start_recinfo,
+                                                  &join->tmp_table_param.
+                                                  recinfo,
+                                                  err, 1, &is_duplicate))
+            DBUG_RETURN(1);
+          if (is_duplicate)
+            continue;
+        }
+      }
+      else
+      {
+        if (join->do_send_rows)
+        {
+          int error;
+          /* result < 0 if row was not accepted and should not be counted */
+          if ((error= join->result->send_data(*join->fields)))
+          {
+            (void) end_scan();
+            DBUG_RETURN(error < 0 ? 0 : -1);
+          }
+        }
+      }
+
+      /* limit handling */
+      if (++join->send_records >= max_limit && join->do_send_rows)
+      {
+        if (!(join->select_options & OPTION_FOUND_ROWS))
+          break;                              // LIMIT reached
+        join->do_send_rows= 0;                // Calculate FOUND_ROWS()
+        if (reset_limit)
+          *reset_limit= HA_POS_ERROR;
+        if (reset_item)
+          *reset_item= 0;
+      }
+    }
+  }
+  if (err != 0 && err != HA_ERR_END_OF_FILE)
+    goto error;
+
+  if ((err= end_scan()))
+    goto error_2;
+  if (!store_data_in_temp_table && join->result->send_eof())
+    DBUG_RETURN(1);                              // Don't send error to client
+
+  DBUG_RETURN(0);
+
+error:
+  (void) end_scan();
+error_2:
+  print_error(err, MYF(0));
+  DBUG_RETURN(-1);                              // Error not sent to client
+}
+
+
+void group_by_handler::print_error(int error, myf errflag)
+{
+  my_error(ER_GET_ERRNO, MYF(0), error, hton_name(ht)->str);
+}