Fixed bug#36006: Optimizer does table scan for SELECT COUNT(*)

                 for ENGINE=MRG_MYISAM (should be optimized out).

Before WL#3281 MERGE engine had the HA_NOT_EXACT_COUNT flag
unset, and it worked with COUNT optimization as desired.
After the removal of the HA_NOT_EXACT_COUNT flag neither
HA_STATS_RECORDS_IS_EXACT (opposite to former HA_NOT_EXACT_COUNT
flag) nor modern HA_HAS_RECORDS flag were not added to MERGE
table flag mask.

1. The HA_HAS_RECORDS table flag has been set.
2. The ha_myisammrg::records method has been overridden to
   calculate total number of records in underlying tables.



storage/myisammrg/myrg_records.c:
  Fixed bug#36006: Optimizer does table scan for select count(*).
  The myrg_records function has been added to calculate total number
  of records in underlying tables.
include/myisammrg.h:
  Fixed bug#36006: Optimizer does table scan for select count(*).
  The myrg_records function declaration has been added.
mysql-test/r/merge.result:
  Added test case for bug#36006.
mysql-test/t/merge.test:
  Added test case for bug#36006.
storage/myisammrg/CMakeLists.txt:
  Fixed bug#36006: Optimizer does table scan for select count(*).
  New myrg_records.c file has been added.
storage/myisammrg/Makefile.am:
  Fixed bug#36006: Optimizer does table scan for select count(*).
  New myrg_records.c file has been added.
storage/myisammrg/ha_myisammrg.cc:
  Fixed bug#36006: Optimizer does table scan for select count(*).
  The ha_myisammrg::records method has been overridden.
storage/myisammrg/ha_myisammrg.h:
  Fixed bug#36006: Optimizer does table scan for select count(*).
  1. The HA_HAS_RECORDS table flag has been set.
  2. The ha_myisammrg::records method has been overridden.

include/myisammrg.h

@@ -112,6 +112,7 @@ extern int myrg_reset(MYRG_INFO *info);
 extern void myrg_extrafunc(MYRG_INFO *info,invalidator_by_filename inv);
 extern ha_rows myrg_records_in_range(MYRG_INFO *info, int inx,
                                      key_range *min_key, key_range *max_key);
+extern ha_rows myrg_records(MYRG_INFO *info);
 
 extern ulonglong myrg_position(MYRG_INFO *info);
 #ifdef	__cplusplus

mysql-test/r/merge.result

@@ -2006,3 +2006,19 @@ test.t1	optimize	status	OK
 FLUSH TABLES m1, t1;
 UNLOCK TABLES;
 DROP TABLE t1, m1;
+CREATE TABLE t1(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2)) ENGINE=MYISAM;
+CREATE TABLE t2(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2)) ENGINE=MYISAM;
+CREATE TABLE t3(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2)) ENGINE=MYISAM;
+CREATE TABLE t4(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2))
+ENGINE=MRG_MYISAM UNION=(t1, t2, t3);
+INSERT INTO t1 VALUES (1,1), (1,2),(1,3), (1,4);
+INSERT INTO t2 VALUES (2,1), (2,2),(2,3), (2,4);
+INSERT INTO t3 VALUES (3,1), (3,2),(3,3), (3,4);
+EXPLAIN SELECT COUNT(*) FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+EXPLAIN SELECT COUNT(*) FROM t4;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+DROP TABLE t1, t2, t3, t4;
+End of 5.1 tests

mysql-test/t/merge.test

@@ -1394,3 +1394,19 @@ FLUSH TABLES m1, t1;
 UNLOCK TABLES;
 DROP TABLE t1, m1;
 
+#
+# Bug#36006 - Optimizer does table scan for select count(*)
+#
+CREATE TABLE t1(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2)) ENGINE=MYISAM;
+CREATE TABLE t2(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2)) ENGINE=MYISAM;
+CREATE TABLE t3(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2)) ENGINE=MYISAM;
+CREATE TABLE t4(C1 INT, C2 INT, KEY C1(C1), KEY C2(C2))
+  ENGINE=MRG_MYISAM UNION=(t1, t2, t3);
+INSERT INTO t1 VALUES (1,1), (1,2),(1,3), (1,4);
+INSERT INTO t2 VALUES (2,1), (2,2),(2,3), (2,4);
+INSERT INTO t3 VALUES (3,1), (3,2),(3,3), (3,4);
+EXPLAIN SELECT COUNT(*) FROM t1;
+EXPLAIN SELECT COUNT(*) FROM t4;
+DROP TABLE t1, t2, t3, t4;
+
+--echo End of 5.1 tests

storage/myisammrg/CMakeLists.txt

@@ -26,7 +26,7 @@ SET(MYISAMMRG_SOURCES  myrg_close.c myrg_create.c myrg_delete.c myrg_extra.c myr
 				myrg_locking.c myrg_open.c myrg_panic.c myrg_queue.c myrg_range.c
 				myrg_rfirst.c myrg_rkey.c myrg_rlast.c myrg_rnext.c myrg_rnext_same.c
 				myrg_rprev.c myrg_rrnd.c myrg_rsame.c myrg_static.c myrg_update.c
-				myrg_write.c)
+				myrg_write.c myrg_records.c)
 
 IF(NOT SOURCE_SUBLIBS)
   ADD_LIBRARY(myisammrg ${MYISAMMRG_SOURCES})

storage/myisammrg/Makefile.am

@@ -35,7 +35,7 @@ libmyisammrg_a_SOURCES = myrg_open.c myrg_extra.c myrg_info.c myrg_locking.c \
 			myrg_rkey.c myrg_rfirst.c myrg_rlast.c myrg_rnext.c \
 			myrg_rprev.c myrg_queue.c myrg_write.c myrg_range.c \
 			ha_myisammrg.cc \
-			myrg_rnext_same.c
+			myrg_rnext_same.c myrg_records.c
 
 
 EXTRA_DIST =		CMakeLists.txt plug.in

storage/myisammrg/ha_myisammrg.cc

@@ -1154,6 +1154,12 @@ int ha_myisammrg::check(THD* thd, HA_CHECK_OPT* check_opt)
 }
 
 
+ha_rows ha_myisammrg::records()
+{
+  return myrg_records(file);
+}
+
+
 extern int myrg_panic(enum ha_panic_function flag);
 int myisammrg_panic(handlerton *hton, ha_panic_function flag)
 {

storage/myisammrg/ha_myisammrg.h

@@ -42,6 +42,7 @@ class ha_myisammrg: public handler
             HA_BINLOG_ROW_CAPABLE | HA_BINLOG_STMT_CAPABLE |
 	    HA_NULL_IN_KEY | HA_CAN_INDEX_BLOBS | HA_FILE_BASED |
             HA_ANY_INDEX_MAY_BE_UNIQUE | HA_CAN_BIT_FIELD |
+            HA_HAS_RECORDS |
             HA_NO_COPY_ON_ALTER);
   }
   ulong index_flags(uint inx, uint part, bool all_parts) const
@@ -94,4 +95,5 @@ class ha_myisammrg: public handler
   TABLE *table_ptr() { return table; }
   bool check_if_incompatible_data(HA_CREATE_INFO *info, uint table_changes);
   int check(THD* thd, HA_CHECK_OPT* check_opt);
+  ha_rows records();
 };

storage/myisammrg/myrg_records.c

+/* Copyright (C) 2008 MySQL AB
+
+   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 */
+
+#include "myrg_def.h"
+
+ha_rows myrg_records(MYRG_INFO *info)
+{
+  ha_rows records=0;
+  MYRG_TABLE *file;
+  DBUG_ENTER("myrg_records");
+
+  for (file=info->open_tables ; file != info->end_table ; file++)
+    records+= file->table->s->state.state.records;
+  DBUG_RETURN(records);
+}