MDEV-5177: ha_partition and innodb index intersection produce fewer rows (MySQL Bug#70703)

MDEV-5555: Incorrect index_merge on BTREE indices
- In ha_partition, make ordered index reads return rows in rowid order
  when index columns are the same.

mysql-test/r/partition.result

@@ -2493,3 +2493,70 @@ i
 3
 4
 DROP TABLE t1;
+#
+# MDEV-5177: ha_partition and innodb index intersection produce fewer rows (MySQL Bug#70703)
+#
+create table t1 (
+a int not null,
+b int not null,
+pk int not null,
+primary key (pk),
+key(a),
+key(b)
+) partition by hash(pk) partitions 10;
+insert into t1 values (1,2,4);
+insert into t1 values (1,0,17);
+insert into t1 values (1,2,25);
+insert into t1 values (10,20,122);
+insert into t1 values (10,20,123);
+create table t2 (a int);
+insert into t2 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+insert into t1 select 1,2, 200 + A.a + 10*B.a + 100*C.a from t2 A, t2 B, t2 C;
+insert into t1 select 10+A.a + 10*B.a + 100*C.a + 1000*D.a,
+10+A.a + 10*B.a + 100*C.a  + 1000*D.a, 
+2000 + A.a + 10*B.a + 100*C.a + 1000*D.a
+from t2 A, t2 B, t2 C ,t2 D;
+explain select * from t1 where a=1 and b=2 and  pk between 1 and 999999 ;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ref	PRIMARY,a,b	b	4	const	982	Using where
+create temporary table t3 as
+select * from t1 where a=1 and b=2 and  pk between 1 and 999 ;
+select count(*) from t3;
+count(*)
+802
+drop table t3;
+create temporary table t3 as
+select * from t1 ignore index(a,b)  where a=1 and b=2 and  pk between 1 and 999 ;
+select count(*) from t3;
+count(*)
+802
+drop table t3;
+drop table t1,t2;
+#
+# MDEV-5555: Incorrect index_merge on BTREE indices
+#
+CREATE TABLE t1 (
+id bigint(20) unsigned NOT NULL,
+id2 bigint(20) unsigned NOT NULL,
+dob date DEFAULT NULL,
+address char(100) DEFAULT NULL,
+city char(35) DEFAULT NULL,
+hours_worked_per_week smallint(5) unsigned DEFAULT NULL,
+weeks_worked_last_year tinyint(3) unsigned DEFAULT NULL,
+KEY dob (dob),
+KEY address (address),
+KEY city (city),
+KEY hours_worked_per_week (hours_worked_per_week),
+KEY weeks_worked_last_year (weeks_worked_last_year)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+PARTITION BY KEY (id) PARTITIONS 5;
+# Insert some rows 
+select * from t1 where hours_worked_per_week = 40 and weeks_worked_last_year = 52 and dob < '1949-11-21';
+id	id2	dob	address	city	hours_worked_per_week	weeks_worked_last_year
+16	16	1949-11-07	address16	city16	40	52
+50	50	1923-09-08	address50	city50	40	52
+select * from t1 IGNORE INDEX(dob, weeks_worked_last_year, hours_worked_per_week) where hours_worked_per_week = 40 and weeks_worked_last_year = 52 and dob < '1949-11-21';
+id	id2	dob	address	city	hours_worked_per_week	weeks_worked_last_year
+16	16	1949-11-07	address16	city16	40	52
+50	50	1923-09-08	address50	city50	40	52
+drop table t1;

mysql-test/r/partition_order.result

@@ -752,8 +752,8 @@ select * from t1 force index (b) where b < 10 ORDER BY b DESC;
 a	b
 6	6
 4	5
-2	4
 30	4
+2	4
 3	3
 35	2
 7	1

mysql-test/t/partition.test

@@ -2494,3 +2494,335 @@ INSERT INTO t1 VALUES (1),(2),(2),(3),(4);
 ALTER TABLE t1 ADD PARTITION PARTITIONS 2;
 SELECT * from t1 order by i;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-5177: ha_partition and innodb index intersection produce fewer rows (MySQL Bug#70703)
+--echo #
+create table t1 (
+  a int not null,
+  b int not null,
+  pk int not null,
+  primary key (pk),
+  key(a),
+  key(b)
+) partition by hash(pk) partitions 10;
+
+insert into t1 values (1,2,4); # both
+insert into t1 values (1,0,17);  # left
+insert into t1 values (1,2,25);   # both
+
+insert into t1 values (10,20,122); 
+insert into t1 values (10,20,123);
+
+# Now, fill in some data so that the optimizer choses index_merge
+create table t2 (a int);
+insert into t2 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+insert into t1 select 1,2, 200 + A.a + 10*B.a + 100*C.a from t2 A, t2 B, t2 C;
+
+insert into t1 select 10+A.a + 10*B.a + 100*C.a + 1000*D.a,
+                       10+A.a + 10*B.a + 100*C.a  + 1000*D.a, 
+                       2000 + A.a + 10*B.a + 100*C.a + 1000*D.a
+                       from t2 A, t2 B, t2 C ,t2 D;
+
+# This should show index_merge, using intersect
+explain select * from t1 where a=1 and b=2 and  pk between 1 and 999999 ;
+# 794 rows in output
+create temporary table t3 as
+select * from t1 where a=1 and b=2 and  pk between 1 and 999 ;
+select count(*) from t3;
+drop table t3;
+
+# 802 rows in output
+create temporary table t3 as
+select * from t1 ignore index(a,b)  where a=1 and b=2 and  pk between 1 and 999 ;
+select count(*) from t3;
+drop table t3;
+
+drop table t1,t2;
+
+--echo #
+--echo # MDEV-5555: Incorrect index_merge on BTREE indices
+--echo #
+
+CREATE TABLE t1 (
+  id bigint(20) unsigned NOT NULL,
+  id2 bigint(20) unsigned NOT NULL,
+  dob date DEFAULT NULL,
+  address char(100) DEFAULT NULL,
+  city char(35) DEFAULT NULL,
+  hours_worked_per_week smallint(5) unsigned DEFAULT NULL,
+  weeks_worked_last_year tinyint(3) unsigned DEFAULT NULL,
+  KEY dob (dob),
+  KEY address (address),
+  KEY city (city),
+  KEY hours_worked_per_week (hours_worked_per_week),
+  KEY weeks_worked_last_year (weeks_worked_last_year)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+PARTITION BY KEY (id) PARTITIONS 5;
+
+--echo # Insert some rows 
+--disable_query_log
+INSERT INTO t1 VALUES (123,123,'2001-04-14','address123','city123',40,51),
+(127,127,'1977-03-30','address127','city127',0,0), 
+(131,131,'1985-07-29','address131','city131',50,52),
+(135,135,'1997-01-20','address135','city135',0,0),
+(139,139,'1963-04-27','address139','city139',27,52),
+(143,143,'1979-01-28','address143','city143',40,52),
+(147,147,'1985-08-28','address147','city147',0,0),
+(151,151,'1997-01-24','address151','city151',40,52),
+(156,156,'1975-02-19','address156','city156',46,52),
+(158,158,'1996-07-06','address158','city158',46,8),
+(164,164,'1925-12-30','address164','city164',0,0),
+(166,166,'2010-12-30','address166','city166',0,0),
+(172,172,'1996-08-15','address172','city172',0,0),
+(174,174,'2008-05-20','address174','city174',40,52),
+(180,180,'1969-09-05','address180','city180',25,52),
+(182,182,'1977-08-11','address182','city182',40,8),
+(188,188,'2012-03-29','address188','city188',0,0),
+(190,190,'1978-02-19','address190','city190',0,0),
+(215,215,'1982-02-07','address215','city215',40,36),
+(223,223,'2005-02-11','address223','city223',55,52),
+(247,247,'2005-07-02','address247','city247',30,51),
+(255,255,'1997-08-15','address255','city255',0,0),
+(2,2,'1973-05-05','address2','city2',25,52),
+(4,4,'2012-07-21','address4','city4',40,12),
+(6,6,'1982-07-15','address6','city6',0,0),
+(8,8,'1979-02-16','address8','city8',0,0),
+(10,10,'1955-10-06','address10','city10',60,52),
+(12,12,'1977-08-09','address12','city12',40,52),
+(14,14,'2005-03-28','address14','city14',0,0),
+(16,16,'1949-11-07','address16','city16',40,52),
+(18,18,'2012-01-04','address18','city18',0,0),
+(20,20,'2011-01-23','address20','city20',0,0),
+(22,22,'1954-10-14','address22','city22',20,52),
+(24,24,'2010-01-22','address24','city24',0,0),
+(26,26,'1999-08-15','address26','city26',0,0),
+(28,28,'1964-07-05','address28','city28',10,20),
+(30,30,'2004-10-13','address30','city30',0,0),
+(32,32,'2007-06-08','address32','city32',0,0),
+(34,34,'1977-02-23','address34','city34',40,52),
+(36,36,'2007-06-11','address36','city36',75,52),
+(38,38,'1932-04-12','address38','city38',0,0),
+(40,40,'1968-11-16','address40','city40',0,0),
+(42,42,'1996-10-01','address42','city42',38,52),
+(44,44,'1977-08-23','address44','city44',18,6),
+(46,46,'1978-11-23','address46','city46',0,0),
+(48,48,'1998-02-27','address48','city48',0,0),
+(50,50,'1923-09-08','address50','city50',40,52),
+(52,52,'1964-09-09','address52','city52',0,0),
+(55,55,'2001-01-27','address55','city55',40,40),
+(56,56,'1938-08-28','address56','city56',48,52),
+(59,59,'1967-12-19','address59','city59',40,52),
+(60,60,'1969-06-30','address60','city60',40,46),
+(63,63,'2002-04-05','address63','city63',0,0),
+(64,64,'1972-11-21','address64','city64',48,52),
+(67,67,'1988-04-04','address67','city67',0,0),
+(68,68,'1964-07-14','address68','city68',4,16),
+(71,71,'1998-03-09','address71','city71',0,0),
+(72,72,'1960-10-28','address72','city72',35,52),
+(75,75,'1968-04-14','address75','city75',40,52),
+(76,76,'1977-05-13','address76','city76',0,0),
+(79,79,'1982-12-19','address79','city79',0,0),
+(80,80,'1966-01-07','address80','city80',15,12),
+(83,83,'1947-02-09','address83','city83',10,18),
+(84,84,'1976-06-23','address84','city84',40,52),
+(87,87,'2000-10-24','address87','city87',25,4),
+(88,88,'2002-05-23','address88','city88',20,52),
+(91,91,'2000-11-28','address91','city91',60,52),
+(92,92,'1965-07-17','address92','city92',40,52),
+(95,95,'1977-09-16','address95','city95',30,52),
+(96,96,'1994-09-26','address96','city96',0,0),
+(99,99,'2008-02-19','address99','city99',0,0),
+(100,100,'1953-01-07','address100','city100',0,0),
+(103,103,'2010-12-29','address103','city103',0,0),
+(104,104,'1990-12-03','address104','city104',40,52),
+(107,107,'2003-10-27','address107','city107',0,0),
+(108,108,'1998-03-05','address108','city108',40,17),
+(111,111,'2002-10-18','address111','city111',0,0),
+(112,112,'1960-04-02','address112','city112',0,0),
+(115,115,'1989-05-28','address115','city115',40,52),
+(116,116,'1985-10-25','address116','city116',15,52),
+(119,119,'1974-04-15','address119','city119',0,0),
+(120,120,'1926-03-21','address120','city120',0,0),
+(157,157,'1972-03-23','address157','city157',0,0),
+(159,159,'2002-11-08','address159','city159',0,0),
+(165,165,'1998-07-10','address165','city165',0,0),
+(167,167,'1973-11-16','address167','city167',0,0),
+(173,173,'1966-06-26','address173','city173',0,0),
+(175,175,'1957-02-02','address175','city175',0,0),
+(181,181,'1964-11-16','address181','city181',45,26),
+(183,183,'1943-12-02','address183','city183',0,0),
+(189,189,'1986-06-30','address189','city189',0,0),
+(191,191,'2005-05-14','address191','city191',0,0),
+(196,196,'1961-03-23','address196','city196',0,0),
+(197,197,'1955-07-13','address197','city197',0,0),
+(198,198,'2006-11-26','address198','city198',0,0),
+(199,199,'1978-02-06','address199','city199',0,0),
+(208,208,'2012-04-13','address208','city208',48,52),
+(210,210,'1989-08-18','address210','city210',0,0),
+(211,211,'1982-08-17','address211','city211',40,52),
+(212,212,'1919-08-29','address212','city212',0,0),
+(213,213,'1987-03-25','address213','city213',0,0),
+(228,228,'1988-05-05','address228','city228',40,52),
+(229,229,'1936-10-15','address229','city229',0,0),
+(230,230,'1973-08-19','address230','city230',40,52),
+(231,231,'2002-06-18','address231','city231',50,52),
+(240,240,'2011-10-17','address240','city240',60,52),
+(242,242,'1981-07-24','address242','city242',0,0),
+(243,243,'1978-10-12','address243','city243',0,0),
+(244,244,'2003-01-15','address244','city244',0,0),
+(245,245,'1950-09-26','address245','city245',0,0),
+(125,125,'1939-08-02','address125','city125',28,32),
+(126,126,'1984-02-10','address126','city126',0,0),
+(129,129,'1992-01-20','address129','city129',0,0),
+(130,130,'1992-09-18','address130','city130',0,0),
+(133,133,'1996-05-07','address133','city133',24,20),
+(134,134,'1987-07-13','address134','city134',0,0),
+(137,137,'2004-03-10','address137','city137',0,0),
+(138,138,'1989-02-10','address138','city138',0,0),
+(141,141,'1970-03-21','address141','city141',0,0),
+(142,142,'1984-05-25','address142','city142',40,50),
+(145,145,'1959-05-24','address145','city145',0,0),
+(146,146,'1946-07-28','address146','city146',35,16),
+(149,149,'1993-09-16','address149','city149',0,0),
+(150,150,'1975-12-18','address150','city150',0,0),
+(153,153,'1993-12-20','address153','city153',0,0),
+(155,155,'1934-10-29','address155','city155',0,0),
+(161,161,'1969-11-04','address161','city161',50,50),
+(163,163,'1976-05-03','address163','city163',40,52),
+(169,169,'1982-12-19','address169','city169',0,0),
+(171,171,'1976-07-01','address171','city171',0,0),
+(177,177,'2002-11-16','address177','city177',0,0),
+(179,179,'1964-02-05','address179','city179',40,32),
+(185,185,'1981-02-06','address185','city185',0,0),
+(187,187,'1962-06-04','address187','city187',40,52),
+(216,216,'1996-05-21','address216','city216',48,52),
+(248,248,'1963-09-06','address248','city248',0,0),
+(256,256,'1966-07-14','address256','city256',0,0),
+(53,53,'1992-05-25','address53','city53',0,0),
+(57,57,'2003-11-12','address57','city57',25,20),
+(61,61,'1953-01-29','address61','city61',0,0),
+(65,65,'1975-05-02','address65','city65',10,10),
+(69,69,'1938-03-20','address69','city69',0,0),
+(73,73,'1969-05-05','address73','city73',0,0),
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+(81,81,'1985-06-22','address81','city81',0,0),
+(85,85,'2002-10-10','address85','city85',0,0),
+(89,89,'1958-06-16','address89','city89',0,0),
+(93,93,'1962-06-16','address93','city93',0,0),
+(97,97,'1964-10-08','address97','city97',0,0),
+(101,101,'1986-06-11','address101','city101',40,52),
+(105,105,'1999-05-14','address105','city105',40,45),
+(109,109,'2000-05-23','address109','city109',0,0),
+(113,113,'1960-08-03','address113','city113',8,15),
+(117,117,'1982-02-15','address117','city117',50,36),
+(121,121,'1998-10-18','address121','city121',24,52),
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+(193,193,'1973-05-03','address193','city193',0,0),
+(194,194,'1980-01-14','address194','city194',40,52),
+(195,195,'1975-07-15','address195','city195',45,52),
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+(201,201,'2008-05-20','address201','city201',3,28),
+(202,202,'2000-06-30','address202','city202',12,52),
+(203,203,'1992-07-08','address203','city203',50,52),
+(204,204,'1988-07-05','address204','city204',14,40),
+(205,205,'1950-10-29','address205','city205',0,0),
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+(214,214,'1973-12-14','address214','city214',0,0),
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+(218,218,'2007-07-20','address218','city218',0,0),
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+(221,221,'2007-03-26','address221','city221',50,52),
+(222,222,'1989-12-24','address222','city222',0,0),
+(224,224,'1975-07-14','address224','city224',0,0),
+(225,225,'1976-02-23','address225','city225',20,52),
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+(232,232,'1958-01-01','address232','city232',0,0),
+(233,233,'1966-08-03','address233','city233',40,32),
+(234,234,'1975-10-22','address234','city234',40,52),
+(235,235,'1983-10-25','address235','city235',0,0),
+(236,236,'1974-03-07','address236','city236',0,0),
+(237,237,'1965-12-31','address237','city237',45,20),
+(238,238,'1971-10-16','address238','city238',0,0),
+(239,239,'1989-07-19','address239','city239',0,0),
+(246,246,'1960-07-08','address246','city246',0,0),
+(249,249,'1943-07-01','address249','city249',40,30),
+(250,250,'1983-10-15','address250','city250',30,52),
+(251,251,'1979-07-03','address251','city251',0,0),
+(252,252,'1985-10-04','address252','city252',15,4),
+(253,253,'1966-10-24','address253','city253',0,0),
+(254,254,'1956-02-02','address254','city254',0,0),
+(1,1,'2003-11-23','address1','city1',40,52),
+(3,3,'1938-01-23','address3','city3',0,0),
+(5,5,'2006-12-27','address5','city5',40,48),
+(7,7,'1969-04-09','address7','city7',0,0),
+(9,9,'2006-06-14','address9','city9',0,0),
+(11,11,'1999-01-12','address11','city11',40,52),
+(13,13,'1968-01-13','address13','city13',50,12),
+(15,15,'1960-04-11','address15','city15',0,0),
+(17,17,'2006-10-13','address17','city17',40,52),
+(19,19,'1950-08-19','address19','city19',0,0),
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+(23,23,'1952-06-09','address23','city23',40,52),
+(25,25,'1934-12-08','address25','city25',32,40),
+(27,27,'1995-04-19','address27','city27',40,45),
+(29,29,'1986-01-14','address29','city29',44,52),
+(31,31,'1978-04-19','address31','city31',10,20),
+(33,33,'1989-11-23','address33','city33',25,10),
+(35,35,'2012-01-02','address35','city35',8,48),
+(37,37,'2005-08-24','address37','city37',40,42),
+(39,39,'1973-11-02','address39','city39',40,52),
+(41,41,'2011-10-12','address41','city41',20,30),
+(43,43,'1960-12-24','address43','city43',0,0),
+(45,45,'1990-04-17','address45','city45',35,40),
+(47,47,'1964-04-02','address47','city47',0,0),
+(49,49,'1957-01-25','address49','city49',40,52),
+(51,51,'1970-10-20','address51','city51',0,0),
+(54,54,'1987-09-30','address54','city54',0,0),
+(58,58,'1975-05-07','address58','city58',0,0),
+(62,62,'1972-08-03','address62','city62',40,52),
+(66,66,'1995-11-04','address66','city66',0,0),
+(70,70,'1985-10-19','address70','city70',40,52),
+(74,74,'1969-06-09','address74','city74',0,0),
+(78,78,'2003-01-16','address78','city78',66,52),
+(82,82,'2012-04-29','address82','city82',50,30),
+(86,86,'2008-02-03','address86','city86',0,0),
+(90,90,'1973-05-15','address90','city90',35,12),
+(94,94,'1987-10-28','address94','city94',40,50),
+(98,98,'1973-06-10','address98','city98',65,50),
+(102,102,'2009-09-13','address102','city102',0,0),
+(106,106,'1986-07-03','address106','city106',0,0),
+(110,110,'1982-06-10','address110','city110',35,52),
+(114,114,'1963-10-08','address114','city114',48,52),
+(118,118,'1948-03-07','address118','city118',0,0),
+(122,122,'1997-12-19','address122','city122',0,0),
+(124,124,'1966-03-25','address124','city124',0,0),
+(128,128,'1968-08-13','address128','city128',0,0),
+(132,132,'1989-09-25','address132','city132',20,20),
+(136,136,'1993-09-02','address136','city136',0,0),
+(140,140,'1981-05-31','address140','city140',48,52),
+(144,144,'1960-09-15','address144','city144',0,0),
+(148,148,'1945-02-13','address148','city148',40,38),
+(152,152,'2010-11-13','address152','city152',20,52),
+(154,154,'1950-11-07','address154','city154',55,52),
+(160,160,'1981-01-17','address160','city160',0,0),
+(162,162,'2001-03-19','address162','city162',0,0),
+(168,168,'2003-03-28','address168','city168',0,0),
+(170,170,'1977-06-18','address170','city170',50,52),
+(176,176,'1967-04-15','address176','city176',30,50),
+(178,178,'1989-10-25','address178','city178',60,12),
+(184,184,'2004-04-21','address184','city184',0,0),
+(186,186,'1952-11-08','address186','city186',50,48),
+(209,209,'1943-03-15','address209','city209',40,30),
+(241,241,'1979-12-02','address241','city241',0,0),
+(257,257,'2010-03-06','address257','city257',40,47);
+--enable_query_log
+
+select * from t1 where hours_worked_per_week = 40 and weeks_worked_last_year = 52 and dob < '1949-11-21';
+select * from t1 IGNORE INDEX(dob, weeks_worked_last_year, hours_worked_per_week) where hours_worked_per_week = 40 and weeks_worked_last_year = 52 and dob < '1949-11-21';
+
+drop table t1;

sql/ha_partition.cc

@@ -80,6 +80,7 @@ static handler *partition_create_handler(handlerton *hton,
 static uint partition_flags();
 static uint alter_table_flags(uint flags);
 
+extern "C" int cmp_key_then_part_id(void *key_p, uchar *ref1, uchar *ref2);
 
 static int partition_initialize(void *p)
 {
@@ -4540,8 +4541,8 @@ bool ha_partition::init_record_priority_queue()
     } while (++i < m_tot_parts);
     m_start_key.key= (const uchar*)ptr;
     /* Initialize priority queue, initialized to reading forward. */
-    if (init_queue(&m_queue, used_parts, (uint) PARTITION_BYTES_IN_POS,
-                   0, key_rec_cmp, (void*)m_curr_key_info, 0, 0))
+    if (init_queue(&m_queue, used_parts, 0,
+                   0, cmp_key_then_part_id, (void*)m_curr_key_info, 0, 0))
     {
       my_free(m_ordered_rec_buffer);
       m_ordered_rec_buffer= NULL;
@@ -4737,6 +4738,52 @@ int ha_partition::index_read_map(uchar *buf, const uchar *key,
 }
 
 
+/*
+  @brief
+  Provide ordering by (key_value, partition_id). 
+  
+  @detail
+  Ordering by partition id is required so that key scans on key=const
+  return rows in rowid order (this is required for some variants of 
+  index_merge to work).  
+  
+  In ha_partition, rowid is a (partition_id, underlying_table_rowid). 
+  handle_ordered_index_scan must return rows ordered by (key, rowid).
+
+  If two rows have the same key value and come from different partitions, 
+  it is sufficient to return them in the order of their partition_id.
+*/
+
+extern "C" int cmp_key_then_part_id(void *key_p, uchar *ref1, uchar *ref2)
+{
+  my_ptrdiff_t diff1, diff2;
+  int res;
+
+  if ((res= key_rec_cmp(key_p, ref1 + PARTITION_BYTES_IN_POS, 
+                        ref2 + PARTITION_BYTES_IN_POS)))
+  {
+    return res;
+  }
+  
+  /* The following was taken from ha_partition::cmp_ref */
+  diff1= ref2[1] - ref1[1];
+  diff2= ref2[0] - ref1[0];
+  if (!diff1 && !diff2)
+    return 0;
+
+  if (diff1 > 0)
+    return(-1);
+
+  if (diff1 < 0)
+    return(+1);
+
+  if (diff2 > 0)
+    return(-1);
+
+  return(+1);
+}
+
+
 /**
   Common routine for a number of index_read variants
 
@@ -7647,6 +7694,16 @@ uint ha_partition::min_record_length(uint options) const
     If they belong to different partitions we decide that they are not
     the same record. Otherwise we use the particular handler to decide if
     they are the same. Sort in partition id order if not equal.
+
+  MariaDB note: 
+    Please don't merge the code from MySQL that does this:
+
+    We get two references and need to check if those records are the same.
+    If they belong to different partitions we decide that they are not
+    the same record. Otherwise we use the particular handler to decide if
+    they are the same. Sort in partition id order if not equal.
+
+    It is incorrect, MariaDB has an alternative fix.
 */
 
 int ha_partition::cmp_ref(const uchar *ref1, const uchar *ref2)