diff --git a/mysql-test/main/partition_index_scan,desc.rdiff b/mysql-test/main/partition_index_scan,desc.rdiff
index 99093b20a3982..b54723f1cb10f 100644
--- a/mysql-test/main/partition_index_scan,desc.rdiff
+++ b/mysql-test/main/partition_index_scan,desc.rdiff
@@ -287,8 +287,8 @@
  show status like 'handler_read_%';
  Variable_name	Value
  Handler_read_first	0
--Handler_read_key	6
-+Handler_read_key	7
+-Handler_read_key	8
++Handler_read_key	11
  Handler_read_last	0
  Handler_read_next	0
  Handler_read_prev	0
@@ -301,9 +301,12 @@
  ) PARTITION BY RANGE COLUMNS(d) (
  PARTITION `p1` VALUES LESS THAN (10),
  PARTITION `p2` VALUES LESS THAN (20),
-@@ -688,8 +688,8 @@
+@@ -686,10 +686,10 @@
+ show status like 'handler_read_%';
+ Variable_name	Value
  Handler_read_first	0
- Handler_read_key	1
+-Handler_read_key	1
++Handler_read_key	2
  Handler_read_last	0
 -Handler_read_next	2
 -Handler_read_prev	0
@@ -312,6 +315,15 @@
  Handler_read_retry	0
  Handler_read_rnd	0
  Handler_read_rnd_deleted	0
+@@ -698,7 +698,7 @@
+ set @js='$out';
+ select json_extract(@js,'$**.partitions_index_scan_method');
+ json_extract(@js,'$**.partitions_index_scan_method')
+-["iterate_over_partitions"]
++["merge_ordered_scans"]
+ ## Case 2: Unordered scans in ha_partition::index_read_map
+ flush status;
+ select * from t1 where c = 120 ORDER BY d DESC LIMIT 1;
 @@ -725,8 +725,8 @@
  Handler_read_first	0
  Handler_read_key	1
@@ -479,7 +491,7 @@
  Handler_read_prev	0
  Handler_read_retry	0
  Handler_read_rnd	0
-@@ -1202,18 +1202,18 @@
+@@ -1202,20 +1202,20 @@
  #
  # MDEV-40479: Reverse prefix scan across partitions
  #
@@ -503,3 +515,5 @@
 +11
 +15
  DROP TABLE t1;
+ #
+ # MDEV-41366 Wrong result with Case 2 of non-merging partition index scan
diff --git a/mysql-test/main/partition_index_scan.result b/mysql-test/main/partition_index_scan.result
index 4450568bdc5fb..dba3b683765f6 100644
--- a/mysql-test/main/partition_index_scan.result
+++ b/mysql-test/main/partition_index_scan.result
@@ -630,8 +630,8 @@ select json_extract(@js,'$**.partitions_index_scan_method');
 json_extract(@js,'$**.partitions_index_scan_method')
 ["iterate_over_partitions"]
 DROP TABLE t1;
-## Case 2: calls to ha_partition::read_range_first and
-## ha_partition::index_read_map, via loose index scan
+## Case 2 loose index scan: calls to read_range_first iterates;
+## index_read_map with AFTER_KEY is merged
 CREATE TABLE t1 (
 a INT,
 b INT,
@@ -651,7 +651,7 @@ a	MIN(b)
 show status like 'handler_read_%';
 Variable_name	Value
 Handler_read_first	0
-Handler_read_key	6
+Handler_read_key	8
 Handler_read_last	0
 Handler_read_next	0
 Handler_read_prev	0
@@ -662,7 +662,7 @@ Handler_read_rnd_next	0
 set @js='$out';
 select json_extract(@js,'$**.partitions_index_scan_method');
 json_extract(@js,'$**.partitions_index_scan_method')
-["iterate_over_partitions"]
+["both"]
 DROP TABLE t1;
 CREATE TABLE t1 (
 c int,
@@ -676,7 +676,8 @@ PARTITION `p4` VALUES LESS THAN (40),
 PARTITION `p5` VALUES LESS THAN MAXVALUE
 );
 insert into t1 select t2.seq, t3.seq from seq_100_to_200 t2, seq_1_to_50 t3;
-## Case 2: Unordered scans with MRR
+## Case 2: iterated partition scans with MRR with ASC key;
+## merged scan with index_read_map with DESC key
 flush status;
 select * from t1 where c=113 and d between 25 and 35 order by d limit 3;
 c	d
@@ -1217,3 +1218,227 @@ c
 11
 6
 DROP TABLE t1;
+#
+# MDEV-41366 Wrong result with Case 2 of non-merging partition index scan
+#
+# prefix-only PREFIX_LAST_OR_PREV, merged scan
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c), KEY(b,c)) ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,5),(2,0,12);
+SELECT c FROM t1 FORCE INDEX(b) WHERE b=1 AND c<=15 ORDER BY c DESC;
+c
+5
+set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+json_extract(@js,'$**.partitions_index_scan_method')
+["merge_ordered_scans"]
+DROP TABLE t1;
+# same but with desc key
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c), KEY(b desc,c desc)) ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,5),(2,0,12);
+SELECT c FROM t1 FORCE INDEX(b) WHERE b=0 AND c>=2 ORDER BY c;
+c
+12
+set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+json_extract(@js,'$**.partitions_index_scan_method')
+["merge_ordered_scans"]
+DROP TABLE t1;
+# prefix-only KEY_OR_NEXT / PREFIX_LAST_OR_PREV (opt_sum)
+# merged scan
+CREATE TABLE t1 (a INT, b INT, c INT NOT NULL, PRIMARY KEY(a,c), KEY k(b,c))
+ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+PARTITION p1 VALUES LESS THAN (20),
+PARTITION p2 VALUES LESS THAN (30));
+# no b=1 in p0 or p2
+INSERT INTO t1 VALUES (1,2,5),(2,1,15),(3,0,22);
+EXPLAIN
+SELECT MIN(c) FROM t1 WHERE b=1;
+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
+FLUSH STATUS;
+SELECT MIN(c) FROM t1 WHERE b=1;
+MIN(c)
+15
+# 3 (merged scan: p0, p1, p2)
+SHOW STATUS LIKE 'Handler_read_key';
+Variable_name	Value
+Handler_read_key	3
+EXPLAIN
+SELECT MAX(c) FROM t1 WHERE b=1;
+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
+FLUSH STATUS;
+SELECT MAX(c) FROM t1 WHERE b=1;
+MAX(c)
+15
+# 3 (merged scan: p0, p1, p2)
+SHOW STATUS LIKE 'Handler_read_key';
+Variable_name	Value
+Handler_read_key	3
+DROP TABLE t1;
+# index_first with prefix does not qualify Case 2
+# neither does index_read_map with AFTER_KEY
+CREATE TABLE t1 (a INT, b INT, KEY(a,b)) ENGINE=InnoDB
+PARTITION BY RANGE COLUMNS(b) (PARTITION p1 VALUES LESS THAN (100),
+PARTITION p2 VALUES LESS THAN (200),
+PARTITION p3 VALUES LESS THAN MAXVALUE);
+INSERT INTO t1 SELECT seq % 3 + 1, seq FROM seq_1_to_300;
+ANALYZE TABLE t1 PERSISTENT FOR ALL;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	status	OK
+EXPLAIN SELECT a, MIN(b) FROM t1 GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	NULL	a	10	NULL	4	Using index for group-by
+SELECT a, MIN(b) FROM t1 GROUP BY a;
+a	MIN(b)
+1	3
+2	1
+3	2
+set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+json_extract(@js,'$**.partitions_index_scan_method')
+["merge_ordered_scans"]
+DROP TABLE t1;
+# index_first with prefix does not qualify Case 2
+# neither does index_read_map with AFTER_KEY
+CREATE TABLE t1 (a INT, c INT, KEY(a,c)) ENGINE=InnoDB
+PARTITION BY RANGE COLUMNS(c) (PARTITION p1 VALUES LESS THAN (100),
+PARTITION p2 VALUES LESS THAN (200),
+PARTITION p3 VALUES LESS THAN MAXVALUE);
+INSERT INTO t1 SELECT 1, seq FROM seq_1_to_99;
+INSERT INTO t1 SELECT 2, seq FROM seq_100_to_199;
+INSERT INTO t1 SELECT 3, seq FROM seq_1_to_99;
+ANALYZE TABLE t1 PERSISTENT FOR ALL;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Engine-independent statistics collected
+test.t1	analyze	status	OK
+EXPLAIN SELECT a, MIN(c) FROM t1 GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	NULL	a	10	NULL	4	Using index for group-by
+SELECT a, MIN(c) FROM t1 GROUP BY a;
+a	MIN(c)
+1	1
+2	100
+3	1
+set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+json_extract(@js,'$**.partitions_index_scan_method')
+["merge_ordered_scans"]
+DROP TABLE t1;
+# AFTER_KEY / BEFORE_KEY merged scan
+CREATE TABLE t1 (a INT, b INT, c INT NOT NULL, PRIMARY KEY(a,c), KEY k(b,c))
+ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+PARTITION p1 VALUES LESS THAN (20),
+PARTITION p2 VALUES LESS THAN (30));
+INSERT INTO t1 VALUES
+(1,1,5),(2,2,7),(3,2,10),(4,1,15),(5,2,18),(6,0,22),(7,1,25);
+EXPLAIN
+SELECT MIN(c) FROM t1 WHERE b=1 AND c>5;
+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
+FLUSH STATUS;
+SELECT MIN(c) FROM t1 WHERE b=1 AND c>5;
+MIN(c)
+15
+# 3 (merged: p0, p1, p2)
+SHOW STATUS LIKE 'Handler_read_key';
+Variable_name	Value
+Handler_read_key	3
+EXPLAIN
+SELECT MAX(c) FROM t1 WHERE b=1 AND c<25;
+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
+FLUSH STATUS;
+SELECT MAX(c) FROM t1 WHERE b=1 AND c<25;
+MAX(c)
+15
+# 3 (merged: p0, p1, p2)
+SHOW STATUS LIKE 'Handler_read_key';
+Variable_name	Value
+Handler_read_key	3
+DROP TABLE t1;
+# KEY_OR_PREV needs a reverse partition scan.
+# Ordered scan (HASH)
+CREATE TABLE t1 (a INT, KEY k(a)) ENGINE=InnoDB
+PARTITION BY HASH(a) PARTITIONS 3;
+INSERT INTO t1 SELECT seq FROM seq_1_to_9;
+HANDLER t1 OPEN;
+HANDLER t1 READ k <= (5);
+a
+5
+HANDLER t1 READ k PREV;
+a
+4
+HANDLER t1 READ k PREV;
+a
+3
+HANDLER t1 READ k <= (0);
+a
+HANDLER t1 CLOSE;
+DROP TABLE t1;
+# KEY_OR_PREV merged scan
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c), KEY k(b,c))
+ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,5),(2,2,7),(3,1,15);
+HANDLER t1 OPEN;
+HANDLER t1 READ k <= (1,17);
+a	b	c
+3	1	15
+HANDLER t1 READ k PREV;
+a	b	c
+1	1	5
+HANDLER t1 CLOSE;
+DROP TABLE t1;
+# A range spanning several prefix values:
+# QUICK_SELECT_DESC reads PREFIX_LAST_OR_PREV (2,15), keypart_map=b,c
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c), KEY k(b,c)) ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,7),(2,2,7),(3,1,12),(4,2,12);
+EXPLAIN SELECT b, c FROM t1 FORCE INDEX(k)
+WHERE b BETWEEN 1 AND 2 AND c BETWEEN 5 AND 15 ORDER BY b DESC, c DESC;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	k	k	9	NULL	4	Using where; Using index
+SELECT b, c FROM t1 FORCE INDEX(k)
+WHERE b BETWEEN 1 AND 2 AND c BETWEEN 5 AND 15 ORDER BY b DESC, c DESC;
+b	c
+2	12
+2	7
+1	12
+1	7
+set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+json_extract(@js,'$**.partitions_index_scan_method')
+["merge_ordered_scans"]
+DROP TABLE t1;
+# reset m_unordered_prefix_len: single partition scan skips
+# can_skip_merging_scans()
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c),
+KEY k(b,c), KEY kc(c,b)) ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,5),(2,1,12),(3,2,13);
+HANDLER t1 OPEN;
+# 2 partitions, Case 2 on k(b,c): m_unordered_prefix_len = len(b)
+HANDLER t1 READ k = (1);
+a	b	c
+1	1	5
+# full key of kc binds c: single partition p1, reset
+HANDLER t1 READ kc = (13,2);
+a	b	c
+3	2	13
+# expect (2,1,12)
+HANDLER t1 READ kc PREV;
+a	b	c
+2	1	12
+HANDLER t1 CLOSE;
+DROP TABLE t1;
diff --git a/mysql-test/main/partition_index_scan.test b/mysql-test/main/partition_index_scan.test
index 571dc014b7a0d..642a07fe3891d 100644
--- a/mysql-test/main/partition_index_scan.test
+++ b/mysql-test/main/partition_index_scan.test
@@ -1,5 +1,6 @@
 --source include/have_partition.inc
 --source include/have_sequence.inc
+--source include/have_innodb.inc
 --source include/no_view_protocol.inc
 --disable_ps2_protocol
 --disable_cursor_protocol
@@ -251,8 +252,8 @@ select json_extract(@js,'$**.partitions_index_scan_method');
 
 DROP TABLE t1;
 
---echo ## Case 2: calls to ha_partition::read_range_first and
---echo ## ha_partition::index_read_map, via loose index scan
+--echo ## Case 2 loose index scan: calls to read_range_first iterates;
+--echo ## index_read_map with AFTER_KEY is merged
 eval
 CREATE TABLE t1 (
  a INT,
@@ -293,7 +294,8 @@ CREATE TABLE t1 (
 
 insert into t1 select t2.seq, t3.seq from seq_100_to_200 t2, seq_1_to_50 t3;
 
---echo ## Case 2: Unordered scans with MRR
+--echo ## Case 2: iterated partition scans with MRR with ASC key;
+--echo ## merged scan with index_read_map with DESC key
 flush status;
 select * from t1 where c=113 and d between 25 and 35 order by d limit 3;
 show status like 'handler_read_%';
@@ -572,5 +574,180 @@ SELECT c FROM t1 WHERE b = 1 ORDER BY c $opposite_direction;
 
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-41366 Wrong result with Case 2 of non-merging partition index scan
+--echo #
+
+--echo # prefix-only PREFIX_LAST_OR_PREV, merged scan
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c), KEY(b,c)) ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+                       PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,5),(2,0,12);
+let $q=
+SELECT c FROM t1 FORCE INDEX(b) WHERE b=1 AND c<=15 ORDER BY c DESC;
+eval $q;
+let $out=`analyze format=json $q`;
+evalp set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+
+DROP TABLE t1;
+
+--echo # same but with desc key
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c), KEY(b desc,c desc)) ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+                       PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,5),(2,0,12);
+let $q=
+SELECT c FROM t1 FORCE INDEX(b) WHERE b=0 AND c>=2 ORDER BY c;
+eval $q;
+let $out=`analyze format=json $q`;
+evalp set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+
+DROP TABLE t1;
+
+--echo # prefix-only KEY_OR_NEXT / PREFIX_LAST_OR_PREV (opt_sum)
+--echo # merged scan
+CREATE TABLE t1 (a INT, b INT, c INT NOT NULL, PRIMARY KEY(a,c), KEY k(b,c))
+  ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+                       PARTITION p1 VALUES LESS THAN (20),
+                       PARTITION p2 VALUES LESS THAN (30));
+--echo # no b=1 in p0 or p2
+INSERT INTO t1 VALUES (1,2,5),(2,1,15),(3,0,22);
+
+EXPLAIN
+SELECT MIN(c) FROM t1 WHERE b=1;
+FLUSH STATUS;
+SELECT MIN(c) FROM t1 WHERE b=1;
+--echo # 3 (merged scan: p0, p1, p2)
+SHOW STATUS LIKE 'Handler_read_key';
+
+EXPLAIN
+SELECT MAX(c) FROM t1 WHERE b=1;
+FLUSH STATUS;
+SELECT MAX(c) FROM t1 WHERE b=1;
+--echo # 3 (merged scan: p0, p1, p2)
+SHOW STATUS LIKE 'Handler_read_key';
+DROP TABLE t1;
+
+--echo # index_first with prefix does not qualify Case 2
+--echo # neither does index_read_map with AFTER_KEY
+CREATE TABLE t1 (a INT, b INT, KEY(a,b)) ENGINE=InnoDB
+PARTITION BY RANGE COLUMNS(b) (PARTITION p1 VALUES LESS THAN (100),
+                               PARTITION p2 VALUES LESS THAN (200),
+                               PARTITION p3 VALUES LESS THAN MAXVALUE);
+INSERT INTO t1 SELECT seq % 3 + 1, seq FROM seq_1_to_300;
+ANALYZE TABLE t1 PERSISTENT FOR ALL;
+let $q=
+SELECT a, MIN(b) FROM t1 GROUP BY a;
+eval EXPLAIN $q;
+eval $q;
+let $out=`analyze format=json $q`;
+evalp set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+DROP TABLE t1;
+
+--echo # index_first with prefix does not qualify Case 2
+--echo # neither does index_read_map with AFTER_KEY
+CREATE TABLE t1 (a INT, c INT, KEY(a,c)) ENGINE=InnoDB
+PARTITION BY RANGE COLUMNS(c) (PARTITION p1 VALUES LESS THAN (100),
+                               PARTITION p2 VALUES LESS THAN (200),
+                               PARTITION p3 VALUES LESS THAN MAXVALUE);
+INSERT INTO t1 SELECT 1, seq FROM seq_1_to_99;
+INSERT INTO t1 SELECT 2, seq FROM seq_100_to_199;
+INSERT INTO t1 SELECT 3, seq FROM seq_1_to_99;
+ANALYZE TABLE t1 PERSISTENT FOR ALL;
+let $q=
+SELECT a, MIN(c) FROM t1 GROUP BY a;
+eval EXPLAIN $q;
+eval $q;
+let $out=`analyze format=json $q`;
+evalp set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+DROP TABLE t1;
+
+--echo # AFTER_KEY / BEFORE_KEY merged scan
+CREATE TABLE t1 (a INT, b INT, c INT NOT NULL, PRIMARY KEY(a,c), KEY k(b,c))
+  ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+                       PARTITION p1 VALUES LESS THAN (20),
+                       PARTITION p2 VALUES LESS THAN (30));
+INSERT INTO t1 VALUES
+  (1,1,5),(2,2,7),(3,2,10),(4,1,15),(5,2,18),(6,0,22),(7,1,25);
+
+EXPLAIN
+SELECT MIN(c) FROM t1 WHERE b=1 AND c>5;
+FLUSH STATUS;
+SELECT MIN(c) FROM t1 WHERE b=1 AND c>5;
+--echo # 3 (merged: p0, p1, p2)
+SHOW STATUS LIKE 'Handler_read_key';
+
+EXPLAIN
+SELECT MAX(c) FROM t1 WHERE b=1 AND c<25;
+FLUSH STATUS;
+SELECT MAX(c) FROM t1 WHERE b=1 AND c<25;
+--echo # 3 (merged: p0, p1, p2)
+SHOW STATUS LIKE 'Handler_read_key';
+DROP TABLE t1;
+
+--echo # KEY_OR_PREV needs a reverse partition scan.
+--echo # Ordered scan (HASH)
+CREATE TABLE t1 (a INT, KEY k(a)) ENGINE=InnoDB
+PARTITION BY HASH(a) PARTITIONS 3;
+INSERT INTO t1 SELECT seq FROM seq_1_to_9;
+HANDLER t1 OPEN;
+HANDLER t1 READ k <= (5);
+HANDLER t1 READ k PREV;
+HANDLER t1 READ k PREV;
+HANDLER t1 READ k <= (0);
+HANDLER t1 CLOSE;
+DROP TABLE t1;
+
+--echo # KEY_OR_PREV merged scan
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c), KEY k(b,c))
+ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+                       PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,5),(2,2,7),(3,1,15);
+HANDLER t1 OPEN;
+HANDLER t1 READ k <= (1,17);
+HANDLER t1 READ k PREV;
+HANDLER t1 CLOSE;
+DROP TABLE t1;
+
+--echo # A range spanning several prefix values:
+--echo # QUICK_SELECT_DESC reads PREFIX_LAST_OR_PREV (2,15), keypart_map=b,c
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c), KEY k(b,c)) ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+                       PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,7),(2,2,7),(3,1,12),(4,2,12);
+let $q=
+SELECT b, c FROM t1 FORCE INDEX(k)
+WHERE b BETWEEN 1 AND 2 AND c BETWEEN 5 AND 15 ORDER BY b DESC, c DESC;
+eval EXPLAIN $q;
+eval $q;
+let $out=`analyze format=json $q`;
+evalp set @js='$out';
+select json_extract(@js,'$**.partitions_index_scan_method');
+DROP TABLE t1;
+
+--echo # reset m_unordered_prefix_len: single partition scan skips
+--echo # can_skip_merging_scans()
+CREATE TABLE t1 (a INT, b INT, c INT, PRIMARY KEY(a,c),
+                 KEY k(b,c), KEY kc(c,b)) ENGINE=InnoDB
+PARTITION BY RANGE(c) (PARTITION p0 VALUES LESS THAN (10),
+                       PARTITION p1 VALUES LESS THAN (20));
+INSERT INTO t1 VALUES (1,1,5),(2,1,12),(3,2,13);
+HANDLER t1 OPEN;
+--echo # 2 partitions, Case 2 on k(b,c): m_unordered_prefix_len = len(b)
+HANDLER t1 READ k = (1);
+--echo # full key of kc binds c: single partition p1, reset
+HANDLER t1 READ kc = (13,2);
+--echo # expect (2,1,12)
+HANDLER t1 READ kc PREV;
+HANDLER t1 CLOSE;
+DROP TABLE t1;
+
 --enable_cursor_protocol
 --enable_ps2_protocol
diff --git a/sql/ha_partition.cc b/sql/ha_partition.cc
index 343ab395ffc32..143a2bf56ef81 100644
--- a/sql/ha_partition.cc
+++ b/sql/ha_partition.cc
@@ -5991,7 +5991,8 @@ int ha_partition::common_index_read(uchar *buf, bool have_start_key)
   }
 
   if (have_start_key &&
-      (m_start_key.flag == HA_READ_PREFIX_LAST ||
+      (m_start_key.flag == HA_READ_KEY_OR_PREV ||
+       m_start_key.flag == HA_READ_PREFIX_LAST ||
        m_start_key.flag == HA_READ_PREFIX_LAST_OR_PREV ||
        m_start_key.flag == HA_READ_BEFORE_KEY))
   {
@@ -7515,8 +7516,23 @@ bool ha_partition::can_skip_merging_scans()
         (key_part->key_part_flag & HA_REVERSE_SORT) ? true : false;
       if (m_index_scan_type == partition_index_read)
       {
-        return
-          m_start_key.key && (m_start_key.keypart_map & prefix) == prefix;
+        switch (m_start_key.flag) {
+        case HA_READ_AFTER_KEY:
+        case HA_READ_BEFORE_KEY:
+        case HA_READ_KEY_OR_NEXT:
+        case HA_READ_KEY_OR_PREV:
+        case HA_READ_PREFIX_LAST_OR_PREV:
+          return false;
+        case HA_READ_KEY_EXACT:
+        case HA_READ_PREFIX:
+        case HA_READ_PREFIX_LAST:
+          return m_start_key.key &&
+            (m_start_key.keypart_map & prefix) == prefix;
+        default:
+          /* SPATIAL keys have a single key part, so Case 2 cannot apply */
+          DBUG_ASSERT(0);
+          return false;
+        }
       }
       else if (m_index_scan_type == partition_read_range)
       {
@@ -7595,6 +7611,7 @@ int ha_partition::partition_scan_set_up(uchar * buf, bool idx_read_flag)
     DBUG_PRINT("info", ("index scan using the single partition %u",
 			(uint) m_part_spec.start_part));
     m_ordered_scan_ongoing= FALSE;
+    m_unordered_prefix_len= 0;
   }
   else
   {