Amend the previous cset:

Make EXPLAIN better at displaying MRR/BKA:
- Update all .result files
- Extra comments

mysql-test/suite/innodb/r/innodb.result

@@ -1219,7 +1219,7 @@ count(*)
 623
 explain select * from t1 where c between 1 and 2500;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	range	c	c	5	NULL	#	Using index condition; Using MRR
+1	SIMPLE	t1	range	c	c	5	NULL	#	Using index condition; Rowid-ordered scan
 update t1 set c=a;
 explain select * from t1 where c between 1 and 2500;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

mysql-test/suite/maria/r/maria.result

@@ -389,10 +389,10 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ref	a	a	4	test.t2.a	3	
 explain select * from t1 where a=0 or a=2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	range	a	a	4	NULL	4	Using index condition; Using MRR
+1	SIMPLE	t1	range	a	a	4	NULL	4	Using index condition; Rowid-ordered scan
 explain select * from t1 force index (a) where a=0 or a=2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	range	a	a	4	NULL	4	Using index condition; Using MRR
+1	SIMPLE	t1	range	a	a	4	NULL	4	Using index condition; Rowid-ordered scan
 explain select * from t1 where c=1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ref	c,c_2	c	5	const	1	

mysql-test/suite/vcol/r/vcol_misc.result

@@ -144,7 +144,7 @@ explain
 select * from t1,t2 where t1.b=t2.c and d <= 100;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
-1	SIMPLE	t2	ref	idx	idx	5	test.t1.b	2	Using where; Using join buffer (flat, BKA join)
+1	SIMPLE	t2	ref	idx	idx	5	test.t1.b	2	Using where; Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan
 select * from t1,t2 where t1.b=t2.c and d <= 100;
 a	b	c	d	v
 4	20	20	100	101

mysql-test/suite/vcol/r/vcol_select_innodb.result

@@ -44,7 +44,7 @@ a	b	c
 1	-1	-1
 explain select * from t3 where c>=-1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Using MRR
+1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Rowid-ordered scan
 # select_type=SIMPLE, type=ref
 select * from t1,t3 where t1.c=t3.c and t3.c=-1;
 a	b	c	a	b	c
@@ -141,7 +141,7 @@ a	b	c
 2	-2	-2
 explain select * from t3 where c >= -2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Using MRR
+1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Rowid-ordered scan
 # SELECT * FROM tbl_name WHERE <non-vcol expr>
 select * from t3 where a between 1 and 2;
 a	b	c
@@ -165,7 +165,7 @@ a	b	c
 2	-2	-2
 explain select * from t3 where c between -2 and -1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Using MRR
+1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Rowid-ordered scan
 # SELECT * FROM tbl_name WHERE <non-vcol expr> ORDER BY <non-indexed vcol>
 select * from t3 where a between 1 and 2 order by b;
 a	b	c
@@ -205,7 +205,7 @@ a	b	c
 1	-1	-1
 explain select * from t3 where c between -2 and -1 order by b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Using MRR; Using filesort
+1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Rowid-ordered scan; Using filesort
 # SELECT * FROM tbl_name WHERE <non-indexed vcol expr> ORDER BY <indexed vcol>
 select * from t3 where b between -2 and -1 order by c;
 a	b	c

mysql-test/suite/vcol/r/vcol_select_myisam.result

@@ -44,7 +44,7 @@ a	b	c
 1	-1	-1
 explain select * from t3 where c>=-1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	c	c	5	NULL	2	Using index condition; Using MRR
+1	SIMPLE	t3	range	c	c	5	NULL	2	Using index condition; Rowid-ordered scan
 # select_type=SIMPLE, type=ref
 select * from t1,t3 where t1.c=t3.c and t3.c=-1;
 a	b	c	a	b	c
@@ -141,7 +141,7 @@ a	b	c
 1	-1	-1
 explain select * from t3 where c >= -2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	c	c	5	NULL	2	Using index condition; Using MRR
+1	SIMPLE	t3	range	c	c	5	NULL	2	Using index condition; Rowid-ordered scan
 # SELECT * FROM tbl_name WHERE <non-vcol expr>
 select * from t3 where a between 1 and 2;
 a	b	c
@@ -149,7 +149,7 @@ a	b	c
 1	-1	-1
 explain select * from t3 where a between 1 and 2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	PRIMARY	PRIMARY	4	NULL	1	Using index condition; Using MRR
+1	SIMPLE	t3	range	PRIMARY	PRIMARY	4	NULL	1	Using index condition; Rowid-ordered scan
 # SELECT * FROM tbl_name WHERE <non-indexed vcol expr>
 select * from t3 where b between -2 and -1;
 a	b	c
@@ -165,7 +165,7 @@ a	b	c
 1	-1	-1
 explain select * from t3 where c between -2 and -1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Using MRR
+1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Rowid-ordered scan
 # SELECT * FROM tbl_name WHERE <non-vcol expr> ORDER BY <indexed vcol>
 select * from t3 where a between 1 and 2 order by c;
 a	b	c
@@ -173,7 +173,7 @@ a	b	c
 1	-1	-1
 explain select * from t3 where a between 1 and 2 order by c;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	PRIMARY	PRIMARY	4	NULL	1	Using index condition; Using where; Using MRR; Using filesort
+1	SIMPLE	t3	range	PRIMARY	PRIMARY	4	NULL	1	Using index condition; Using where; Rowid-ordered scan; Using filesort
 # SELECT * FROM tbl_name WHERE <non-indexed vcol expr> ORDER BY <non-vcol>
 select * from t3 where b between -2 and -1 order by a;
 a	b	c
@@ -189,7 +189,7 @@ a	b	c
 2	-2	-2
 explain select * from t3 where c between -2 and -1 order by a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Using where; Using MRR; Using filesort
+1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Using where; Rowid-ordered scan; Using filesort
 # SELECT * FROM tbl_name WHERE <non-indexed vcol expr> ORDER BY <non-indexed vcol>
 select * from t3 where b between -2 and -1 order by b;
 a	b	c
@@ -205,7 +205,7 @@ a	b	c
 1	-1	-1
 explain select * from t3 where c between -2 and -1 order by b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Using MRR; Using filesort
+1	SIMPLE	t3	range	c	c	5	NULL	1	Using index condition; Rowid-ordered scan; Using filesort
 # SELECT * FROM tbl_name WHERE <non-indexed vcol expr> ORDER BY <indexed vcol>
 select * from t3 where b between -2 and -1 order by c;
 a	b	c

sql/handler.h

@@ -1358,6 +1358,22 @@ void get_sweep_read_cost(TABLE *table, ha_rows nrows, bool interrupted,
 */
 #define HA_MRR_MATERIALIZED_KEYS 256
 
+/*
+  The following bits are reserved for use by MRR implementation. The intended
+  use scenario:
+
+  * sql layer calls handler->multi_range_read_info[_const]() 
+    - MRR implementation figures out what kind of scan it will perform, saves
+      the result in *mrr_mode parameter.
+  * sql layer remembers what was returned in *mrr_mode
+
+  * the optimizer picks the query plan (which may or may not include the MRR 
+    scan that was estimated by the multi_range_read_info[_const] call)
+
+  * if the query is an EXPLAIN statement, sql layer will call 
+    handler->multi_range_read_explain_info(mrr_mode) to get a text description
+    of the picked MRR scan; the description will be a part of EXPLAIN output.
+*/
 #define HA_MRR_IMPLEMENTATION_FLAG1 512
 #define HA_MRR_IMPLEMENTATION_FLAG2 1024
 #define HA_MRR_IMPLEMENTATION_FLAG3 2048