MDEV-19600: The optimizer should be able to produce rows=1 estimate for unique index with NULLable columnsbb-10.3-mdev19600

Modify best_access_path() to produce rows=1 estimate for null-rejecting
lookups on unique NULL keys.

11 files changed, 276 insertions, 37 deletions

diff --git a/mysql-test/main/invisible_field.result b/mysql-test/main/invisible_field.result
index 876a80814e5..43a8b9d726b 100644
--- a/mysql-test/main/invisible_field.result
+++ b/mysql-test/main/invisible_field.result
@@ -404,8 +404,8 @@ b	int(11)	YES		NULL
 c	int(11)	YES		NULL	
 explain select * from t1,t2 where t1.b = t2.c and t1.c = t2.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	10	
-1	SIMPLE	t1	ALL	b,c	NULL	NULL	NULL	10	Using where; Using join buffer (flat, BNL join)
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	10	Using where
+1	SIMPLE	t1	ref	b,c	b	5	test.t2.c	1	Using where
 select * from t1,t2 where t1.b = t2.c and t1.c = t2.b;
 a	a	b	c
 1	1	1	1
diff --git a/mysql-test/main/join.result b/mysql-test/main/join.result
index cc8e174c8e6..8ca82002855 100644
--- a/mysql-test/main/join.result
+++ b/mysql-test/main/join.result
@@ -1599,3 +1599,33 @@ SELECT STRAIGHT_JOIN * FROM t1, t2 AS t2_1, t2 AS t2_2
 WHERE t2_2.c = t2_1.c AND t2_2.b = t2_1.b AND ( a IS NULL OR t2_1.c = a );
 a	b	c	b	c
 DROP TABLE t1,t2;
+#
+# MDEV-19600: The optimizer should be able to produce rows=1 estimate for unique index with NULLable columns
+#
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t1 (
+pk int not null primary key auto_increment,
+a int,
+b int,
+unique key(a)
+);
+insert into t1 (a,b) select null, 12345 from t0 A, t0 B, t0 C;
+insert into t1 (a,b) select a,a from t0;
+# Simulate InnoDB's persistent statistics (It always uses nulls_equal)
+set @tmp1= @@myisam_stats_method;
+set myisam_stats_method=nulls_equal;
+analyze table t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+set myisam_stats_method=@tmp1;
+show keys from t1;
+Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment	Index_comment
+t1	0	PRIMARY	1	pk	A	1010	NULL	NULL		BTREE		
+t1	0	a	1	a	A	10	NULL	NULL	YES	BTREE		
+# t1 must use ref(t1.a=t0.a) and rows must be 1 (and not 45):
+explain select * from t0,t1 where t0.a=t1.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	10	Using where
+1	SIMPLE	t1	ref	a	a	5	test.t0.a	1	
+drop table t0,t1;
diff --git a/mysql-test/main/join.test b/mysql-test/main/join.test
index 3d2a02e2346..b90a9cc39eb 100644
--- a/mysql-test/main/join.test
+++ b/mysql-test/main/join.test
@@ -1254,3 +1254,33 @@ SELECT STRAIGHT_JOIN * FROM t1, t2 AS t2_1, t2 AS t2_2
   WHERE t2_2.c = t2_1.c AND t2_2.b = t2_1.b AND ( a IS NULL OR t2_1.c = a );                  
 
 DROP TABLE t1,t2;
+
+--echo #
+--echo # MDEV-19600: The optimizer should be able to produce rows=1 estimate for unique index with NULLable columns
+--echo #
+
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+create table t1 (
+  pk int not null primary key auto_increment,
+  a int,
+  b int,
+  unique key(a)
+);
+
+# 10K of null values
+insert into t1 (a,b) select null, 12345 from t0 A, t0 B, t0 C;
+insert into t1 (a,b) select a,a from t0;
+
+--echo # Simulate InnoDB's persistent statistics (It always uses nulls_equal)
+set @tmp1= @@myisam_stats_method;
+set myisam_stats_method=nulls_equal;
+analyze table t1;
+set myisam_stats_method=@tmp1;
+show keys from t1;
+
+--echo # t1 must use ref(t1.a=t0.a) and rows must be 1 (and not 45):
+explain select * from t0,t1 where t0.a=t1.a;
+
+drop table t0,t1;
diff --git a/mysql-test/main/order_by.result b/mysql-test/main/order_by.result
index 8d1e471f618..8692e727c60 100644
--- a/mysql-test/main/order_by.result
+++ b/mysql-test/main/order_by.result
@@ -1515,7 +1515,7 @@ WHERE t2.b=14 AND t2.a=t1.a AND 5.1<t2.c AND t1.b='DE'
 ORDER BY t2.c LIMIT 1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	range	a,b,c	c	5	NULL	420	Using where
-1	SIMPLE	t1	ref	a	a	39	test.t2.a,const	10	Using where; Using index
+1	SIMPLE	t1	ref	a	a	39	test.t2.a,const	1	Using where; Using index
 SELECT d FROM t3 AS t1, t2 AS t2 
 WHERE t2.b=14 AND t2.a=t1.a AND 5.1<t2.c AND t1.b='DE'
 ORDER BY t2.c LIMIT 1;
diff --git a/mysql-test/main/subselect_sj.result b/mysql-test/main/subselect_sj.result
index 13d5224566b..ad43a09c7ff 100644
--- a/mysql-test/main/subselect_sj.result
+++ b/mysql-test/main/subselect_sj.result
@@ -2555,33 +2555,89 @@ CREATE TABLE t1 ( a INT PRIMARY KEY, b INT, KEY(b) );
 INSERT INTO t1 VALUES
 (1,2),(2,1),(3,3),(4,2),(5,5),
 (6,3),(7,1),(8,4),(9,3),(10,2);
-CREATE TABLE t2 ( c INT, d INT, UNIQUE KEY(c) );
+CREATE TABLE t2 ( c INT, d INT, KEY(c) );
 INSERT INTO t2 VALUES
 (1,2),(2,1),(3,3),(4,2),(5,5),(6,3),(7,1);
+analyze table t1,t2;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+test.t2	analyze	status	OK
+explain
+SELECT a, b, d FROM t1, t2
+WHERE ( b, d ) IN
+( SELECT b, d FROM t1, t2 WHERE b = c );
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	7	
+1	PRIMARY	t1	index	b	b	5	NULL	10	Using where; Using index; LooseScan
+1	PRIMARY	t2	ref	c	c	5	test.t1.b	1	Using where; FirstMatch(t1)
+1	PRIMARY	t1	ref	b	b	5	test.t1.b	2	
 SELECT a, b, d FROM t1, t2
 WHERE ( b, d ) IN
 ( SELECT b, d FROM t1, t2 WHERE b = c );
 a	b	d
 2	1	2
 7	1	2
-2	1	2
-7	1	2
-1	2	1
-4	2	1
-10	2	1
+8	4	2
 1	2	1
 4	2	1
 10	2	1
 3	3	3
 6	3	3
 9	3	3
+2	1	2
+7	1	2
+8	4	2
+5	5	5
 3	3	3
 6	3	3
 9	3	3
-8	4	2
-8	4	2
-5	5	5
+1	2	1
+4	2	1
+10	2	1
 DROP TABLE t1, t2;
+# Another testcase for the above that still uses LooseScan:
+create table t0(a int primary key);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t10(a int primary key);
+insert into t10 select A.a + B.a* 10 + C.a * 100 from t0 A, t0 B, t0 C;
+create table t1 (
+pk int primary key auto_increment,
+kp1 int,
+kp2 int,
+filler char(100),
+key (kp1, kp2)
+);
+insert into t1 (kp1, kp2, filler) 
+select 
+A.a, B.a, 'filler-data'
+from t0 A, t0 B;
+create table t2 (a int, filler char(100), key(a));
+create table t3 (a int);
+insert into t3 values (1),(2);
+insert into t2 
+select (A.a+1000*B.a)/20, 'filler_data' from t10 A, t0 B;
+analyze table t1,t2,t3;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Table is already up to date
+test.t2	analyze	status	Table is already up to date
+test.t3	analyze	status	OK
+delete from t1 where kp2 in (1,3);
+# Ref + LooseScan on t1:
+explain select sum(t2.a)
+from t2,t3
+where (t3.a,t2.a) in (select kp1,kp2 from t1,t0 where t0.a=2);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t0	const	PRIMARY	PRIMARY	4	const	1	Using index
+1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	2	Using where
+1	PRIMARY	t1	ref	kp1	kp1	5	test.t3.a	10	Using where; Using index; LooseScan
+1	PRIMARY	t2	ref	a	a	5	test.t1.kp2	20	Using index
+select sum(t2.a)
+from t2,t3
+where (t3.a,t2.a) in (select kp1,kp2 from t1,t0 where t0.a=2);
+sum(t2.a)
+1640
+drop table t0,t10;
+drop table t1,t2,t3;
 #
 # BUG#920713: Wrong result (missing rows) with firstmatch+BNL, IN subquery, ...
 #
diff --git a/mysql-test/main/subselect_sj.test b/mysql-test/main/subselect_sj.test
index 29211cadd9c..81cca95092d 100644
--- a/mysql-test/main/subselect_sj.test
+++ b/mysql-test/main/subselect_sj.test
@@ -2285,16 +2285,66 @@ INSERT INTO t1 VALUES
   (1,2),(2,1),(3,3),(4,2),(5,5),
   (6,3),(7,1),(8,4),(9,3),(10,2);
 
-CREATE TABLE t2 ( c INT, d INT, UNIQUE KEY(c) );
+CREATE TABLE t2 ( c INT, d INT, KEY(c) );
 INSERT INTO t2 VALUES
   (1,2),(2,1),(3,3),(4,2),(5,5),(6,3),(7,1);
 
+analyze table t1,t2;
+explain
+SELECT a, b, d FROM t1, t2
+WHERE ( b, d ) IN
+  ( SELECT b, d FROM t1, t2 WHERE b = c );
 SELECT a, b, d FROM t1, t2
 WHERE ( b, d ) IN
   ( SELECT b, d FROM t1, t2 WHERE b = c );
 
 DROP TABLE t1, t2;
 
+--echo # Another testcase for the above that still uses LooseScan:
+
+create table t0(a int primary key);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+create table t10(a int primary key);
+insert into t10 select A.a + B.a* 10 + C.a * 100 from t0 A, t0 B, t0 C;
+
+create table t1 (
+   pk int primary key auto_increment,
+   kp1 int,
+   kp2 int,
+   filler char(100),
+   key (kp1, kp2)
+);
+
+# 10 groups, each has 10 elements.
+insert into t1 (kp1, kp2, filler) 
+select 
+  A.a, B.a, 'filler-data'
+from t0 A, t0 B;
+
+create table t2 (a int, filler char(100), key(a));
+
+create table t3 (a int);
+insert into t3 values (1),(2);
+
+insert into t2 
+select (A.a+1000*B.a)/20, 'filler_data' from t10 A, t0 B;
+
+analyze table t1,t2,t3;
+delete from t1 where kp2 in (1,3);
+
+--echo # Ref + LooseScan on t1:
+explain select sum(t2.a)
+from t2,t3
+where (t3.a,t2.a) in (select kp1,kp2 from t1,t0 where t0.a=2);
+
+select sum(t2.a)
+from t2,t3
+where (t3.a,t2.a) in (select kp1,kp2 from t1,t0 where t0.a=2);
+
+drop table t0,t10;
+drop table t1,t2,t3;
+
 --echo #
 --echo # BUG#920713: Wrong result (missing rows) with firstmatch+BNL, IN subquery, ...
 --echo #
diff --git a/mysql-test/main/subselect_sj_jcl6.result b/mysql-test/main/subselect_sj_jcl6.result
index 014c44d1181..0026924b605 100644
--- a/mysql-test/main/subselect_sj_jcl6.result
+++ b/mysql-test/main/subselect_sj_jcl6.result
@@ -2569,9 +2569,22 @@ CREATE TABLE t1 ( a INT PRIMARY KEY, b INT, KEY(b) );
 INSERT INTO t1 VALUES
 (1,2),(2,1),(3,3),(4,2),(5,5),
 (6,3),(7,1),(8,4),(9,3),(10,2);
-CREATE TABLE t2 ( c INT, d INT, UNIQUE KEY(c) );
+CREATE TABLE t2 ( c INT, d INT, KEY(c) );
 INSERT INTO t2 VALUES
 (1,2),(2,1),(3,3),(4,2),(5,5),(6,3),(7,1);
+analyze table t1,t2;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+test.t2	analyze	status	OK
+explain
+SELECT a, b, d FROM t1, t2
+WHERE ( b, d ) IN
+( SELECT b, d FROM t1, t2 WHERE b = c );
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	7	
+1	PRIMARY	t1	index	b	b	5	NULL	10	Using where; Using index; LooseScan
+1	PRIMARY	t2	ref	c	c	5	test.t1.b	1	Using where; FirstMatch(t1)
+1	PRIMARY	t1	ref	b	b	5	test.t1.b	2	Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan
 SELECT a, b, d FROM t1, t2
 WHERE ( b, d ) IN
 ( SELECT b, d FROM t1, t2 WHERE b = c );
@@ -2596,6 +2609,49 @@ a	b	d
 10	2	1
 10	2	1
 DROP TABLE t1, t2;
+# Another testcase for the above that still uses LooseScan:
+create table t0(a int primary key);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t10(a int primary key);
+insert into t10 select A.a + B.a* 10 + C.a * 100 from t0 A, t0 B, t0 C;
+create table t1 (
+pk int primary key auto_increment,
+kp1 int,
+kp2 int,
+filler char(100),
+key (kp1, kp2)
+);
+insert into t1 (kp1, kp2, filler) 
+select 
+A.a, B.a, 'filler-data'
+from t0 A, t0 B;
+create table t2 (a int, filler char(100), key(a));
+create table t3 (a int);
+insert into t3 values (1),(2);
+insert into t2 
+select (A.a+1000*B.a)/20, 'filler_data' from t10 A, t0 B;
+analyze table t1,t2,t3;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	Table is already up to date
+test.t2	analyze	status	Table is already up to date
+test.t3	analyze	status	OK
+delete from t1 where kp2 in (1,3);
+# Ref + LooseScan on t1:
+explain select sum(t2.a)
+from t2,t3
+where (t3.a,t2.a) in (select kp1,kp2 from t1,t0 where t0.a=2);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t0	const	PRIMARY	PRIMARY	4	const	1	Using index
+1	PRIMARY	t3	ALL	NULL	NULL	NULL	NULL	2	Using where
+1	PRIMARY	t1	ref	kp1	kp1	5	test.t3.a	10	Using where; Using index; LooseScan
+1	PRIMARY	t2	ref	a	a	5	test.t1.kp2	20	Using index
+select sum(t2.a)
+from t2,t3
+where (t3.a,t2.a) in (select kp1,kp2 from t1,t0 where t0.a=2);
+sum(t2.a)
+1640
+drop table t0,t10;
+drop table t1,t2,t3;
 #
 # BUG#920713: Wrong result (missing rows) with firstmatch+BNL, IN subquery, ...
 #
diff --git a/mysql-test/main/subselect_sj_nonmerged.result b/mysql-test/main/subselect_sj_nonmerged.result
index 47970668ae5..4d9a70e6bba 100644
--- a/mysql-test/main/subselect_sj_nonmerged.result
+++ b/mysql-test/main/subselect_sj_nonmerged.result
@@ -67,9 +67,9 @@ insert into t4 select A.a + 10*B.a, A.a + 10*B.a, 'filler' from t0 A, t0 B;
 explain select * from t0, t4 where 
 t4.b=t0.a and t4.a in (select max(t2.a) from t1, t2 group by t2.b);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	t0	ALL	NULL	NULL	NULL	NULL	10	
-1	PRIMARY	t4	ALL	a	NULL	NULL	NULL	100	Using where; Using join buffer (flat, BNL join)
-1	PRIMARY	<subquery2>	eq_ref	distinct_key	distinct_key	4	test.t4.a	1	
+1	PRIMARY	<subquery2>	ALL	distinct_key	NULL	NULL	NULL	5	
+1	PRIMARY	t0	ALL	NULL	NULL	NULL	NULL	10	Using where; Using join buffer (flat, BNL join)
+1	PRIMARY	t4	ref	a	a	10	<subquery2>.max(t2.a),test.t0.a	1	
 2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	5	Using temporary
 2	MATERIALIZED	t1	ALL	NULL	NULL	NULL	NULL	10	Using join buffer (flat, BNL join)
 insert into t4 select 100 + (B.a *100 + A.a), 100 + (B.a*100 + A.a), 'filler' from t4 A, t0 B;
@@ -77,9 +77,9 @@ explain select * from t4 where
 t4.a in (select max(t2.a) from t1, t2 group by t2.b) and 
 t4.b in (select max(t2.a) from t1, t2 group by t2.b);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	PRIMARY	<subquery2>	ALL	distinct_key	NULL	NULL	NULL	5	
-1	PRIMARY	t4	ref	a	a	5	<subquery2>.max(t2.a)	12	Using index condition
-1	PRIMARY	<subquery3>	eq_ref	distinct_key	distinct_key	4	test.t4.b	1	
+1	PRIMARY	<subquery3>	ALL	distinct_key	NULL	NULL	NULL	5	
+1	PRIMARY	<subquery2>	ALL	distinct_key	NULL	NULL	NULL	5	Using join buffer (flat, BNL join)
+1	PRIMARY	t4	ref	a	a	10	<subquery2>.max(t2.a),<subquery3>.max(t2.a)	1	
 3	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	5	Using temporary
 3	MATERIALIZED	t1	ALL	NULL	NULL	NULL	NULL	10	Using join buffer (flat, BNL join)
 2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	5	Using temporary
diff --git a/mysql-test/main/table_elim.result b/mysql-test/main/table_elim.result
index cf9a4a38779..7780e165451 100644
--- a/mysql-test/main/table_elim.result
+++ b/mysql-test/main/table_elim.result
@@ -279,7 +279,7 @@ insert into t2 values
 explain select t1.* from t1 left join t2 on t2.a=t1.a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	PRIMARY	10	NULL	2	Using index
-1	SIMPLE	t2	ref	a	a	3	test.t1.a	2	Using where
+1	SIMPLE	t2	ref	a	a	3	test.t1.a	1	Using where
 drop table t1, t2;
 #
 # check UPDATE/DELETE that look like they could be eliminated
diff --git a/mysql-test/suite/gcol/r/gcol_select_myisam.result b/mysql-test/suite/gcol/r/gcol_select_myisam.result
index d0fe7fbd0d4..280618ed76e 100644
--- a/mysql-test/suite/gcol/r/gcol_select_myisam.result
+++ b/mysql-test/suite/gcol/r/gcol_select_myisam.result
@@ -1102,7 +1102,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 EXPLAIN SELECT * FROM t1 AS t2 STRAIGHT_JOIN t1 FORCE INDEX(b) WHERE t1.b=t2.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	b	NULL	NULL	NULL	2	Using where
-1	SIMPLE	t1	ref	b	b	5	test.t2.b	2	
+1	SIMPLE	t1	ref	b	b	5	test.t2.b	1	
 EXPLAIN SELECT b FROM t1 FORCE INDEX(b);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	b	5	NULL	2	Using index
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 0fbc4bac7d8..13a5cd88c93 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -5507,18 +5507,16 @@ add_key_field(JOIN *join,
   (*key_fields)->level=         and_level;
   (*key_fields)->optimize=      optimize;
   /*
-    If the condition has form "tbl.keypart = othertbl.field" and 
-    othertbl.field can be NULL, there will be no matches if othertbl.field 
-    has NULL value.
-    We use null_rejecting in add_not_null_conds() to add
-    'othertbl.field IS NOT NULL' to tab->select_cond.
+    If the condition we are analyzing is NULL-rejecting and at least
+    one side of the equalities is NULLable, mark the KEY_FIELD object as
+    null-rejecting. This property is used by:
+    - add_not_null_conds() to add "column IS NOT NULL" conditions
+    - best_access_path() to produce better estimates for NULL-able unique keys.
   */
   {
-    Item *real= (*value)->real_item();
-    if (((cond->functype() == Item_func::EQ_FUNC) ||
-         (cond->functype() == Item_func::MULT_EQUAL_FUNC)) &&
-        (real->type() == Item::FIELD_ITEM) &&
-        ((Item_field*)real)->field->maybe_null())
+    if ((cond->functype() == Item_func::EQ_FUNC ||
+         cond->functype() == Item_func::MULT_EQUAL_FUNC) &&
+        ((*value)->maybe_null || field->real_maybe_null()))
       (*key_fields)->null_rejecting= true;
     else
       (*key_fields)->null_rejecting= false;
@@ -6834,6 +6832,7 @@ best_access_path(JOIN      *join,
       ulong key_flags;
       uint key_parts;
       key_part_map found_part= 0;
+      key_part_map notnull_part=0; // key parts which won't have NULL in lookup tuple.
       table_map found_ref= 0;
       uint key= keyuse->key;
       bool ft_key=  (keyuse->keypart == FT_KEYPART);
@@ -6892,6 +6891,9 @@ best_access_path(JOIN      *join,
             if (!(keyuse->used_tables & ~join->const_table_map))
               const_part|= keyuse->keypart_map;
 
+            if (!keyuse->val->maybe_null || keyuse->null_rejecting)
+              notnull_part|=keyuse->keypart_map;
+
             double tmp2= prev_record_reads(join->positions, idx,
                                            (found_ref | keyuse->used_tables));
             if (tmp2 < best_prev_record_reads)
@@ -6942,12 +6944,19 @@ best_access_path(JOIN      *join,
         loose_scan_opt.check_ref_access_part1(s, key, start_key, found_part);
 
         /* Check if we found full key */
-        if (found_part == PREV_BITS(uint, key_parts) &&
-            !ref_or_null_part)
+        const key_part_map all_key_parts= PREV_BITS(uint, key_parts);
+        if (found_part == all_key_parts && !ref_or_null_part)
         {                                         /* use eq key */
           max_key_part= (uint) ~0;
-          if ((key_flags & (HA_NOSAME | HA_NULL_PART_KEY)) == HA_NOSAME ||
-              MY_TEST(key_flags & HA_EXT_NOSAME))
+          /*
+            If the index is a unique index (1), and
+            - all its columns are not null (2), or
+            - equalities we are using reject NULLs (3)
+            then the estimate is rows=1.
+          */
+          if ((key_flags & (HA_NOSAME | HA_EXT_NOSAME)) &&   // (1)
+              (!(key_flags & HA_NULL_PART_KEY) ||            //  (2)
+               all_key_parts == notnull_part))               //  (3)
           {
             tmp = prev_record_reads(join->positions, idx, found_ref);
             records=1.0;
@@ -9991,8 +10000,16 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
       uint maybe_null= MY_TEST(keyinfo->key_part[i].null_bit);
       j->ref.items[i]=keyuse->val;		// Save for cond removal
       j->ref.cond_guards[i]= keyuse->cond_guard;
-      if (keyuse->null_rejecting) 
+
+      /*
+        Set ref.null_rejecting to true only if we are going to inject a
+        "keyuse->val IS NOT NULL" predicate.
+      */
+      Item *real= (keyuse->val)->real_item();
+      if (keyuse->null_rejecting && (real->type() == Item::FIELD_ITEM) &&
+          ((Item_field*)real)->field->maybe_null())
         j->ref.null_rejecting|= (key_part_map)1 << i;
+
       keyuse_uses_no_tables= keyuse_uses_no_tables && !keyuse->used_tables;
       /*
         We don't want to compute heavy expressions in EXPLAIN, an example would