MDEV-15433: Optimizer does not use group by optimization with distinct

After the commit b76b69cd
loose index scan for queries with DISTINCT stopped working.
That is why that commit has to be reverted.
Additionally this patch  fixes the problem of MDEV-10880.

mysql-test/r/bench_count_distinct.result

@@ -5,7 +5,7 @@ count(distinct n)
 100
 explain extended select count(distinct n) from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	SIMPLE	t1	index	NULL	n	4	NULL	200	100.00	Using index
+1	SIMPLE	t1	range	NULL	n	4	NULL	10	100.00	Using index for group-by
 Warnings:
 Note	1003	select count(distinct `test`.`t1`.`n`) AS `count(distinct n)` from `test`.`t1`
 drop table t1;

mysql-test/r/distinct.result

@@ -212,7 +212,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	1	Using index
 explain SELECT distinct a from t3 order by a desc limit 2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t3	index	NULL	a	5	NULL	2	Using index
+1	SIMPLE	t3	index	NULL	a	5	NULL	40	Using index
 explain SELECT distinct a,b from t3 order by a+1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	204	Using temporary; Using filesort

mysql-test/r/explain_json.result

@@ -1030,10 +1030,10 @@ Table	Op	Msg_type	Msg_text
 test.t1	analyze	status	Table is already up to date
 explain select count(distinct a1,a2,b) from t1 where (a2 >= 'b') and (b = 'a');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx_t1_2	147	NULL	128	Using where; Using index
+1	SIMPLE	t1	range	NULL	idx_t1_1	147	NULL	17	Using where; Using index for group-by
 explain select count(distinct a1,a2,b,c) from t1 where (a2 >= 'b') and (b = 'a') and (c = 'i121');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx_t1_1	163	NULL	128	Using where; Using index
+1	SIMPLE	t1	range	NULL	idx_t1_1	163	NULL	65	Using where; Using index for group-by (scanning)
 explain format=json select count(distinct a1,a2,b) from t1 where (a2 >= 'b') and (b = 'a');
 EXPLAIN
 {
@@ -1041,14 +1041,14 @@ EXPLAIN
     "select_id": 1,
     "table": {
       "table_name": "t1",
-      "access_type": "index",
-      "key": "idx_t1_2",
+      "access_type": "range",
+      "key": "idx_t1_1",
       "key_length": "147",
       "used_key_parts": ["a1", "a2", "b"],
-      "rows": 128,
+      "rows": 17,
       "filtered": 100,
       "attached_condition": "((t1.b = 'a') and (t1.a2 >= 'b'))",
-      "using_index": true
+      "using_index_for_group_by": true
     }
   }
 }
@@ -1059,14 +1059,14 @@ EXPLAIN
     "select_id": 1,
     "table": {
       "table_name": "t1",
-      "access_type": "index",
+      "access_type": "range",
       "key": "idx_t1_1",
       "key_length": "163",
       "used_key_parts": ["a1", "a2", "b", "c"],
-      "rows": 128,
+      "rows": 65,
       "filtered": 100,
       "attached_condition": "((t1.b = 'a') and (t1.c = 'i121') and (t1.a2 >= 'b'))",
-      "using_index": true
+      "using_index_for_group_by": "scanning"
     }
   }
 }

mysql-test/r/group_min_max.result

@@ -1707,13 +1707,13 @@ select a1,a2,b from t2 where (a1 > 'a') and (a2 > 'a') and (b = 'c') group by a1
 a1	a2	b
 explain select distinct a1,a2,b from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx_t1_2	147	NULL	128	Using index
+1	SIMPLE	t1	range	NULL	idx_t1_1	147	NULL	17	Using index for group-by
 explain select distinct a1,a2,b from t1 where (a2 >= 'b') and (b = 'a');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx_t1_2	147	NULL	128	Using where; Using index
+1	SIMPLE	t1	range	NULL	idx_t1_1	147	NULL	17	Using where; Using index for group-by
 explain extended select distinct a1,a2,b,c from t1 where (a2 >= 'b') and (b = 'a') and (c = 'i121');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	SIMPLE	t1	index	NULL	idx_t1_1	163	NULL	128	100.00	Using where; Using index
+1	SIMPLE	t1	index	NULL	idx_t1_1	163	NULL	128	50.78	Using where; Using index
 Warnings:
 Note	1003	select distinct `test`.`t1`.`a1` AS `a1`,`test`.`t1`.`a2` AS `a2`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c` from `test`.`t1` where ((`test`.`t1`.`b` = 'a') and (`test`.`t1`.`c` = 'i121') and (`test`.`t1`.`a2` >= 'b'))
 explain select distinct a1,a2,b from t1 where (a1 > 'a') and (a2 > 'a') and (b = 'c');
@@ -1724,13 +1724,13 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	idx_t1_2	147	NULL	128	Using where; Using index
 explain select distinct a1,a2,b from t2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	index	NULL	idx_t2_2	146	NULL	#	Using index
+1	SIMPLE	t2	range	NULL	idx_t2_1	146	NULL	#	Using index for group-by
 explain select distinct a1,a2,b from t2 where (a2 >= 'b') and (b = 'a');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	index	NULL	idx_t2_2	146	NULL	#	Using where; Using index
+1	SIMPLE	t2	range	NULL	idx_t2_1	146	NULL	#	Using where; Using index for group-by
 explain extended select distinct a1,a2,b,c from t2 where (a2 >= 'b') and (b = 'a') and (c = 'i121');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	SIMPLE	t2	index	NULL	idx_t2_1	163	NULL	164	100.00	Using where; Using index
+1	SIMPLE	t2	index	NULL	idx_t2_1	163	NULL	164	50.61	Using where; Using index
 Warnings:
 Note	1003	select distinct `test`.`t2`.`a1` AS `a1`,`test`.`t2`.`a2` AS `a2`,`test`.`t2`.`b` AS `b`,`test`.`t2`.`c` AS `c` from `test`.`t2` where ((`test`.`t2`.`b` = 'a') and (`test`.`t2`.`c` = 'i121') and (`test`.`t2`.`a2` >= 'b'))
 explain select distinct a1,a2,b from t2 where (a1 > 'a') and (a2 > 'a') and (b = 'c');
@@ -1855,7 +1855,7 @@ c	e
 d	e
 explain select distinct a1,a2,b from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx_t1_2	147	NULL	128	Using index
+1	SIMPLE	t1	range	NULL	idx_t1_1	147	NULL	17	Using index for group-by
 explain select distinct a1,a2,b from t1 where (a2 >= 'b') and (b = 'a') group by a1,a2,b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	NULL	idx_t1_1	147	NULL	17	Using where; Using index for group-by
@@ -1870,7 +1870,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	range	NULL	idx_t1_1	147	NULL	17	Using where; Using index for group-by; Using temporary; Using filesort
 explain select distinct a1,a2,b from t2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	index	NULL	idx_t2_2	146	NULL	#	Using index
+1	SIMPLE	t2	range	NULL	idx_t2_1	146	NULL	#	Using index for group-by
 explain select distinct a1,a2,b from t2 where (a2 >= 'b') and (b = 'a') group by a1,a2,b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	range	NULL	idx_t2_1	146	NULL	#	Using where; Using index for group-by
@@ -1953,10 +1953,10 @@ b
 a
 explain select count(distinct a1,a2,b) from t1 where (a2 >= 'b') and (b = 'a');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx_t1_2	147	NULL	128	Using where; Using index
+1	SIMPLE	t1	range	NULL	idx_t1_1	147	NULL	17	Using where; Using index for group-by
 explain select count(distinct a1,a2,b,c) from t1 where (a2 >= 'b') and (b = 'a') and (c = 'i121');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx_t1_1	163	NULL	128	Using where; Using index
+1	SIMPLE	t1	range	NULL	idx_t1_1	163	NULL	65	Using where; Using index for group-by (scanning)
 explain extended select count(distinct a1,a2,b) from t1 where (a1 > 'a') and (a2 > 'a') and (b = 'c');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	range	idx_t1_0,idx_t1_1,idx_t1_2	idx_t1_1	147	NULL	14	100.00	Using where; Using index for group-by
@@ -2173,7 +2173,7 @@ c
 d
 explain select distinct a1 from t1 where a2 = 'b';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx_t1_2	147	NULL	128	Using where; Using index
+1	SIMPLE	t1	range	NULL	idx_t1_1	130	NULL	5	Using where; Using index for group-by
 select distinct a1 from t1 where a2 = 'b';
 a1
 a
@@ -2283,7 +2283,7 @@ INSERT INTO t1 (a) VALUES
 ('SOUTH EAST'), ('SOUTH WEST'), ('WESTERN');
 EXPLAIN SELECT DISTINCT a,a FROM t1 ORDER BY a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	a	66	NULL	11	Using index
+1	SIMPLE	t1	range	NULL	a	66	NULL	6	Using index for group-by
 SELECT DISTINCT a,a FROM t1 ORDER BY a;
 a	a
 	
@@ -2499,7 +2499,7 @@ INSERT INTO t1 VALUES
 (4), (2), (1), (2), (2), (4), (1), (4);
 EXPLAIN SELECT DISTINCT(a) FROM t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx	5	NULL	16	Using index
+1	SIMPLE	t1	range	NULL	idx	5	NULL	9	Using index for group-by
 SELECT DISTINCT(a) FROM t1;
 a
 1
@@ -2507,7 +2507,7 @@ a
 4
 EXPLAIN SELECT SQL_BIG_RESULT DISTINCT(a) FROM t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	idx	5	NULL	16	Using index
+1	SIMPLE	t1	range	NULL	idx	5	NULL	9	Using index for group-by
 SELECT SQL_BIG_RESULT DISTINCT(a) FROM t1;
 a
 1
@@ -2646,7 +2646,7 @@ INSERT INTO t1 SELECT * FROM t1;
 INSERT INTO t1 SELECT a,b,c+1,d FROM t1;
 EXPLAIN SELECT DISTINCT c FROM t1 WHERE d=4;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	foo	20	NULL	32	Using where; Using index
+1	SIMPLE	t1	range	NULL	foo	10	NULL	9	Using where; Using index for group-by
 SELECT DISTINCT c FROM t1 WHERE d=4;
 c
 1
@@ -3339,19 +3339,19 @@ INSERT INTO t2 SELECT a, b + 4, c,d,e,f FROM t2;
 INSERT INTO t2 SELECT a + 1, b, c,d,e,f FROM t2;
 EXPLAIN SELECT COUNT(DISTINCT a) FROM t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	a	10	NULL	16	Using index
+1	SIMPLE	t1	range	NULL	a	5	NULL	9	Using index for group-by
 SELECT COUNT(DISTINCT a) FROM t1;
 COUNT(DISTINCT a)
 2
 EXPLAIN SELECT COUNT(DISTINCT a,b) FROM t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	a	10	NULL	16	Using index
+1	SIMPLE	t1	range	NULL	a	10	NULL	9	Using index for group-by
 SELECT COUNT(DISTINCT a,b) FROM t1;
 COUNT(DISTINCT a,b)
 16
 EXPLAIN SELECT COUNT(DISTINCT b,a) FROM t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	a	10	NULL	16	Using index
+1	SIMPLE	t1	range	NULL	a	10	NULL	9	Using index for group-by
 SELECT COUNT(DISTINCT b,a) FROM t1;
 COUNT(DISTINCT b,a)
 16
@@ -3414,7 +3414,7 @@ COUNT(DISTINCT a)
 2
 EXPLAIN SELECT 1 FROM t1 HAVING COUNT(DISTINCT a) < 10;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	a	10	NULL	16	Using index
+1	SIMPLE	t1	range	NULL	a	5	NULL	9	Using index for group-by
 SELECT 1 FROM t1 HAVING COUNT(DISTINCT a) < 10;
 1
 1
@@ -3435,19 +3435,19 @@ COUNT(DISTINCT t1_1.a)
 1
 EXPLAIN SELECT COUNT(DISTINCT a), 12 FROM t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	a	10	NULL	16	Using index
+1	SIMPLE	t1	range	NULL	a	5	NULL	9	Using index for group-by
 SELECT COUNT(DISTINCT a), 12 FROM t1;
 COUNT(DISTINCT a)	12
 2	12
 EXPLAIN SELECT COUNT(DISTINCT a, b, c) FROM t2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	index	NULL	a	15	NULL	16	Using index
+1	SIMPLE	t2	range	NULL	a	15	NULL	9	Using index for group-by
 SELECT COUNT(DISTINCT a, b, c) FROM t2;
 COUNT(DISTINCT a, b, c)
 16
 EXPLAIN SELECT COUNT(DISTINCT a), SUM(DISTINCT a), AVG(DISTINCT a) FROM t2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	index	NULL	a	15	NULL	16	Using index
+1	SIMPLE	t2	range	NULL	a	5	NULL	9	Using index for group-by
 SELECT COUNT(DISTINCT a), SUM(DISTINCT a), AVG(DISTINCT a) FROM t2;
 COUNT(DISTINCT a)	SUM(DISTINCT a)	AVG(DISTINCT a)
 2	3	1.5000
@@ -3459,7 +3459,7 @@ COUNT(DISTINCT a)	SUM(DISTINCT a)	AVG(DISTINCT f)
 2	3	1.0000
 EXPLAIN SELECT COUNT(DISTINCT a, b), COUNT(DISTINCT b, a) FROM t2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	index	NULL	a	15	NULL	16	Using index
+1	SIMPLE	t2	range	NULL	a	10	NULL	9	Using index for group-by
 SELECT COUNT(DISTINCT a, b), COUNT(DISTINCT b, a) FROM t2;
 COUNT(DISTINCT a, b)	COUNT(DISTINCT b, a)
 16	16
@@ -3893,5 +3893,22 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	a	a	13	NULL	2	Using where; Using index
 drop table t1;
 #
+# MDEV-15433: Optimizer does not use group by optimization with distinct
+#
+CREATE TABLE t1 (id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, a INT NOT NULL, KEY(a));
+OPTIMIZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	optimize	status	OK
+EXPLAIN SELECT DISTINCT a FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	NULL	a	4	NULL	5	Using index for group-by
+SELECT DISTINCT a FROM t1;
+a
+1
+2
+3
+4
+drop table t1;
+#
 # End of 10.1 tests
 #

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

@@ -133,7 +133,7 @@ count(distinct c)
 3
 explain select count(distinct c) from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	c	5	NULL	5	Using index
+1	SIMPLE	t1	range	NULL	c	5	NULL	6	Using index for group-by (scanning)
 ###
 ### filesort & range-based utils
 ###

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

@@ -133,7 +133,7 @@ count(distinct c)
 3
 explain select count(distinct c) from t1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	c	5	NULL	5	Using index
+1	SIMPLE	t1	range	NULL	c	5	NULL	6	Using index for group-by (scanning)
 ###
 ### filesort & range-based utils
 ###

mysql-test/t/group_min_max.test

@@ -1608,6 +1608,33 @@ explain select min(a) from t1 where a between "a" and "Cafeeeeeeeeeeeeeeeeeeeeee
 explain select min(a) from t1 where a between "abbbbbbbbbbbbbbbbbbbb" and "Cafe2";
 drop table t1;
 
+--echo #
+--echo # MDEV-15433: Optimizer does not use group by optimization with distinct
+--echo #
+
+CREATE TABLE t1 (id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, a INT NOT NULL, KEY(a));
+--disable_query_log
+INSERT INTO t1(a) VALUES (1), (2), (3), (4);
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+INSERT INTO t1(a) SELECT a FROM t1;
+--enable_query_log
+OPTIMIZE TABLE t1;
+EXPLAIN SELECT DISTINCT a FROM t1;
+SELECT DISTINCT a FROM t1;
+drop table t1;
+
 --echo #
 --echo # End of 10.1 tests
 --echo #

sql/sql_select.cc

@@ -1388,6 +1388,14 @@ JOIN::optimize_inner()
       error= 1;
       DBUG_RETURN(1);
     }
+    if (!group_list)
+    {
+      /* The output has only one row */
+      order=0;
+      simple_order=1;
+      group_optimized_away= 1;
+      select_distinct=0;
+    }
   }
   
   /* Calculate how to do the join */
@@ -5754,7 +5762,7 @@ add_group_and_distinct_keys(JOIN *join, JOIN_TAB *join_tab)
   Item_field *cur_item;
   key_map possible_keys(0);
 
-  if (join->group_list || join->simple_group)
+  if (join->group_list)
   { /* Collect all query fields referenced in the GROUP clause. */
     for (cur_group= join->group_list; cur_group; cur_group= cur_group->next)
       (*cur_group->item)->walk(&Item::collect_item_field_processor, 0,