Adjust costs for doing index scan in cost_group_min_max()

The idea is that when doing a tree dive (once per group), we need to
compare key values, which is fast.  For each new group, we have to
compare the full where clause for the row.
Compared to original code, the cost of group_min_max() has slightly
increased which affects some test with only a few rows.
main.group_min_max and main.distinct have been modified to show the
effect of the change.

The patch also adjust the number of groups in case of quick selects:
- For simple WHERE clauses, ensure that we have at least as many groups
  as we have conditions on the used group-by key parts.
  The assumption is that each condition will create at least one group.
- Ensure that there are no more groups than rows found by quick_select

Test changes:
- For some small tables there has been a change of
  Using index for group-by -> Using index for group-by (scanning)
  Range -> Index and Using index for group-by -> Using index

1 files changed, 3 insertions, 3 deletions

diff --git a/mysql-test/main/subselect_mat.result b/mysql-test/main/subselect_mat.result
index 83213e0750d..647124e5581 100644
--- a/mysql-test/main/subselect_mat.result
+++ b/mysql-test/main/subselect_mat.result
@@ -1142,7 +1142,7 @@ a
 explain extended
 select a from t1 group by a having a in (select c from t2 where d >= 20);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	PRIMARY	t1	range	NULL	it1a	4	NULL	7	100.00	Using index for group-by
+1	PRIMARY	t1	index	NULL	it1a	4	NULL	7	100.00	Using index
 2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	7	100.00	Using where
 Warnings:
 Note	1003	/* select#1 */ select `test`.`t1`.`a` AS `a` from `test`.`t1` group by `test`.`t1`.`a` having <expr_cache><`test`.`t1`.`a`>(<in_optimizer>(`test`.`t1`.`a`,`test`.`t1`.`a` in ( <materialize> (/* select#2 */ select `test`.`t2`.`c` from `test`.`t2` where `test`.`t2`.`d` >= 20 ), <primary_index_lookup>(`test`.`t1`.`a` in <temporary table> on distinct_key where `test`.`t1`.`a` = `<subquery2>`.`c`))))
@@ -1154,7 +1154,7 @@ create index iab on t1(a, b);
 explain extended
 select a from t1 group by a having a in (select c from t2 where d >= 20);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	PRIMARY	t1	range	NULL	it1a	4	NULL	7	100.00	Using index for group-by
+1	PRIMARY	t1	index	NULL	it1a	4	NULL	7	100.00	Using index
 2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	7	100.00	Using where
 Warnings:
 Note	1003	/* select#1 */ select `test`.`t1`.`a` AS `a` from `test`.`t1` group by `test`.`t1`.`a` having <expr_cache><`test`.`t1`.`a`>(<in_optimizer>(`test`.`t1`.`a`,`test`.`t1`.`a` in ( <materialize> (/* select#2 */ select `test`.`t2`.`c` from `test`.`t2` where `test`.`t2`.`d` >= 20 ), <primary_index_lookup>(`test`.`t1`.`a` in <temporary table> on distinct_key where `test`.`t1`.`a` = `<subquery2>`.`c`))))
@@ -1166,7 +1166,7 @@ explain extended
 select a from t1 group by a
 having a in (select c from t2 where d >= some(select e from t3 where max(b)=e));
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
-1	PRIMARY	t1	range	NULL	iab	4	NULL	7	100.00	Using index for group-by
+1	PRIMARY	t1	index	NULL	iab	8	NULL	7	100.00	Using index
 2	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	7	100.00	Using where
 3	DEPENDENT SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	4	100.00	Using where
 Warnings: