// 5.6.23  - Not affected

mysql> DROP TABLE IF EXISTS count_distinct;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> CREATE TABLE count_distinct (
    ->     a INT NOT NULL,
    ->     b INT NOT NULL
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.03 sec)

mysql> INSERT INTO count_distinct (a,b) VALUES
    ->     (11,22),
    ->     (11,33),
    ->     (11,22);
SELECT
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        1 |        2 |
+----------+----------+
1 row in set (0.00 sec)

mysql> ALTER TABLE count_distinct ADD index(a,b);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        1 |        2 |
+----------+----------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table          | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | count_distinct | index | NULL          | a    | 8       | NULL |    3 | Using index |
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM count_distinct;
+----+----+
| a  | b  |
+----+----+
| 11 | 22 |
| 11 | 22 |
| 11 | 33 |
+----+----+
3 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE '%big%';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| big_tables      | OFF   |
| sql_big_selects | OFF   |
+-----------------+-------+
2 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'query_cache%';
+------------------------------+---------+
| Variable_name                | Value   |
+------------------------------+---------+
| query_cache_limit            | 1048576 |
| query_cache_min_res_unit     | 4096    |
| query_cache_size             | 1048576 |
| query_cache_type             | OFF     |
| query_cache_wlock_invalidate | OFF     |
+------------------------------+---------+
5 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'version%';
+-------------------------+---------------------------------------------------------+
| Variable_name           | Value                                                   |
+-------------------------+---------------------------------------------------------+
| version                 | 5.6.23-enterprise-commercial-advanced                   |
| version_comment         | MySQL Enterprise Server - Advanced Edition (Commercial) |
| version_compile_machine | x86_64                                                  |
| version_compile_os      | Linux                                                   |
+-------------------------+---------------------------------------------------------+
4 rows in set (0.00 sec)


// 5.7.6  - Not affected

mysql> DROP TABLE IF EXISTS count_distinct;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> CREATE TABLE count_distinct (
    ->     a INT NOT NULL,
    ->     b INT NOT NULL
    -> ) ENGINE=InnoDB;
INSERT INTO count_distinct (a,b) VALUES
    (11,22),
    (11,33),
    (11,22);
Query OK, 0 rows affected (0.04 sec)

mysql> INSERT INTO count_distinct (a,b) VALUES
    ->     (11,22),
    ->     (11,33),
    ->     (11,22);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        1 |        2 |
+----------+----------+
1 row in set (0.00 sec)

mysql> ALTER TABLE count_distinct ADD index(a,b);
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        1 |        2 |
+----------+----------+
1 row in set (0.01 sec)

mysql> EXPLAIN SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----+-------------+----------------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table          | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+----------------+------------+-------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | count_distinct | NULL       | index | NULL          | a    | 8       | NULL |    3 |   100.00 | Using index |
+----+-------------+----------------+------------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

mysql> SELECT * FROM count_distinct;
+----+----+
| a  | b  |
+----+----+
| 11 | 22 |
| 11 | 22 |
| 11 | 33 |
+----+----+
3 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE '%big%';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| big_tables      | OFF   |
| sql_big_selects | OFF   |
+-----------------+-------+
2 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'query_cache%';
+------------------------------+---------+
| Variable_name                | Value   |
+------------------------------+---------+
| query_cache_limit            | 1048576 |
| query_cache_min_res_unit     | 4096    |
| query_cache_size             | 1048576 |
| query_cache_type             | OFF     |
| query_cache_wlock_invalidate | OFF     |
+------------------------------+---------+
5 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'version%';
+-------------------------+---------------------------------------------------------+
| Variable_name           | Value                                                   |
+-------------------------+---------------------------------------------------------+
| version                 | 5.7.6-m16-enterprise-commercial-advanced                |
| version_comment         | MySQL Enterprise Server - Advanced Edition (Commercial) |
| version_compile_machine | x86_64                                                  |
| version_compile_os      | Linux                                                   |
+-------------------------+---------------------------------------------------------+
4 rows in set (0.00 sec)


// 5.5.42  - Not affected

mysql> DROP TABLE IF EXISTS count_distinct;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> CREATE TABLE count_distinct (
    ->     a INT NOT NULL,
    ->     b INT NOT NULL
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO count_distinct (a,b) VALUES
    ->     (11,22),
    ->     (11,33),
    ->     (11,22);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        1 |        2 |
+----------+----------+
1 row in set (0.01 sec)

mysql> ALTER TABLE count_distinct ADD index(a,b);
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        1 |        2 |
+----------+----------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table          | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | count_distinct | index | NULL          | a    | 8       | NULL |    3 | Using index |
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM count_distinct;
+----+----+
| a  | b  |
+----+----+
| 11 | 22 |
| 11 | 22 |
| 11 | 33 |
+----+----+
3 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE '%big%';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| big_tables      | OFF   |
| sql_big_selects | ON    |
| sql_big_tables  | OFF   |
+-----------------+-------+
3 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'query_cache%';
+------------------------------+---------+
| Variable_name                | Value   |
+------------------------------+---------+
| query_cache_limit            | 1048576 |
| query_cache_min_res_unit     | 4096    |
| query_cache_size             | 0       |
| query_cache_type             | ON      |
| query_cache_wlock_invalidate | OFF     |
+------------------------------+---------+
5 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'version%';
+-------------------------+------------------------------+
| Variable_name           | Value                        |
+-------------------------+------------------------------+
| version                 | 5.5.42                       |
| version_comment         | MySQL Community Server (GPL) |
| version_compile_machine | x86_64                       |
| version_compile_os      | Linux                        |
+-------------------------+------------------------------+
4 rows in set (0.00 sec)


// 5.6.21 - Not affected


mysql> DROP TABLE IF EXISTS count_distinct;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> CREATE TABLE count_distinct (
    ->     a INT NOT NULL,
    ->     b INT NOT NULL
    -> ) ENGINE=InnoDB;
INSERT INTO count_distinct (a,b) VALUES
    (11,22),
Query OK, 0 rows affected (0.02 sec)

mysql> INSERT INTO count_distinct (a,b) VALUES
    ->     (11,22),
    ->     (11,33),
    ->     (11,22);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        1 |        2 |
+----------+----------+
1 row in set (0.00 sec)

mysql> ALTER TABLE count_distinct ADD index(a,b);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
EXPLAIN SELECT
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        1 |        2 |
+----------+----------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table          | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | count_distinct | index | NULL          | a    | 8       | NULL |    3 | Using index |
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM count_distinct;
+----+----+
| a  | b  |
+----+----+
| 11 | 22 |
| 11 | 22 |
| 11 | 33 |
+----+----+
3 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE '%big%';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| big_tables      | OFF   |
| sql_big_selects | ON    |
+-----------------+-------+
2 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'query_cache%';
+------------------------------+---------+
| Variable_name                | Value   |
+------------------------------+---------+
| query_cache_limit            | 1048576 |
| query_cache_min_res_unit     | 4096    |
| query_cache_size             | 1048576 |
| query_cache_type             | OFF     |
| query_cache_wlock_invalidate | OFF     |
+------------------------------+---------+
5 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'version%';
+-------------------------+------------------------------+
| Variable_name           | Value                        |
+-------------------------+------------------------------+
| version                 | 5.6.21                       |
| version_comment         | MySQL Community Server (GPL) |
| version_compile_machine | x86_64                       |
| version_compile_os      | linux-glibc2.5               |
+-------------------------+------------------------------+
4 rows in set (0.00 sec)

// 5.6.12 - Affected

mysql> DROP TABLE IF EXISTS count_distinct;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> CREATE TABLE count_distinct (
    ->     a INT NOT NULL,
    ->     b INT NOT NULL
    -> ) ENGINE=InnoDB;
INSERT INTO count_distinct (a,b) VALUES
    (11,22),
    (11,33),
Query OK, 0 rows affected (0.03 sec)

mysql> INSERT INTO count_distinct (a,b) VALUES
    ->     (11,22),
    ->     (11,33),
    ->     (11,22);
SELECT
    COUNT(DISTINCT a)   AS expect_1,
    COUNT(DISTINCT a,b) AS expect_2
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        1 |        2 |
+----------+----------+
1 row in set (0.00 sec)

mysql> ALTER TABLE count_distinct ADD index(a,b);
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----------+----------+
| expect_1 | expect_2 |
+----------+----------+
|        2 |        2 |
+----------+----------+
1 row in set (0.02 sec)

mysql> EXPLAIN SELECT
    ->     COUNT(DISTINCT a)   AS expect_1,
    ->     COUNT(DISTINCT a,b) AS expect_2
    ->     FROM count_distinct;
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------------------------------+
| id | select_type | table          | type  | possible_keys | key  | key_len | ref  | rows | Extra                               |
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------------------------------+
|  1 | SIMPLE      | count_distinct | range | a             | a    | 8       | NULL |    4 | Using index for group-by (scanning) |
+----+-------------+----------------+-------+---------------+------+---------+------+------+-------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM count_distinct;
+----+----+
| a  | b  |
+----+----+
| 11 | 22 |
| 11 | 22 |
| 11 | 33 |
+----+----+
3 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE '%big%';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| big_tables      | OFF   |
| sql_big_selects | ON    |
+-----------------+-------+
2 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'query_cache%';
+------------------------------+---------+
| Variable_name                | Value   |
+------------------------------+---------+
| query_cache_limit            | 1048576 |
| query_cache_min_res_unit     | 4096    |
| query_cache_size             | 1048576 |
| query_cache_type             | OFF     |
| query_cache_wlock_invalidate | OFF     |
+------------------------------+---------+
5 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'version%';
+-------------------------+------------------------------+
| Variable_name           | Value                        |
+-------------------------+------------------------------+
| version                 | 5.6.12                       |
| version_comment         | MySQL Community Server (GPL) |
| version_compile_machine | x86_64                       |
| version_compile_os      | linux-glibc2.5               |
+-------------------------+------------------------------+
4 rows in set (0.00 sec)