MySQL 中的 distinct 和 group by 哪个效率更高？

一、数据库版本：Server version: 5.7.44-log

没有索引的情况下：

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mysql> select * from test1;
+----+--------------+
| id | name2        |
+----+--------------+
|  1 | test_data_1  |
|  2 | test_data_2  |
|  3 | test_data_3  |
|  4 | test_data_4  |
|  5 | test_data_5  |
|  6 | test_data_6  |
|  7 | test_data_7  |
|  8 | test_data_8  |
|  9 | test_data_9  |
| 10 | test_data_10 |
+----+--------------+
10 rows in set (0.00 sec)


mysql> explain select distinct(name2) from test1 ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | SIMPLE      | test1 | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   10 |   100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select name2 from test1  group by name2;;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                           |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
|  1 | SIMPLE      | test1 | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   10 |   100.00 | Using temporary; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
1 row in set, 1 warning (0.00 sec)

有索引的情况下：

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mysql> create index idx_name2 on test1(name2);
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select name2 from test1  group by name2;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | test1 | NULL       | index | idx_name2     | idx_name2 | 103     | NULL |   10 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select distinct(name2) from test1 ;
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key       | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | test1 | NULL       | index | idx_name2     | idx_name2 | 103     | NULL |   10 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

二、数据库版本：8.0.46

没索引的情况下：

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mysql> show create table test\G
*************************** 1. row ***************************
       Table: test
Create Table: CREATE TABLE `test` (
  `id` int DEFAULT NULL,
  `a` int DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)


mysql> select * from test;
+------+------+
| id   | a    |
+------+------+
|    1 |    2 |
|    2 |   10 |
|    3 |    1 |
|    4 | NULL |
|    6 | NULL |
|    7 | NULL |
|    8 | NULL |
|   10 | NULL |
|   11 | NULL |
+------+------+
9 rows in set (0.00 sec)

mysql> explain  select a from test group by a;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | SIMPLE      | test  | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select distinct(a) from test;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | SIMPLE      | test  | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)

总结：

在语义相同，有索引的情况下：

group by和distinct都能使用索引，效率相同。因为group by和distinct近乎等价，distinct可以被看做是特殊的group by。

在语义相同，无索引的情况下：

distinct效率高于group by。原因是distinct 和 group by都会进行分组操作，但group by在Mysql8.0之前会进行隐式排序，导致触发filesort，sql执行效率低下。

但从Mysql8.0开始，Mysql就删除了隐式排序，所以，此时在语义相同，无索引的情况下，group by和distinct的执行效率也是近乎等价的。