添加唯一约束会减慢速度吗？

Question

我的桌子上有三列.

+-----------+-----------------------+------+-----+---------+-------+
| Field     | Type                  | Null | Key | Default | Extra |
+-----------+-----------------------+------+-----+---------+-------+
| hash      | mediumint(8) unsigned | NO   | PRI | 0       |       | 
| nums      | int(10) unsigned      | NO   | PRI | 0       |       | 
| acc       | smallint(5) unsigned  | NO   | PRI | 0       |       | 
+-----------+-----------------------+------+-----+---------+-------+

我期待我的数据重复,所以我继续并添加了一个独特的约束:

ALTER TABLE nt_accs ADD UNIQUE(hash,nums,acc);

我有大约5亿行要插入到这个表中,并且这个表已经使用nums上的RANGE分成大约20个分区.

1. 唯一约束是否会减慢插入？仅仅制作一个主键而不是强加一个独特的约束,这有何不同？
2. 我有很多GROUP BY使用hash和nums列的类型查询.我是否继续添加转换索引,或者我只是添加单个索引？

编辑:

分区并插入一些测试数据后解释计划

1. mysql> explain partitions select * from nt_accs;
+----+-------------+-----------+---------------------------------------------------------------------------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table     | partitions                                                                | type  | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+-------------+-----------+---------------------------------------------------------------------------+-------+---------------+----------+---------+------+------+-------------+
|  1 | SIMPLE      | nt_accs   | p0,p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p11,p12,p13,p14,p15,p16,p17,p18,p19,p20 | index | NULL          | hash     | 7       | NULL |   10 | Using index | 
+----+-------------+-----------+---------------------------------------------------------------------------+-------+---------------+----------+---------+------+------+-------------+
1 row in set (0.00 sec)



2. mysql> explain partitions select * from nt_accs WHERE nums=1504887570;
+----+-------------+-----------+------------+-------+---------------+----------+---------+------+------+--------------------------+
| id | select_type | table     | partitions | type  | possible_keys | key      | key_len | ref  | rows | Extra                    |
+----+-------------+-----------+------------+-------+---------------+----------+---------+------+------+--------------------------+
|  1 | SIMPLE      | nt_accs   | p7         | index | NULL          | hash     | 7       | NULL |   10 | Using where; Using index | 
+----+-------------+-----------+------------+-------+---------------+----------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

3. mysql> explain partitions select * from nt_accs WHERE hash=2347200;
+----+-------------+-----------+---------------------------------------------------------------------------+------+---------------+----------+---------+-------+------+-------------+
| id | select_type | table     | partitions                                                                | type | possible_keys | key      | key_len | ref   | rows | Extra       |
+----+-------------+-----------+---------------------------------------------------------------------------+------+---------------+----------+---------+-------+------+-------------+
|  1 | SIMPLE      | nt_accs  | p0,p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p11,p12,p13,p14,p15,p16,p17,p18,p19,p20 | ref  | hash          | hash     | 3       | const |   27 | Using index | 
+----+-------------+-----------+---------------------------------------------------------------------------+------+---------------+----------+---------+-------+------+-------------+
1 row in set (0.00 sec)

4. mysql> EXPLAIN PARTITIONS SELECT hash, count(distinct nums) FROM nt_accs GROUP BY hash;
+----+-------------+-----------+---------------------------------------------------------------------------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table     | partitions                                                                | type  | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+-------------+-----------+---------------------------------------------------------------------------+-------+---------------+----------+---------+------+------+-------------+
|  1 | SIMPLE      | nt_accs   | p0,p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p11,p12,p13,p14,p15,p16,p17,p18,p19,p20 | index | NULL          | hash     | 7       | NULL |   10 | Using index | 
+----+-------------+-----------+---------------------------------------------------------------------------+-------+---------------+----------+---------+------+------+-------------+
1 row in set (0.00 sec)

5. mysql> EXPLAIN PARTITIONS SELECT nums, count(distinct hash) FROM nt_accs GROUP BY nums;
+----+-------------+-----------+---------------------------------------------------------------------------+-------+---------------+----------+---------+------+------+-----------------------------+
| id | select_type | table     | partitions                                                                | type  | possible_keys | key      | key_len | ref  | rows | Extra                       |
+----+-------------+-----------+---------------------------------------------------------------------------+-------+---------------+----------+---------+------+------+-----------------------------+
|  1 | SIMPLE      | nt_accs   | p0,p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p11,p12,p13,p14,p15,p16,p17,p18,p19,p20 | index | NULL          | hash     | 7       | NULL |   10 | Using index; Using filesort | 
+----+-------------+-----------+---------------------------------------------------------------------------+-------+---------------+----------+---------+------+------+-----------------------------+
1 row in set (0.00 sec)

第一次和第二次查询我完全没问题,但我不确定第3,第4和第5次的表现.在这一点上我还能做些什么来优化这个吗？

Answer 1

唯一约束是否会减慢插入？仅仅制作一个主键而不是强加一个独特的约束,这有何不同？

是的,索引(MySQL实现一个唯一约束作为索引)将减慢插入速度.
同样是主键,这就是为什么表期望高插入负载(IE:用于日志记录)没有定义主键的原因 - 使插入更快.

我使用hash和nums列有很多GROUP BY类型查询.我是否继续添加转换索引,或者我只是添加单个索引？

绝对知道的唯一方法是测试和检查EXPLAIN计划.

UPDATE

根据提供的解释计划,我没有看到对第3和第4版本的关注.MySQL每个select_type只能使用一个索引.第五个版本可能受益于覆盖指数.

附录

只是想确保你知道:

ALTER TABLE nt_accs ADD UNIQUE(hash, nums, acc);

...表示三列值的组合将是唯一的.IE:这些是有效的,唯一约束将允许:

hash  nums  acc
----------------
1     1     1
1     1     2
1     2     1
2     1     1