MySQL基于时间字段进行分区的方案总结

MySQL支持的分区类型一共有四种：RANGE，LIST，HASH，KEY。其中，RANGE又可分为原生RANGE和RANGE COLUMNS，LIST分为原生LIST和LIST COLUMNS，HASH分为原生HASH和LINEAR HASH，KEY包含原生KEY和LINEAR HASH。关于这些分区之间的差别，改日另写文章进行阐述。

最近，碰到一个需求，要对表的时间字段（类型：datetime）基于天进行分区。于是遍历MySQL官方文档分区章节，总结如下：

实现方式

主要是以下几种：

1. 基于RANGE

2. 基于RANGE COLUMNS

3. 基于HASH

测试数据 

为了测试以上三种方案，特构造了100万的测试数据，放在test表中，test表只有两列：id和hiredate，其中hiredate只包含10天的数据，从2015-12-01到2015-12-10。具体信息如下：

mysql> show create table test\G
*************************** 1. row ***************************
       Table: test
Create Table: CREATE TABLE `test` (
  `id` int(11) DEFAULT NULL,
  `hiredate` datetime DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> select min(hiredate),max(hiredate) from test;
+---------------------+---------------------+
| min(hiredate)       | max(hiredate)       |
+---------------------+---------------------+
| 2015-12-01 00:00:00 | 2015-12-10 23:59:56 |
+---------------------+---------------------+
1 row in set (0.44 sec)

mysql> select date(hiredate),count(*) from test group by date(hiredate);
+----------------+----------+
| date(hiredate) | count(*) |
+----------------+----------+
| 2015-12-01     |    99963 |
| 2015-12-02     |   100032 |
| 2015-12-03     |   100150 |
| 2015-12-04     |    99989 |
| 2015-12-05     |    99908 |
| 2015-12-06     |    99897 |
| 2015-12-07     |   100137 |
| 2015-12-08     |   100171 |
| 2015-12-09     |    99851 |
| 2015-12-10     |    99902 |
+----------------+----------+
10 rows in set (0.98 sec)

测试的维度

测试的维度主要从两个方面进行，

一、分区剪裁

针对特定的查询，是否能进行分区剪裁（即只查询相关的分区，而不是所有分区）

二、查询时间

鉴于该批测试数据是静止的（即没有并发进行的insert，update和delete操作），数据量也不太大，从这个维度来考量貌似意义也不是很大。

因此，重点测试第一个维度。

基于RANGE的分区方案

在这里，选用了TO_DAYS函数

CREATE TABLE range_datetime(
    id INT,
    hiredate DATETIME
)
PARTITION BY RANGE (TO_DAYS(hiredate) ) (
    PARTITION p1 VALUES LESS THAN ( TO_DAYS('') ),
    PARTITION p2 VALUES LESS THAN ( TO_DAYS('') ),
    PARTITION p3 VALUES LESS THAN ( TO_DAYS('') ),
    PARTITION p4 VALUES LESS THAN ( TO_DAYS('') ),
    PARTITION p5 VALUES LESS THAN ( TO_DAYS('') ),
    PARTITION p6 VALUES LESS THAN ( TO_DAYS('') ),
    PARTITION p7 VALUES LESS THAN ( TO_DAYS('') ),
    PARTITION p8 VALUES LESS THAN ( TO_DAYS('') ),
    PARTITION p9 VALUES LESS THAN ( TO_DAYS('') ),
    PARTITION p10 VALUES LESS THAN ( TO_DAYS('') )
);

插入数据并查看特定查询的执行计划

mysql> insert into range_datetime select * from test;
Query OK, 1000000 rows affected (8.15 sec)
Records: 1000000  Duplicates: 0  Warnings: 0

mysql> explain partitions select * from range_datetime where hiredate >= '' and hiredate<='';
+----+-------------+----------------+--------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table          | partitions   | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+----------------+--------------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | range_datetime | p7,p8,p9,p10 | ALL  | NULL          | NULL | NULL    | NULL | 400061 | Using where |
+----+-------------+----------------+--------------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.03 sec)

注意执行计划中的partitions的内容，只查询了p7，p8，p9，p10三个分区，由此来看，使用to_days函数确实可以实现分区裁剪。

基于RANGE COLUMNS的分区方案

RANGE COLUMNS可以直接基于列，而无需像上述RANGE那种，分区的对象只能为整数。

创表语句如下：

CREATE TABLE range_columns (
    id INT,
    hiredate DATETIME
)
PARTITION BY RANGE COLUMNS(hiredate) (
    PARTITION p1 VALUES LESS THAN ( '' ),
    PARTITION p2 VALUES LESS THAN ( '' ),
    PARTITION p3 VALUES LESS THAN ( '' ),
    PARTITION p4 VALUES LESS THAN ( '' ),
    PARTITION p5 VALUES LESS THAN ( '' ),
    PARTITION p6 VALUES LESS THAN ( '' ),
    PARTITION p7 VALUES LESS THAN ( '' ),
    PARTITION p8 VALUES LESS THAN ( '' ),
    PARTITION p9 VALUES LESS THAN ( '' ),
    PARTITION p10 VALUES LESS THAN ('' )
);

插入数据并查看上述查询的执行计划

mysql> insert into range_columns select * from test;
Query OK, 1000000 rows affected (9.20 sec)
Records: 1000000  Duplicates: 0  Warnings: 0

mysql> explain partitions select * from range_columns where hiredate >= '' and hiredate<='';
+----+-------------+---------------+--------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table         | partitions   | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+---------------+--------------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | range_columns | p7,p8,p9,p10 | ALL  | NULL          | NULL | NULL    | NULL | 400210 | Using where |
+----+-------------+---------------+--------------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.11 sec)

同样，使用该分区方案也实现了分区剪裁。

基于HASH的分区方案

因HASH分区对象同样只能为整数，所以我们无法像上述RANGE COLUMNS那种直接引用列，在这里，同样用了TO_DAYS函数进行转换。

创表语句如下：

CREATE TABLE hash_datetime (
   id INT,
   hiredate DATETIME
)
PARTITION BY HASH( TO_DAYS(hiredate) )
PARTITIONS 10;

插入数据并查看上述查询的执行计划

mysql> insert into hash_datetime select * from test;
Query OK, 1000000 rows affected (9.43 sec)
Records: 1000000  Duplicates: 0  Warnings: 0

mysql> explain partitions select * from hash_datetime where hiredate >= '' and hiredate<='';
+----+-------------+---------------+-------------------------------+------+---------------+------+---------+------+---------+-------------+
| id | select_type | table         | partitions                    | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+---------------+-------------------------------+------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | hash_datetime | p0,p1,p2,p3,p4,p5,p6,p7,p8,p9 | ALL  | NULL          | NULL | NULL    | NULL | 1000500 | Using where |
+----+-------------+---------------+-------------------------------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

不难看出，使用hash分区并不能有效的实现分区裁剪，至少在本例，基于天的需求中如此。

以上三种方案都是基于datetime的，那么，对于timestamp类型，又该如何选择呢？

事实上，MySQL提供了一种基于UNIX_TIMESTAMP函数的RANGE分区方案，而且，只能使用UNIX_TIMESTAMP函数，如果使用其它函数，譬如to_days，会报如下错误：“ERROR 1486 (HY000): Constant, random or timezone-dependent expressions in (sub)partitioning function are not allowed”。

而且官方文档中也提到“Any other expressions involving TIMESTAMP values are not permitted. (See Bug #42849.)”。

下面来测试一下基于UNIX_TIMESTAMP函数的RANGE分区方案，看其能否实现分区裁剪。

针对TIMESTAMP的分区方案

创表语句如下：

CREATE TABLE range_timestamp (
    id INT,
    hiredate TIMESTAMP
)
PARTITION BY RANGE ( UNIX_TIMESTAMP(hiredate) ) (
    PARTITION p1 VALUES LESS THAN ( UNIX_TIMESTAMP('2015-12-02 00:00:00') ),
    PARTITION p2 VALUES LESS THAN ( UNIX_TIMESTAMP('2015-12-03 00:00:00') ),
    PARTITION p3 VALUES LESS THAN ( UNIX_TIMESTAMP('2015-12-04 00:00:00') ),
    PARTITION p4 VALUES LESS THAN ( UNIX_TIMESTAMP('2015-12-05 00:00:00') ),
    PARTITION p5 VALUES LESS THAN ( UNIX_TIMESTAMP('2015-12-06 00:00:00') ),
    PARTITION p6 VALUES LESS THAN ( UNIX_TIMESTAMP('2015-12-07 00:00:00') ),
    PARTITION p7 VALUES LESS THAN ( UNIX_TIMESTAMP('2015-12-08 00:00:00') ),
    PARTITION p8 VALUES LESS THAN ( UNIX_TIMESTAMP('2015-12-09 00:00:00') ),
    PARTITION p9 VALUES LESS THAN ( UNIX_TIMESTAMP('2015-12-10 00:00:00') ),
    PARTITION p10 VALUES LESS THAN (UNIX_TIMESTAMP('2015-12-11 00:00:00') )
);

插入数据并查看上述查询的执行计划

mysql> insert into range_timestamp select * from test;
Query OK, 1000000 rows affected (13.25 sec)
Records: 1000000  Duplicates: 0  Warnings: 0

mysql> explain partitions select * from range_timestamp where hiredate >= '' and hiredate<='';
+----+-------------+-----------------+--------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table           | partitions   | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-----------------+--------------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | range_timestamp | p7,p8,p9,p10 | ALL  | NULL          | NULL | NULL    | NULL | 400448 | Using where |
+----+-------------+-----------------+--------------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)

同样也能实现分区裁剪。

总结：

1. 经过对比，个人倾向于第二种方案，即基于RANGE COLUMNS的分区实现。

2. 在5.7版本之前，对于DATA和DATETIME类型的列，如果要实现分区裁剪，只能使用YEAR() 和TO_DAYS()函数，在5.7版本中，又新增了TO_SECONDS()函数。

3. 其实LIST也能实现基于天的分区方案，但在这个需求上，相比于RANGE，还是显得很鸡肋。

4. TIMESTAMP类型的列，只能基于UNIX_TIMESTAMP函数进行分区，切记！

参考：

http://dev.mysql.com/doc/refman/5.7/en/partitioning.html