mysql 5.1新功能 －－ 按日期分区

mysql 5.1已经到了beta版，官方网站上也陆续有一些文章介绍，比如上次看到的Improving Database Performance with Partitioning。在使用分区的前提下，可以用mysql实现非常大的数据量存储。今天在mysql的站上又看到一篇进阶的文章 ——按日期分区存储。如果能够实现按日期分区，这对某些时效性很强的数据存储是相当实用的功能。下面是从这篇文章中摘录的一些内容。

错误的按日期分区例子

最直观的方法，就是直接用年月日这种日期格式来进行常规的分区：

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CODE:

mysql>  create table rms(d date)
-> partition by range (d)
->(partition p0 values less than('1995-01-01'),
-> partition p1 VALUES LESS THAN ('2010-01-01'));

上面的例子中，就是直接用"Y-m-d"的格式来对一个table进行分区，可惜想当然往往不能奏效，会得到一个错误信息:

ERROR 1064 (42000): VALUES value must be of same type as partition function near '),

partition p1 VALUES LESS THAN ('2010-01-01'))' at line 3

上述分区方式没有成功，而且明显的不经济，老练的DBA会用整型数值来进行分区：

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mysql> CREATE TABLE part_date1
->     (  c1 int default NULL,
-> c2 varchar(30) default NULL,
-> c3 date default NULL) engine=myisam
->     partition by range (cast(date_format(c3,'%Y%m%d') as signed))
->(PARTITION p0 VALUES LESS THAN(19950101),
->PARTITION p1 VALUES LESS THAN (19960101) ,
->PARTITION p2 VALUES LESS THAN (19970101) ,
->PARTITION p3 VALUES LESS THAN (19980101) ,
->PARTITION p4 VALUES LESS THAN (19990101) ,
->PARTITION p5 VALUES LESS THAN (20000101) ,
->PARTITION p6 VALUES LESS THAN (20010101) ,
->PARTITION p7 VALUES LESS THAN (20020101) ,
->PARTITION p8 VALUES LESS THAN (20030101) ,
->PARTITION p9 VALUES LESS THAN (20040101) ,
->PARTITION p10 VALUES LESS THAN (20100101),
->PARTITION p11 VALUES LESS THAN MAXVALUE );
Query OK,0 rows affected (0.01 sec)

搞定？接着往下分析

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mysql> explain partitions
->select count(*) from part_date1 where
->     c3> date '1995-01-01' and c3 <date '1995-12-31'\G
***************************1. row ***************************
id:1
select_type: SIMPLE
table: part_date1
partitions: p0,p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p11
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows:8100000
Extra: Using where
1 row in set(0.00 sec)

万恶的mysql居然对上面的sql使用全表扫描，而不是按照我们的日期分区分块查询。原文中解释到MYSQL的优化器并不认这种日期形式的分区，花了大量的篇幅来引诱俺走上歧路，过分。

正确的日期分区例子

mysql优化器支持以下两种内置的日期函数进行分区：

TO_DAYS()
YEAR()

看个例子：

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mysql> CREATE TABLE part_date3
->     (  c1 int default NULL,
-> c2 varchar(30) default NULL,
-> c3 date default NULL) engine=myisam
->     partition by range (to_days(c3))
->(PARTITION p0 VALUES LESS THAN(to_days('1995-01-01')),
->PARTITION p1 VALUES LESS THAN (to_days('1996-01-01')) ,
->PARTITION p2 VALUES LESS THAN (to_days('1997-01-01')) ,
->PARTITION p3 VALUES LESS THAN (to_days('1998-01-01')) ,
->PARTITION p4 VALUES LESS THAN (to_days('1999-01-01')) ,
->PARTITION p5 VALUES LESS THAN (to_days('2000-01-01')) ,
->PARTITION p6 VALUES LESS THAN (to_days('2001-01-01')) ,
->PARTITION p7 VALUES LESS THAN (to_days('2002-01-01')) ,
->PARTITION p8 VALUES LESS THAN (to_days('2003-01-01')) ,
->PARTITION p9 VALUES LESS THAN (to_days('2004-01-01')) ,
->PARTITION p10 VALUES LESS THAN (to_days('2010-01-01')),
->PARTITION p11 VALUES LESS THAN MAXVALUE );
Query OK,0 rows affected (0.00 sec)

以to_days()函数分区成功，我们分析一下看看：

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mysql> explain partitions
->select count(*) from part_date3 where
->     c3> date '1995-01-01' and c3 <date '1995-12-31'\G
***************************1. row ***************************
id:1
select_type: SIMPLE
table: part_date3
partitions: p1
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows:808431
Extra: Using where
1 row in set(0.00 sec)

可以看到，mysql优化器这次不负众望，仅仅在p1分区进行查询。在这种情况下查询，真的能够带来提升查询效率么？下面分别对这次建立的part_date3和之前分区失败的part_date1做一个查询对比：

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mysql> select count(*) from part_date3 where
->     c3> date '1995-01-01' and c3 <date '1995-12-31';
+----------+
| count(*) |
+----------+
|   805114 |
+----------+
1 row in set(4.11 sec)

mysql> select count(*) from part_date1 where
->     c3> date '1995-01-01' and c3 <date '1995-12-31';
+----------+
| count(*) |
+----------+
|   805114 |
+----------+
1 row in set(40.33 sec)

可以看到，分区正确的话query花费时间为4秒，而分区错误则花费时间40秒（相当于没有分区），效率有90％的提升！所以我们千万要正确的使用分区功能，分区后务必用explain验证，这样才能获得真正的性能提升。