MySQL 分表优化试验代码

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这里的分表逻辑是根据t_group表的user_name组的个数来分的。
因为这种情况单独user_name字段上的索引就属于烂索引。起不了啥名明显的效果。

1、试验PROCEDURE.
DELIMITER $$
DROP PROCEDURE `t_girl`.`sp_split_table`$$
CREATE PROCEDURE `t_girl`.`sp_split_table`()
BEGIN
declare done int default 0;
declare v_user_name varchar(20) default '';
declare v_table_name varchar(64) default '';
-- Get all users' name.
declare cur1 cursor for select user_name from t_group group by user_name;
-- Deal with error or warnings.
declare continue handler for 1329 set done = 1;
-- Open cursor.
open cur1;
while done <> 1
do
fetch cur1 into v_user_name;
if not done then
-- Get table name.
set v_table_name = concat('t_group_',v_user_name);
-- Create new extra table.
set @stmt = concat('create table ',v_table_name,' like t_group');
prepare s1 from @stmt;
execute s1;
drop prepare s1;
-- Load data into it.
set @stmt = concat('insert into ',v_table_name,' select * from t_group where user_name = ''',v_user_name,'''');
prepare s1 from @stmt;
execute s1;
drop prepare s1;
end if;
end while;
-- Close cursor.
close cur1;
-- Free variable from memory.
set @stmt = NULL;
END$$

DELIMITER ;
2、试验表。
我们用一个有一千万条记录的表来做测试。

mysql> select count(*) from t_group;
+----------+
| count(*) |
+----------+
| 10388608 |
+----------+
1 row in set (0.00 sec)

表结构。
mysql> desc t_group;
+-------------+------------------+------+-----+-------------------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+------------------+------+-----+-------------------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| money | decimal(10,2) | NO | | | |
| user_name | varchar(20) | NO | MUL | | |
| create_time | timestamp | NO | | CURRENT_TIMESTAMP | |
+-------------+------------------+------+-----+-------------------+----------------+
4 rows in set (0.00 sec)

索引情况。

mysql> show index from t_group;
+---------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| t_group | 0 | PRIMARY | 1 | id | A | 10388608 | NULL | NULL | | BTREE | |
| t_group | 1 | idx_user_name | 1 | user_name | A | 8 | NULL | NULL | | BTREE | |
| t_group | 1 | idx_combination1 | 1 | user_name | A | 8 | NULL | NULL | | BTREE | |
| t_group | 1 | idx_combination1 | 2 | money | A | 3776 | NULL | NULL | | BTREE | |
+---------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
4 rows in set (0.00 sec)

PS:
idx_combination1 这个索引是必须的，因为要对user_name来GROUP BY。此时属于松散索引扫描！当然完了后你可以干掉她。
idx_user_name 这个索引是为了加快单独执行constant这种类型的查询。
我们要根据用户名来分表。

mysql> select user_name from t_group where 1 group by user_name;
+-----------+
| user_name |
+-----------+
| david |
| leo |
| livia |
| lucy |
| sarah |
| simon |
| sony |
| sunny |
+-----------+
8 rows in set (0.00 sec)

所以结果表应该是这样的。
mysql> show tables like 't_group_%';
+------------------------------+
| Tables_in_t_girl (t_group_%) |
+------------------------------+
| t_group_david |
| t_group_leo |
| t_group_livia |
| t_group_lucy |
| t_group_sarah |
| t_group_simon |
| t_group_sony |
| t_group_sunny |
+------------------------------+
8 rows in set (0.00 sec)

3、对比结果。


mysql> select count(*) from t_group where user_name = 'david';
+----------+
| count(*) |
+----------+
| 1298576 |
+----------+
1 row in set (1.71 sec)

执行了将近2秒。

mysql> select count(*) from t_group_david;
+----------+
| count(*) |
+----------+
| 1298576 |
+----------+
1 row in set (0.00 sec)
几乎是瞬间的。

mysql> select count(*) from t_group where user_name <> 'david';
+----------+
| count(*) |
+----------+
| 9090032 |
+----------+
1 row in set (9.26 sec)
执行了将近10秒，可以想象，这个是实际的项目中是不能忍受的。
mysql> select (select count(*) from t_group) - (select count(*) from t_group_david) as total;
+---------+
| total |
+---------+
| 9090032 |
+---------+
1 row in set (0.00 sec)
几乎是瞬间的。


我们来看看聚集函数。
对于原表的操作。

mysql> select min(money),max(money) from t_group where user_name = 'david';
+------------+------------+
| min(money) | max(money) |
+------------+------------+
| -6.41 | 500.59 |
+------------+------------+
1 row in set (0.00 sec)
最小，最大值都是FULL INDEX SCAN。所以是瞬间的。
mysql> select sum(money),avg(money) from t_group where user_name = 'david';
+--------------+------------+
| sum(money) | avg(money) |
+--------------+------------+
| 319992383.84 | 246.417910 |
+--------------+------------+
1 row in set (2.15 sec)
其他聚集函数的结果就不是FULL INDEX SCAN了。耗时2.15秒。

对于小表的操作。
mysql> select min(money),max(money) from t_group_david;
+------------+------------+
| min(money) | max(money) |
+------------+------------+
| -6.41 | 500.59 |
+------------+------------+
1 row in set (1.50 sec)
最大最小值完全是FULL TABLE SCAN,耗时1.50秒，不划算。以此看来。
mysql> select sum(money),avg(money) from t_group_david;
+--------------+------------+
| sum(money) | avg(money) |
+--------------+------------+
| 319992383.84 | 246.417910 |
+--------------+------------+
1 row in set (1.68 sec)

取得这两个结果也是花了快2秒，快了一点。

我们来看看这个小表的结构。
mysql> desc t_group_david;
+-------------+------------------+------+-----+-------------------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+------------------+------+-----+-------------------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| money | decimal(10,2) | NO | | | |
| user_name | varchar(20) | NO | MUL | | |
| create_time | timestamp | NO | | CURRENT_TIMESTAMP | |
+-------------+------------------+------+-----+-------------------+----------------+
4 rows in set (0.00 sec)

明显的user_name属性是多余的。那么就干掉它。
mysql> alter table t_group_david drop user_name;
Query OK, 1298576 rows affected (7.58 sec)
Records: 1298576 Duplicates: 0 Warnings: 0

现在来重新对小表运行查询

mysql> select min(money),max(money) from t_group_david;
+------------+------------+
| min(money) | max(money) |
+------------+------------+
| -6.41 | 500.59 |
+------------+------------+
1 row in set (0.00 sec)

此时是瞬间的。
mysql> select sum(money),avg(money) from t_group_david;
+--------------+------------+
| sum(money) | avg(money) |
+--------------+------------+
| 319992383.84 | 246.417910 |
+--------------+------------+
1 row in set (0.94 sec)

这次算是控制在一秒以内了。

mysql> Aborted

小总结一下：分出的小表的属性尽量越少越好。大胆的去干吧。

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