mysql的hash分区_MySQL中hash和key分区值的计算方法
MySQL中hash和key分区值的计算方法
mysql中有一种叫作key作为partition key的类型.来看看记录是怎么分布的
对于hash 分区，使用%操作符，每个partition key只能是int类型,通过
partition key%3(比如定义了三个分区)来把记录分布三个不同的artition里面
mysql> create table t13 ( a int,b int) partition by hash(a) partitions 3
mysql>insert into t14 values(10,1);
mysql>insert into t14 values(11,1);
mysql>insert into t14 values(12,1);
10%3=1 所以第一条记录是在p1里面，11%3=2在第二个分区p2里面，以此类推.
mysql> explain partitions select * from t13 where a=10;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t13 | p1 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain partitions select * from t13 where a=11;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t13 | p2 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain partitions select * from t13 where a=12;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t13 | p0 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
对于使用key partition 的方法，官方文档说是使用了一种password的方法.
mysql>create table t14 (a int,b int) partition by key(a) partitions 3
insert into t14 values(10,1);
insert into t14 values(11,1);
insert into t14 values(12,1);
insert into t14 values(13,1);
insert into t14 values(14,1);
insert into t14 values(15,1);
insert into t14 values(16,1);
mysql> explain partitions select * from t14 where a=10;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t14 | p0 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain partitions select * from t14 where a=11;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t14 | p0 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain partitions select * from t14 where a=12;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t14 | p1 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain partitions select * from t14 where a=13;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t14 | p1 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain partitions select * from t14 where a=14;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t14 | p2 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain partitions select * from t14 where a=15;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t14 | p2 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+-------------+
我发现对于key partition的规律是每两个值落在同一步分区里面，其他没有什么规律字，只是为什么样这样分配到不同的
分区里面，没有很好的解释。如果使用password函数，这些值的结果根本就不一样.有知道的朋友可以和我交流一下.
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