mysql中的key 分区为什么总是一半有数据，一半没有数据？

CREATE TABLE `pis_supply_chain_a` (
  `ID` varchar(50) NOT NULL,
  `CMMDTY_CODE` varchar(18) NOT NULL,
  `CITY_TO` varchar(12) DEFAULT NULL,
  `LOGISTICS_RAD_MODE` varchar(3) DEFAULT NULL,
  `CITY_FROM` varchar(12) DEFAULT NULL,
  `VERSION_NO` varchar(18) DEFAULT NULL,
  `CREATE_TIME` datetime DEFAULT NULL,
  `UPDATE_TIME` datetime DEFAULT NULL,
  `DELETE_FLAG` varchar(4) DEFAULT "N",
  PRIMARY KEY (`ID`,`CMMDTY_CODE`),
  UNIQUE KEY `supply_chain_a_index` (`CMMDTY_CODE`,`CITY_TO`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT="1\n3000W10"
PARTITION BY KEY (CMMDTY_CODE) PARTITIONS 10 
mysql> SELECT TABLE_NAME,PARTITION_NAME,TABLE_ROWS FROM INFORMATION_SCHEMA.PARTITIONS WHERE TABLE_NAME ="pis_supply_chain_a";
+--+--+--+
| TABLE_NAME         | PARTITION_NAME | TABLE_ROWS |
+--+--+--+
| pis_supply_chain_a | p0             |    3630867 |
| pis_supply_chain_a | p1             |          0 |
| pis_supply_chain_a | p2             |    4537399 |
| pis_supply_chain_a | p3             |          0 |
| pis_supply_chain_a | p4             |    5225392 |
| pis_supply_chain_a | p5             |          0 |
| pis_supply_chain_a | p6             |    5495859 |
| pis_supply_chain_a | p7             |          1 |
| pis_supply_chain_a | p8             |    4757965 |
| pis_supply_chain_a | p9             |          0 |
+--+--+--+
10 rows in set (0.35 sec)
mysql> select distinct CMMDTY_CODE from pis_supply_chain_a limit 20;
+--+
| CMMDTY_CODE        |
+--+
| 000000000100003931 |
| 000000000100004594 |
| 000000000100005662 |
| 000000000100019670 |
| 000000000100025155 |
| 000000000100028229 |
| 000000000100028764 |
| 000000000100030935 |
| 000000000100032337 |
| 000000000100032338 |
| 000000000100033242 |
| 000000000100034839 |
| 000000000100039735 |
| 000000000100039837 |
| 000000000100039848 |
| 000000000100040161 |
| 000000000100044946 |
| 000000000100044950 |
| 000000000100048556 |
| 000000000100050836 |
+--+
20 rows in set (0.02 sec)

解决方案

引用

17.2.4. KEY Partitioning
Partitioning by key is similar to partitioning by hash, except that where hash partitioning employs a user-defined expression, the hashing function for key partitioning is supplied by the MySQL server. MySQL Cluster uses MD5() for this purpose; for tables using other storage engines, the server employs its own internal hashing function which is based on the same algorithm as PASSWORD().

算法问题。