HASH

Application Scenarios

This algorithm features even distribution of data and sharding tables. Arithmetic operators such as equal sign (=) and IN are often used in SQL queries.

Instructions

The sharding key must be CHAR, VARCHAR, INT, INTEGER, BIGINT, MEDIUMINT, SMALLINT, TINYINT, or DECIMAL (the precision can be 0). The sharding key must be DATE, DATETIME, or TIMESTAMP if you use HASH together with date functions.
If you use a sharding key of the string type, do not convert the data type of its value in SQL statements. Changing the data type may cause a failure of routing calculation, and data will be routed to the default shard and cannot be found when you query it.

Data Routing

Determine the range of each database or table shard using 102400.

For example, if there are 8 shards in each schema, use formula 102400/8=12800 to calculate the range of each shard as follows: 0=0–12799, 1=12800–25599, 2=25600–38399, 3=38400–51199, 4=51200–63999, 5=64000–76799, 6=76800–89599, and 7=89600–102399

To determine the route, calculate CRC32 value based on the sharding key value and divide the CRC value by 102400. Then check which range the remainder belongs to.

Calculation Method

Method 1: Use a Non-date Sharding Key

**Table 1** Required calculation methods when the sharding key is not the DATE type
Condition	Calculation Method	Example
Non-date sharding key	Database routing result = crc32(Database sharding key) % 102400 Table routing result = crc32(Table sharding key) % 102400	Database/Table shard: crc32(16) % 102400 = 49364; 49364 belongs to range 3=38400-51199, so data is routed to shard 3.

Method 2: Use a Date Sharding Key

**Table 2** Supported date functions
Date Function	Calculation Method	Example
year()	year(yyyy-MM-dd)=yyyy	year('2019-10-11')=2019
month()	month(yyyy-MM-dd)=MM	month('2019-10-11')=10
weekofyear()	weekofyear(yyyy-MM-dd)=Week number of the current year NOTE: For the definition of the week number in a year, see WEEKOFYEAR(date).	weekofyear('2019-10-11')=41
day()	day(yyyy-MM-dd)=Day number of the current month	day('2019-10-11')=11

**Table 3** Required calculation methods when the sharding key is the DATE type
Condition	Calculation Method	Example
Date sharding key	Database routing result = crc32(Date function(Database sharding key)) % 102400 Table routing result = crc32(Date function(Table sharding key)) % 102400	Database/Table shard: crc32(year('2019-10-11')) % 102400 = 5404 5404 belongs to range 0=0-12799, so data is routed to shard 0.

Syntax for Creating Tables

Assume that you use field ID as the sharding key and the HASH algorithm to shard databases:

create table hash_tb (
    id int,
    name varchar(30) DEFAULT NULL,  
    create_time datetime DEFAULT NULL,
    primary key(id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 dbpartition by hash (ID);

Assume that you use field ID as the sharding key and the hash algorithm to shard databases and tables:

create table mod_hash_tb (
    id int,
    name varchar(30) DEFAULT NULL,  
    create_time datetime DEFAULT NULL,
    primary key(id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 
dbpartition by hash (ID)
tbpartition by hash (ID) tbpartitions 4;