Updated on 2022-09-16 GMT+08:00

HASH

Application Scenarios

This algorithm features even distribution of data or sharding tables by year, month, week, day, or a combination of them. Arithmetic operators such as equality (=) and IN operators 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.

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

weekofyear ('2019-10-11')=41

day()

day(yyyy-MM-dd)=Day number of the current year

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(Database 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;

Precautions

None