MOD_HASH

Application Scenarios

This algorithm applies if you want to route data to different database shards by user ID or order ID.

Instructions

The sharding key must be CHAR, VARCHAR, INT, INTEGER, BIGINT, MEDIUMINT, SMALLINT, TINYINT, or DECIMAL (the precision can be 0).

Data Routing

The data route depends on the remainder of the sharding key value divided by database or table shards. If the value is a string, convert the string into a hashed value and calculate the data route based on the value.

For example, if the sharding key value is 8, MOD_HASH('8') is equivalent to 8 % D. D is the number of database or table shards.

Calculation Method

Method 1: Use an Integer as the Sharding Key

Table 1 Required calculation methods when the sharding key is the integer data type

Condition	Calculation Method	Example
Database sharding key ≠ Table sharding key	Database routing result = Database sharding key value % Database shards Table routing result = Table sharding key value % Table shards	Database shard: 16 % 8 = 0 Table shard: 16 % 3 = 1
Database sharding key = Table sharding key	Table routing result = Sharding key value % (Database shards x Table shards) Database routing result = Table routing result/Table shards Note Database routing result is rounded off to the nearest integer.	Table shard: 16 % (8 x 3) = 16 Database shard: 16/3 = 5

Method 2: Use a String as the Sharding Key

Table 2 Required calculation methods when the sharding key is the string data type

Condition	Calculation Method	Example
Database sharding key ≠ Table sharding key	Database routing result = hash(Database sharding key value) % Database shards Table routing result = hash(Table sharding key value) % Table shards	hash('abc') = 'abc'.hashCode()=96354 Database shard: 96354 % 8 = 2; Table shard: 96354 % 3 = 0;
Database sharding key = Table sharding key	Table routing result = hash(Sharding key value) % (Database shards x Table shards) Database routing result = Table routing result/Table shards Note Database routing result is rounded off to the nearest integer.	hash('abc') = 'abc'.hashCode()=96354 Table shard: 96354 % (8 x 3) = 18 Database shard: 18/3 = 6

Syntax for Creating Tables

• Assume that you use field ID as the sharding key to shard databases based on MOD_HASH:

  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 mod_hash(ID);
  

• Assume that you use field ID as the sharding key to shard databases and tables based on MOD_HASH:

  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 mod_hash(ID) tbpartition by mod_hash(ID) tbpartitions 4;
  

Precautions

The MOD_HASH algorithm is a simple way to find the remainder of the sharding key value divided by shards. This algorithm features even distribution of sharding key values to ensure even results.

last updated: 2024-12-11 13:43 UTC - commit: 0e04c4a111cf440cbe530f17da84f5c1ac074d8f