Help Center/ Distributed Database Middleware/ User Guide/ SQL Syntax/ DDL/ Sharding Algorithms/ MOD_HASH_CI
Updated on 2022-09-16 GMT+08:00
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MOD_HASH_CI

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. MOD_HASH is case-sensitive, but MOD_HASH_CI is not.

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

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'.toUpperCase().hashCode()=64578

Database shard: 64578 % 8 = 2;

Table shard: 64578 % 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

hash('abc') = 'abc'.toUpperCase().hashCode()=64578

Table shard: 64578% (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_CI: 
    create table mod_hash_ci_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_ci(ID);
  • Assume that you use field ID as the sharding key to shard databases and tables based on MOD_HASH_CI: 
    create table mod_hash_ci_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_ci(ID)
tbpartition by mod_hash_ci(ID) tbpartitions 4;

Precautions

  • The MOD_HASH_CI 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.
Parent topic: Sharding Algorithms