PolarDB:HASH

How it works

HASH routing depends on whether the database shard key and the table shard key are the same column or different columns.

Different shard keys for database sharding and table sharding

The shard key value is divided by the number of database shards to determine which database shard receives the row:

• Integer key: HASH(8) is equivalent to 8 % D, where D is the number of database shards.

• String key: HASH("ABC") is equivalent to hashcode("ABC").abs() % D. The string is first converted to a hash value, then used for routing.

Same shard key for database sharding and table sharding

The shard key value is divided by the total number of table shards across all database shards.

Example: Two database shards, four table shards each (eight total table shards).

• Database shard 0 stores table shards 0 to 3.

• Database shard 1 stores table shards 4 to 7.

• Key value 15: 15 % (2 × 4) = 7 routes the row to table shard 7 in database shard 1.

Supported data types

Use cases

• Database sharding based on user IDs or order IDs.

• Shard key values are strings.

Example

The following DDL statement creates a table that uses HASH sharding on the id column for database sharding only:

CREATE TABLE test_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);