w公共查询表复制至每一个碎片
http://www.agildata.com/database-sharding/
In the Bookstore example, the Primary Shard Table is the ‘customer’ entity. This is the table that is used to shard the data. The ‘customer’ table is the parent of the shard hierarchy, with the ‘customer_order’ and ‘order_item’ entities as child tables. The data is sharded by the ‘customer.id’ attribute, and all related rows in the child tables associated with a given ‘customer.id’ are sharded as well. The Global Tables are the common lookup tables, which have relatively low activity, and these tables are replicated to all shards to avoid cross-shard joins.
While this example is very basic, it does provide the basic considerations for determining how to shard a given database application. By using this evaluation approach, you can determine if sharding is applicable to your particular environment, and what benefits can be realized by implementing Database Sharding.