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1.E-R關系策略的由來 join是關系數據庫最常用的一個特性，然而在分布式環境中，跨分片的join最復雜，最難解決。 這是官方文檔的描述。 具體點，比如： mycat邏輯庫hello，兩張表格t1,t2。做了分庫策略，t1放到了datanode1，t2放到了datanode2。如果我t1 join t2檢索數據， 怎麽辦？ 這就是E-R關系策略要解決的問題。 mycat借鑒了table group的概念，將子表的存儲位置依賴於子表，並且在物理上緊鄰存放，解決了join的效率和性能問題。E-R關系的數據分片策略，根據這一思路，將子表的記錄和所關聯的父表記錄存放在同一個數據分片上。 2.測試官方教程文檔上的E-R關系表 customer采用sharding-by-intfile（分片枚舉）策略，分片在dn1,dn2上，orders依賴父表進行分片，兩個表的關聯關系為orders.customer_id=customer.id。示意圖如下：  <table name="customer" primaryKey="ID" dataNode="dn1,dn2" rule="sharding-by-intfile"> <childTable name="orders" joinKey="customer_id" parentKey="id"/> </table> 解釋： <table name="customer" primaryKey="ID" dataNode="dn1,dn2" rule="sharding-by-intfile"> 這一行是定義customer表，主鍵是id,分片部署在dn1,dn2,分片規則是sharding-by-intfile <childTable name="orders" joinKey="customer_id" parentKey="id"/> 這一行是定義orders是childtable。 childtable是依賴父表的結構，就是前面時候的E-R關系的表。 childtable的joinkey會按照父表的parentkey一起切分。 </table> 這是對應 <table name= 的結束格式，參考xml格式。 3. 表格設計： customer表 id(primarykey) name city （用city做分片） orders表 customer_id(primary key) orders 兩表格關系： customer表的主鍵id為orders表主鍵customer_id的外鍵 4.mycat上實際測試： 停止mycat服務，修改配置文件，如下： [root@ha1 conf]# cat schema.xml <?xml version="1.0"?> <!DOCTYPE mycat:schema SYSTEM "schema.dtd"> <mycat:schema xmlns:mycat="http://io.mycat/"> <schema name="hello" checkSQLschema="false" sqlMaxLimit="100"> <!-- auto sharding by id (long) --> <table name="t1" dataNode="dn1,dn2" rule="sharding-by-intfile" /> <!-- global table is auto cloned to all defined data nodes ,so can join with any table whose sharding node is in the same data node --> <table name="t2" primaryKey="ID" type="global" dataNode="dn1,dn2" /> <table name="t3" dataNode="dn1" /> <table name="t4" dataNode="dn2" /> <table name="customer" primaryKey="id" dataNode="dn1,dn2" rule="sharding-by-intfile"> <childTable name="orders" joinKey="customer_id" parentKey="id"/> </table> </schema> <!-- <dataNode name="dn1$0-743" dataHost="localhost1" database="db$0-743" /> --> <dataNode name="dn1" dataHost="mysql1" database="db1" /> <dataNode name="dn2" dataHost="mysql3" database="db2" /> <dataHost name="mysql1" maxCon="1000" minCon="10" balance="3" writeType="0" dbType="mysql" dbDriver="native" switchType="1" slaveThreshold="100"> <heartbeat>select user()</heartbeat> <!-- can have multi write hosts --> <writeHost host="hostM1" url="192.168.211.138:3306" user="root" password="Alex2010@"> </writeHost> <writeHost host="hostS1" url="192.168.211.139:3306" user="root" password="Alex2010@"> </writeHost> </dataHost> <dataHost name="mysql3" maxCon="1000" minCon="10" balance="0" writeType="0" dbType="mysql" dbDriver="native" switchType="1" slaveThreshold="100"> <heartbeat>select user()</heartbeat> <!-- can have multi write hosts --> <writeHost host="hostM1" url="192.168.211.142:3306" user="root" password="Alex2010@"> <!-- can have multi read hosts --> <readHost host="hostS2" url="192.168.211.142:3306" user="root" password="Alex2010@"/> </writeHost> </dataHost> </mycat:schema> 定義分片規則： <tableRule name="sharding-by-intfile"> <rule> <columns>city</columns> <algorithm>hash-int</algorithm> </rule> </tableRule> <function name="hash-int" class="io.mycat.route.function.PartitionByFileMap"> <property name="mapFile">partition-hash-int.txt</property> <property name="type">1</property> <property name="defaultNode">0</property> </function> [root@ha2 conf]# cat partition-hash-int.txt gz=0 sz=1 啟動mycat，創建表格： mysql> create table customer(id int not null primary key,name varchar(10),city varchar(20)); Query OK, 0 rows affected (0.11 sec) mysql> create table orders (customer_id int not null primary key,orders int not null,foreign key(customer_id) references customer(id) on delete cascade on update cascade); Query OK, 0 rows affected (0.25 sec) customer插入數據測試： mysql> insert into customer(id,name,city) values(1,'am1','gz'),(2,'am2','gz'),(3,'am3','sz'); mysql> select * from customer where city='gz'; +----+------+------+ | id | name | city | +----+------+------+ | 1 | am1 | gz | | 2 | am2 | gz | +----+------+------+ 2 rows in set (0.08 sec) mysql> explain select * from customer where city='gz'; +-----------+----------------------------------------------------+ | DATA_NODE | SQL | +-----------+----------------------------------------------------+ | dn1 | SELECT * FROM customer WHERE city = 'gz' LIMIT 100 | +-----------+----------------------------------------------------+ 1 row in set (0.01 sec) gz的數據都在dn1實現了分片。 orders插入數據測試： mysql> insert into orders(customer_id,orders) values(1,10001); Query OK, 1 row affected (0.33 sec) mysql> insert into orders(customer_id,orders) values(2,10002); Query OK, 1 row affected (0.29 sec) mysql> insert into orders(customer_id,orders) values(3,10003); Query OK, 1 row affected (0.48 sec) 根據E-R分片規則，orders表格根據外鍵的值也就是customer的主鍵值切分， 也就是orders.customer_id=customer.id的數據分在一個區。 分別在db1,db2檢索數據，看看是否達到E-R分片的設計要求。

mycat E-R關系分片策略測試