Local users that only exist in the shard catalog database do not have the privileges to create schema objects in the Oracle Globally Distributed Database. The first step of creating the distributed database schema is to create an all-shards user.

Create an all-shards user by connecting to the shard catalog database as a privileged user, enabling SHARD DDL, and running the CREATE USER command. When the all-shards user connects to the shard catalog database, the SHARD DDL mode is enabled by default.

Create a sharded table family with the SQL CREATE TABLE statement. You can specify parent-child relationships between tables using reference partitioning or equi-partitioning.

Use Reference Partitioning to Specify Parent-Child Relationships Between Tables

The recommended way to create a sharded table family is to specify parent-child relationships between tables using reference partitioning.

Partitioning by reference simplifies the syntax since the partitioning scheme is only specified for the root table. Also, partition management operations that are performed on the root table are automatically propagated to its descendents. For example, when adding a partition to the root table, a new partition is created on all its descendents.

The appropriate CREATE TABLE statements for Customers–Orders–LineItems schema using a system-managed data distribution methodology are shown below. The first statement creates the root table of the table family, Customers.

CREATE SHARDED TABLE Customers 
( CustNo      NUMBER NOT NULL
, Name        VARCHAR2(50)
, Address     VARCHAR2(250) 
, CONSTRAINT RootPK PRIMARY KEY(CustNo)
)
PARTITION BY CONSISTENT HASH (CustNo)
PARTITIONS AUTO
TABLESPACE SET ts1
;

The following two statements create the Orders and LineItems tables, which are a child and grandchild of the Customers table.

CREATE SHARDED TABLE Orders 
( OrderNo   NUMBER NOT NULL
, CustNo    NUMBER NOT NULL
, OrderDate DATE
, CONSTRAINT OrderPK PRIMARY KEY (CustNo, OrderNo)
, CONSTRAINT CustFK  FOREIGN KEY (CustNo) REFERENCES Customers(CustNo) 
)
PARTITION BY REFERENCE (CustFK)
;

CREATE SHARDED TABLE LineItems 
( CustNo    NUMBER NOT NULL
, LineNo    NUMBER(2) NOT NULL
, OrderNo   NUMBER(5) NOT NULL
, StockNo   NUMBER(4)
, Quantity  NUMBER(2)
, CONSTRAINT LinePK  PRIMARY KEY (CustNo, OrderNo, LineNo)
, CONSTRAINT LineFK  FOREIGN KEY (CustNo, OrderNo) REFERENCES Orders(CustNo, OrderNo)
)
PARTITION BY REFERENCE (LineFK)
;

In the example statements above, corresponding partitions of all tables in the family are stored in the same tablespace set, TS1. However, it is possible to specify separate tablespace sets for each table.

Note that in the example statements above, the partitioning column CustNo used as the sharding key is present in all three tables. This is despite the fact that reference partitioning, in general, allows a child table to be equi-partitioned with the parent table without having to duplicate the key columns in the child table. The reason for this is that reference partitioning requires a primary key in a parent table because the primary key must be specified in the foreign key constraint of a child table used to link the child to its parent. However, a primary key on a sharded table must be the same as, or contain, the sharding key. This makes it possible to enforce global uniqueness of a primary key without coordination with other shards, a critical requirement for linear scalability.

To summarize, the use of reference-partitioned tables in a distributed database requires adhering to the following rules:

• A primary key on a sharded table must either be the same as the sharding key, or contain the sharding key. This is required to enforce global uniqueness of a primary key without coordination with other shards.

• Reference partitioning requires a primary key in a parent table, because the primary key must be specified in the foreign key constraint of a child table to link the child to its parent. It is also possible to have a foreign key constraint when the parent table has just UNIQUE constraint, but no PRIMARY KEY. The sharding key must also be NOT NULL.

  For example, to link the LineItems (child) table to the Orders (parent) table, you need a primary key in the Orders table. The second rule implies that the primary key in the Orders table contains the CustNo value. (This is an existing partitioning rule not specific to Oracle Globally Distributed Database.)

Use Equi-Partitioning to Specify Parent-Child Relationships Between Tables

In some cases it is impossible or undesirable to create primary and foreign key constraints that are required for reference partitioning. For such cases, specifying parent-child relationships in a table family requires that all tables are explicitly equi-partitioned. Each child table is created with the PARENT clause in CREATE SHARDED TABLE that contains the name of its parent. An example of the syntax is shown below.

 CREATE SHARDED TABLE Customers 
( CustNo      NUMBER NOT NULL
, Name        VARCHAR2(50)
, Address     VARCHAR2(250) 
, region      VARCHAR2(20)
, class       VARCHAR2(3)
, signup      DATE
)
PARTITION BY CONSISTENT HASH (CustNo)
PARTITIONS AUTO
TABLESPACE SET ts1
;

CREATE SHARDED TABLE Orders 
( OrderNo   NUMBER 
, CustNo    NUMBER NOT NULL
, OrderDate DATE
)
PARENT Customers
PARTITION BY CONSISTENT HASH (CustNo)
PARTITIONS AUTO
TABLESPACE SET ts1
;

CREATE SHARDED TABLE LineItems 
( LineNo    NUMBER
, OrderNo   NUMBER
, CustNo    NUMBER NOT NULL
, StockNo   NUMBER
, Quantity  NUMBER
)
PARENT Customers
PARTITION BY CONSISTENT HASH (CustNo)
PARTITIONS AUTO
TABLESPACE SET ts1
;

Because the partitioning scheme is fully specified in all of the CREATE SHARDED TABLE statements, any table can be independently subpartitioned. This is not permitted with reference partitioning where subpartitions can only be specified for the root table and the subpartitioning scheme is the same for all tables in a table family.

Note that this method only supports two-level table families, that is, all children must have the same parent and grandchildren cannot exist. This is not a limitation as long as the partitioning column from the parent table exists in all of the child tables.

An Oracle Globally Distributed Database schema can have multiple table families, where all of the data from different table families reside in the same chunks, which contain partitions from different table families sharing the same hash key range.

To create a new table family, create a root sharded table and specify tablespace sets that are not used by existing tablespace families. Each table family is identified by its root table. Tables in the different table families should not be related to each other.

Each table family should have its own sharding key definition, while the same restriction on having the same sharding key columns in child tables still holds true within each table family. This means that all tables from different table families are sharded the same way with consistent hash into the same number of chunks, with each chunk containing data from all the table families.

Design your table families such that queries between different table-families are minimal and only carried out on the sharding coordinator, as many such joins will have an effect on performance

The following example shows you how to create multiple table families using the PARENT clause with a system-managed sharding methodology (PARTITION BY CONSISTENT HASH).

CREATE SHARDED TABLE Customers <=== Table Family #1
( CustId NUMBER NOT NULL
, Name VARCHAR2(50)
, Address VARCHAR2(250)
, region VARCHAR2(20)
, class VARCHAR2(3)
, signup DATE
)
PARTITION BY CONSISTENT HASH (CustId)
PARTITIONS AUTO
TABLESPACE SET ts1
;

CREATE SHARDED TABLE Orders
( OrderNo NUMBER
, CustId NUMBER
, OrderDate DATE
)
PARENT Customers
PARTITION BY CONSISTENT HASH (CustId)
PARTITIONS AUTO
TABLESPACE SET ts1
;

CREATE SHARDED TABLE LineItems
( LineNo NUMBER
, OrderNo NUMBER
, CustId NUMBER
, StockNo NUMBER
, Quantity NUMBER
)
)
PARENT Customers
PARTITION BY CONSISTENT HASH (CustId)
PARTITIONS AUTO
TABLESPACE SET ts1
;

CREATE SHARDED TABLE Products <=== Table Family #2
( ProdId NUMBER NOT NULL,
  CONSTRAINT pk_products PRIMARY KEY (ProdId)
)
PARTITION BY CONSISTENT HASH (ProdId)
PARTITIONS AUTO
TABLESPACE SET ts_2
;

Associating Global Services With Multiple Table Families

Each table family should be associated with a different global service. Applications from different table families each have their own connection pool and service, and use their own sharding key for routing to the correct shard.

When you create the first root table (that is, the first table family) all of the existing global services are automatically associated with it. You can use the GDSCTL MODIFY SERVICE command to change the services associated with a table family after more table families are created, as shown in this example.

GDSCTL> MODIFY SERVICE –GDSPOOL shdpool –TABLE_FAMILY sales.customer -SERVICE sales

A sharded table is a table that is partitioned into smaller and more manageable pieces among multiple databases, called shards.

The following topics guide your decisions and provide instructions for creating sharded tables:

• Tablespace Set Sizing

• Sharded Tables for System-Managed Sharding

• Sharded Tables for User-Defined Sharding

• Sharded Tables for Composite Sharding

• Sharded Tables for Directory-Based Sharding

The number of database requests handled by a single shard can be maximized by duplicating read-only or read-mostly tables across all shards.

CREATE DUPLICATED TABLE Products 
( StockNo     NUMBER PRIMARY KEY
, Description VARCHAR2(20)
, Price       NUMBER(6,2))
;

CREATE DUPLICATED TABLE Products 
( StockNo     NUMBER PRIMARY KEY
, Description VARCHAR2(20)
, Price       NUMBER(6,2))
SYNCHRONOUS;