Learn about the physical organization of a sharded database.
The following topics describe the physical organization of a sharded database:
Sharding is a database scaling technique based on horizontal partitioning of data across multiple independent physical databases. Each physical database in such a configuration is called a shard.
Even though a sharded database (SDB) looks like a single database to applications and application developers, from the perspective of a database administrator, it is a set of discrete Oracle databases, each of which is called a shard. A sharded table is partitioned across all shards of the SDB. Table partitions on each shard are not different from partitions that could be used in an Oracle database that is not sharded.
The following figure shows the difference between partitioning on a single logical database and partitions distributed across multiple shards.
Figure 51-1 Sharding as Distributed Partitioning
Oracle Sharding automatically distributes the partitions across shards when you execute the
CREATE SHARDED TABLE statement, and the distribution of partitions is transparent to applications. The figure above shows the logical view of a sharded table and its physical implementation.
Distribution of partitions across shards is achieved by creating partitions in tablespaces that reside on different shards.
Each partition of a sharded table is stored in a separate tablespace, making the tablespace the unit of data distribution in an SDB.
As described in Sharded Table Family, to minimize the number of multi-shard joins, corresponding partitions of all tables in a table family are always stored in the same shard. This is guaranteed when tables in a table family are created in the same set of distributed tablespaces as shown in the syntax examples where tablespace set
ts1 is used for all tables.
However, it is possible to create different tables from a table family in different tablespace sets, for example the Customers table in tablespace set
ts1 and Orders in tablespace set
ts2. In this case, it must be guaranteed that the tablespace that stores partition 1 of Customers always resides in the same shard as the tablespace that stores partition 1 of Orders. To support this functionality, a set of corresponding partitions from all of the tables in a table family, called a chunk, is formed. A chunk contains a single partition from each table of a table family.
A chunk that contains corresponding partitions from the tables of Cutomers-Orders-LineItems schema is shown in the following figure.
Figure 51-2 Chunk as a Set of Partitions
Each shard contains multiple chunks as shown in the following figure.
Figure 51-3 Contents of a Shard
In addition to sharded tables, a shard can also contain one or more duplicated tables. Duplicated tables cannot be stored in tablespaces that are used for sharded tables.