Columnstore replica

A columnstore replica (C replica) is a type of read-only replica in OceanBase Database. It is primarily used for analytical (AP) read-only queries. Data in a C replica is stored in columnar storage. It works with all-purpose replicas (F replicas) and read-only replicas (R replicas): TP writes and regular reads are mainly performed on F and R replicas, while AP scans are performed on C replicas. In a C replica, all user tables (including replicated tables, but not index tables, internal tables, or system tables) are stored in columnar storage. OLAP services access C replicas through a dedicated proxy and perform weak reads for decision analysis.

A columnstore replica has the following characteristics:

• Contains full logs, a MemTable, and an SSTable.

• Baseline (major SSTable): The baseline data of user tables in the same log stream is stored in columnar storage in a C replica. This facilitates large-scale scans and AP computations.

• Incremental and hybrid storage: Incremental data such as MemTables and minor SSTables are still stored in row-based storage. A C replica asynchronously synchronizes logs from the source replica and replays them locally.

• Cannot join a Paxos group or participate in log voting as a member of the Paxos group. Instead, it works as a listener that synchronizes logs from the Paxos group members and then locally replays the logs. It does not act as a voting member and therefore causes no latency in transaction commits.

• Provides read-only services if an application does not require strong-consistency reads.

• Stores user tables in the same log stream in columnar storage mode at the major SSTable layer.

• Cannot be converted into a replica of another type.

The following table describes more features of such a replica.

Scenarios where a C replica stores data in rowstore format

In OceanBase Database, if a table is created as a rowstore table, the system creates a pure columnstore table on the C replica. If a table is created as a columnstore table, the storage method on the C replica is the same as that on the F replica. Therefore, the C replica only converts user partitions of rowstore tables on the F replica to columnstore format, and this description only indicates that the final state is columnstore. On the C replica, not all partitions are in columnstore format at any given time. In the following scenarios, user partitions on the C replica will temporarily be in rowstore format, and the system will automatically schedule row-to-column conversion tasks to convert the latest baseline data to columnstore format.

In the scenarios described above, during the row-to-column conversion process, although the optimizer generates a columnstore query plan, the actual execution is still against the rowstore baseline. Users can query the CDB_OB_CS_REPLICA_STATS (system tenant) and DBA_OB_CS_REPLICA_STATS (user tenant) views to obtain information about the C replica and the progress of the row-to-column conversion. After the row-to-column conversion task is completed, users can then query the C replica.

In the sys tenant, an example query to retrieve the row-to-column conversion progress of all tenants' C replicas in their log streams is as follows:

obclient[oceanbase]> SELECT * FROM oceanbase.CDB_OB_CS_REPLICA_STATS;

The query results are as follows:

+-----------+----------------+----------+-------+------------------+----------------------+-----------------------+---------------------------+-----------+
| TENANT_ID | SVR_IP         | SVR_PORT | LS_ID | TOTAL_TABLET_CNT | AVAILABLE_TABLET_CNT | TOTAL_MACRO_BLOCK_CNT | AVAILABLE_MACRO_BLOCK_CNT | AVAILABLE |
+-----------+----------------+----------+-------+------------------+----------------------+-----------------------+---------------------------+-----------+
|      1004 | xx.xxx.xxx.212 |    63000 |  1001 |             1019 |                 1019 |                 10706 |                     10706 | TRUE      |
|      1006 | xx.xxx.xxx.212 |    63000 |  1001 |              133 |                  133 |                   875 |                       875 | TRUE      |
+-----------+----------------+----------+-------+------------------+----------------------+-----------------------+---------------------------+-----------+
2 rows in set

From the first row of the query results, we can see that for tenant ID 1004, the C replica is located on the server xx.xxx.xxx.212 with port 63000; the log stream ID of the C replica is 1001; the total number of partitions to be converted to columnstore format is 1019, and the number of available partitions is 1019; the total number of baseline macroblocks is 10706, and the number of available macroblocks is 10706. Users can estimate the progress of the row-to-column conversion using AVAILABLE_TABLET_CNT / TOTAL_TABLET_CNT or AVAILABLE_MACRO_BLOCK_CNT / TOTAL_MACRO_BLOCK_CNT. A log stream is considered fully available only when all tablets in the log stream are available.