Automatic management of materialized view logs

For OceanBase Database V4.3.5, automatic management of materialized view logs (mlogs) is supported starting with V4.3.5 BP4.

Automatic management of mlogs includes two main aspects:

1. When a materialized view is created, the system analyzes its dependencies on base tables and automatically creates the required mlogs.
2. The system periodically trims mlogs in the background, deletes mlogs that are no longer needed, and optimizes the column sets in mlogs to reduce maintenance overhead.

Limitations

Mlogs are automatically created or updated only when a materialized view is explicitly declared as a materialized view with incremental refresh (by specifying REFRESH FAST) or a real-time materialized view (by specifying ENABLE ON QUERY COMPUTATION).

Considerations

• When updating an mlog definition, OceanBase Database creates a new mlog table and replaces the original one. During this process, an incremental refresh is performed on all associated incremental refresh materialized views to flush incremental data from the original mlog into the materialized views, ensuring safe replacement. If a base table is associated with many materialized views and contains substantial incremental data, updating the mlog definition may be time-consuming. Please be aware of this potential impact in advance.

• It is recommended not to set the mlog_trim_interval too short, as this may cause mlogs to be mistakenly trimmed. If this occurs, you may need to recreate or update the mlog when creating a materialized view.

Related configuration

OceanBase Database provides two parameters to control mlog automatic management:

• enable_mlog_auto_maintenance: Controls whether automatic management of materialized view logs is enabled.
• mlog_trim_interval: Specifies the interval for scheduling automatic mlog trimming tasks.

Example

Enable the mlog auto management feature.

obclient> ALTER SYSTEM SET enable_mlog_auto_maintenance = True;

Example 1: Automatically create an mlog

1. Create a table named test_tbl1.

   obclient> CREATE TABLE test_tbl1(col1 INT, col2 INT, col3 INT);
   

2. Create a materialized view with incremental refresh named mv_test_tbl1 on the test_tbl1 table. OceanBase Database automatically creates an mlog for the col2 column of the test_tbl1 table.

   obclient> CREATE MATERIALIZED VIEW mv_test_tbl1
       REFRESH FAST
       AS SELECT
              col2,
              count(*) cnt
          FROM test_tbl1
          GROUP BY col2;
   

3. View the information about the materialized view logs on the test_tbl1 table.

   obclient> DESC mlog$_test_tbl1;
   

   The return result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | Field      | Type            | Null | Key  | Default | Extra |
   +------------+-----------------+------+------+---------+-------+
   | col2       | int(11)         | YES  |      | NULL    |       |
   | SEQUENCE$$ | bigint(20)      | NO   | PRI  | NULL    |       |
   | DMLTYPE$$  | varchar(1)      | YES  |      | NULL    |       |
   | OLD_NEW$$  | varchar(1)      | YES  |      | NULL    |       |
   | M_ROW$$    | bigint unsigned | NO   | PRI  | NULL    |       |
   +------------+-----------------+------+------+---------+-------+
   5 rows in set
   

Example 2: Automatically update the mlog definition

1. Create a table named test_tbl2.

   obclient> CREATE TABLE test_tbl2(col1 INT, col2 INT, col3 INT);
   

2. Create a materialized view with incremental refresh named mv1_test_tbl2 on the test_tbl2 table. OceanBase Database automatically creates an mlog for the col2 column on the test_tbl2 table.

   obclient> CREATE MATERIALIZED VIEW mv1_test_tbl2
       REFRESH FAST
       AS SELECT
              col2,
              count(*) cnt
          FROM test_tbl2
          GROUP BY col2;
   

3. View the information about the materialized view log on the test_tbl2 table.

   obclient> DESC mlog$_test_tbl2;
   

   The return result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | Field      | Type            | Null | Key  | Default | Extra |
   +------------+-----------------+------+------+---------+-------+
   | col2       | int(11)         | YES  |      | NULL    |       |
   | SEQUENCE$$ | bigint(20)      | NO   | PRI  | NULL    |       |
   | DMLTYPE$$  | varchar(1)      | YES  |      | NULL    |       |
   | OLD_NEW$$  | varchar(1)      | YES  |      | NULL    |       |
   | M_ROW$$    | bigint unsigned | NO   | PRI  | NULL    |       |
   +------------+-----------------+------+------+---------+-------+
   5 rows in set
   

4. Create a materialized view with incremental refresh named mv2_test_tbl2 on the test_tbl2 table. OceanBase Database detects that the current mlog table contains only the col2 column and modifies the existing mlog definition to add the col3 column to the mlog table.

   obclient> CREATE MATERIALIZED VIEW mv2_test_tbl2
       REFRESH FAST
       AS SELECT
              col3,
              count(*) cnt
          FROM test_tbl2
          GROUP BY col3;
   

5. View the information about the materialized view log on the test_tbl2 table again. You can observe that the mlog table definition for the test_tbl2 table contains the col2 and col3 columns.

   obclient> DESC mlog$_test_tbl2;
   

   The return result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | Field      | Type            | Null | Key  | Default | Extra |
   +------------+-----------------+------+------+---------+-------+
   | col2       | int(11)         | YES  |      | NULL    |       |
   | col3       | int(11)         | YES  |      | NULL    |       |
   | SEQUENCE$$ | bigint(20)      | NO   | PRI  | NULL    |       |
   | DMLTYPE$$  | varchar(1)      | YES  |      | NULL    |       |
   | OLD_NEW$$  | varchar(1)      | YES  |      | NULL    |       |
   | M_ROW$$    | bigint unsigned | NO   | PRI  | NULL    |       |
   +------------+-----------------+------+------+---------+-------+
   6 rows in set
   

Example 3: Automatic mlog pruning

1. Create a table named test_tbl3.

   obclient> CREATE TABLE test_tbl3(col1 INT, col2 INT, col3 INT);
   

2. Create a materialized view with incremental refresh named mv1_test_tbl3 on the test_tbl3 table. OceanBase Database automatically creates an mlog for the col2 column on the test_tbl3 table.

   obclient> CREATE MATERIALIZED VIEW mv1_test_tbl3
       REFRESH FAST
       AS SELECT
              col2,
              count(*) cnt
          FROM test_tbl3
          GROUP BY col2;
   

3. Create a materialized view with incremental refresh named mv2_test_tbl3 on the test_tbl3 table. OceanBase Database detects that the current mlog table contains only the col2 column and modifies the existing mlog definition by adding the col3 column to the mlog table.

   obclient> CREATE MATERIALIZED VIEW mv2_test_tbl3
       REFRESH FAST
       AS SELECT
           col3,
           count(*) cnt
       FROM test_tbl3
       GROUP BY col3;
   

4. View the information about the materialized view logs on the test_tbl3 table. You can observe that the mlog table definition for the test_tbl3 table contains the col2 and col3 columns.

   obclient> DESC mlog$_test_tbl3;
   

   The return result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | Field      | Type            | Null | Key  | Default | Extra |
   +------------+-----------------+------+------+---------+-------+
   | col2       | int(11)         | YES  |      | NULL    |       |
   | col3       | int(11)         | YES  |      | NULL    |       |
   | SEQUENCE$$ | bigint(20)      | NO   | PRI  | NULL    |       |
   | DMLTYPE$$  | varchar(1)      | YES  |      | NULL    |       |
   | OLD_NEW$$  | varchar(1)      | YES  |      | NULL    |       |
   | M_ROW$$    | bigint unsigned | NO   | PRI  | NULL    |       |
   +------------+-----------------+------+------+---------+-------+
   6 rows in set
   

5. Change the mlog pruning interval to observe the pruning result faster.

   obclient> ALTER SYSTEM SET mlog_trim_interval = '5s';
   

6. Drop the materialized view mv1_test_tbl3.

   obclient> DROP MATERIALIZED VIEW mv1_test_tbl3;
   

7. After 5 seconds, check the information about the materialized view logs on the test_tbl3 table. You can observe that the mlog table definition for the test_tbl3 table contains only the col3 column.

   obclient> DESC mlog$_test_tbl3;
   

   The return result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | Field      | Type            | Null | Key  | Default | Extra |
   +------------+-----------------+------+------+---------+-------+
   | col3       | int(11)         | YES  |      | NULL    |       |
   | SEQUENCE$$ | bigint(20)      | NO   | PRI  | NULL    |       |
   | DMLTYPE$$  | varchar(1)      | YES  |      | NULL    |       |
   | OLD_NEW$$  | varchar(1)      | YES  |      | NULL    |       |
   | M_ROW$$    | bigint unsigned | NO   | PRI  | NULL    |       |
   +------------+-----------------+------+------+---------+-------+
   5 rows in set
   

8. Drop the materialized view mv2_test_tbl3.

   obclient> DROP MATERIALIZED VIEW mv2_test_tbl3;
   

9. After 5 seconds, check the information about the materialized view logs on the test_tbl3 table. You can observe that the mlog table for the test_tbl3 table no longer exists.

   obclient> DESC mlog$_test_tbl3;
   

   The return result is as follows:

   ERROR 1146 (42S02): Table 'test_db.mlog$_test_tbl3' doesn't exist
   

10. Restore the mlog pruning interval.

    obclient> ALTER SYSTEM SET mlog_trim_interval = '1d';
    

References

• Overview of materialized views
• Materialized view logs
• Create a materialized view
• Drop a materialized view