Automatic management of materialized view logs

OceanBase Database supports automatic management of materialized view logs (mlogs). This feature covers two main aspects:

1. When you create a materialized view, OceanBase Database analyzes its dependencies on the base table and automatically creates the required mlogs.
2. OceanBase Database periodically trims mlogs in the background. Specifically, it removes mlogs that are no longer referenced and reduces the number of columns in mlogs to lower the maintenance cost.

Limitations

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

Considerations

• When you update the definition of an mlog, OceanBase Database actually creates a new mlog table and replaces the original mlog. During the replacement, the incremental data in the original mlog is refreshed to the incremental refresh materialized views associated with the original mlog. Therefore, when a base table is associated with a large number of materialized views and contains a large amount of incremental data, updating the mlog definition may be a time-consuming operation.
• We recommend that you set the mlog_trim_interval parameter to a relatively large value. Otherwise, the mlog may be incorrectly trimmed, and you may need to recreate or update the mlog when you create a materialized view.

Related parameters

OceanBase Database provides two parameters to control the automatic management of mlogs:

• enable_mlog_auto_maintenance: specifies whether to enable the automatic management of mlogs.
• mlog_trim_interval: specifies the scheduling cycle of the background automatic trimming task of mlogs.

Examples

Enable the mlog auto-maintenance 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 an incremental refresh materialized view named mv_test_tbl1 on the test_tbl1 table. OceanBase Database automatically creates an mlog on 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. Query the information about the materialized view logs of the test_tbl1 table.

   obclient> DESC mlog$_test_tbl1;
   

   The returned 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 definition of an mlog

1. Create a table named test_tbl2.

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

2. Create an incremental refresh materialized view named mv1_test_tbl2 on the test_tbl2 table. OceanBase Database automatically creates an mlog on the col2 column of 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. Query the information about the materialized view logs of the test_tbl2 table.

   obclient> DESC mlog$_test_tbl2;
   

   The returned 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 an incremental refresh materialized view 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. Query the information about the materialized view logs of the test_tbl2 table again. You can find that the mlog table definition of the test_tbl2 table contains the col2 and col3 columns.

   obclient> DESC mlog$_test_tbl2;
   

   The returned 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: Automatically prune an mlog

1. Create a table named test_tbl3.

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

2. Create an incremental refresh materialized view named mv1_test_tbl3 on the test_tbl3 table. OceanBase Database automatically creates an mlog on the col2 column of 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 an incremental refresh materialized view 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 to add 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. Query the information about the materialized view logs of the test_tbl3 table again. You can find that the mlog table definition of the test_tbl3 table contains the col2 and col3 columns.

   obclient> DESC mlog$_test_tbl3;
   

   The returned 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 more quickly.

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

6. Drop the mv1_test_tbl3 materialized view.

   obclient> DROP MATERIALIZED VIEW mv1_test_tbl3;
   

7. Wait for 5 seconds and query the information about the materialized view logs of the test_tbl3 table again. You can find that the mlog table definition of the test_tbl3 table contains only the col3 column.

   obclient> DESC mlog$_test_tbl3;
   

   The returned 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 mv2_test_tbl3 materialized view.

   obclient> DROP MATERIALIZED VIEW mv2_test_tbl3;
   

9. Wait for 5 seconds and query the information about the materialized view logs of the test_tbl3 table again. You can find that the mlog table of the test_tbl3 table no longer exists.

   obclient> DESC mlog$_test_tbl3;
   

   The returned 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