Automatic management of materialized view logs

OceanBase Database supports automatic management of materialized view logs (mlogs). This feature includes the following two aspects:

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

Limitations

When you create a materialized view, an mlog is automatically created or the definition of an mlog is automatically updated only if you explicitly declare the materialized view as an incremental refresh materialized view (by specifying REFRESH FAST) or 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, it performs an incremental refresh on the associated incremental refresh materialized views to transfer the incremental data from the original mlog to the materialized views. This ensures a safe replacement of the mlog. Therefore, when a base table is associated with a large number of materialized views and contains a significant amount of incremental data, updating the mlog definition may be a time-consuming operation. You need to be aware of this in advance.

• We recommend that you do not set the mlog_trim_interval parameter to a very small value. Otherwise, the mlog may be mistakenly trimmed, and you may need to recreate or update the mlog when you create a materialized view.

Related configurations

OceanBase Database provides two parameters to control the automated behavior 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 an mlog.

Examples

Enable mlog auto-maintenance.

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 for the col2 column in 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 log of the test_tbl1 table. The query result is as follows:

   obclient> DESC mlog$_test_tbl1;
   

   The query result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | FIELD      | TYPE            | NULL | KEY  | DEFAULT | EXTRA |
   +------------+-----------------+------+------+---------+-------+
   | COL2       | NUMBER(38)      | YES  | NULL | NULL    | NULL  |
   | SEQUENCE$$ | BIGINT(20)      | NO   | PRI  | NULL    | NULL  |
   | DMLTYPE$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | OLD_NEW$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | M_ROW$$    | BIGINT UNSIGNED | NO   | PRI  | NULL    | 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 an incremental refresh materialized view named mv1_test_tbl2 on the test_tbl2 table. OceanBase Database automatically creates an mlog for the col2 column in 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 log of the test_tbl2 table. The query result is as follows:

   obclient> DESC mlog$_test_tbl2;
   

   The query result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | FIELD      | TYPE            | NULL | KEY  | DEFAULT | EXTRA |
   +------------+-----------------+------+------+---------+-------+
   | COL2       | NUMBER(38)      | YES  | NULL | NULL    | NULL  |
   | SEQUENCE$$ | BIGINT(20)      | NO   | PRI  | NULL    | NULL  |
   | DMLTYPE$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | OLD_NEW$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | M_ROW$$    | BIGINT UNSIGNED | NO   | PRI  | NULL    | 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 updates the 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 log of the test_tbl2 table. The query result is as follows:

   obclient> DESC mlog$_test_tbl2;
   

   The query result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | FIELD      | TYPE            | NULL | KEY  | DEFAULT | EXTRA |
   +------------+-----------------+------+------+---------+-------+
   | COL2       | NUMBER(38)      | YES  | NULL | NULL    | NULL  |
   | COL3       | NUMBER(38)      | YES  | NULL | NULL    | NULL  |
   | SEQUENCE$$ | BIGINT(20)      | NO   | PRI  | NULL    | NULL  |
   | DMLTYPE$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | OLD_NEW$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | M_ROW$$    | BIGINT UNSIGNED | NO   | PRI  | NULL    | 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 for the col2 column in 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 updates the 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 log of the test_tbl3 table. The query result is as follows:

   obclient> DESC mlog$_test_tbl3;
   

   The query result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | FIELD      | TYPE            | NULL | KEY  | DEFAULT | EXTRA |
   +------------+-----------------+------+------+---------+-------+
   | COL2       | NUMBER(38)      | YES  | NULL | NULL    | NULL  |
   | COL3       | NUMBER(38)      | YES  | NULL | NULL    | NULL  |
   | SEQUENCE$$ | BIGINT(20)      | NO   | PRI  | NULL    | NULL  |
   | DMLTYPE$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | OLD_NEW$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | M_ROW$$    | BIGINT UNSIGNED | NO   | PRI  | NULL    | NULL  |
   +------------+-----------------+------+------+---------+-------+
   6 rows in set
   

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

   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 log of the test_tbl3 table. The query result is as follows:

   obclient> DESC mlog$_test_tbl3;
   

   The query result is as follows:

   +------------+-----------------+------+------+---------+-------+
   | FIELD      | TYPE            | NULL | KEY  | DEFAULT | EXTRA |
   +------------+-----------------+------+------+---------+-------+
   | COL3       | NUMBER(38)      | YES  | NULL | NULL    | NULL  |
   | SEQUENCE$$ | BIGINT(20)      | NO   | PRI  | NULL    | NULL  |
   | DMLTYPE$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | OLD_NEW$$  | VARCHAR2(1 )    | YES  | NULL | NULL    | NULL  |
   | M_ROW$$    | BIGINT UNSIGNED | NO   | PRI  | NULL    | 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 log of the test_tbl3 table. The query result is as follows:

   obclient> DESC mlog$_test_tbl3;
   

   The query result is as follows:

   OBE-04043: object TEST_USER001.MLOG$_TEST_TBL3 does not exist
   

10. Restore the mlog pruning interval.

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

References

• Overview of materialized views
• Materialized view logs
• Create materialized views
• Delete materialized views