CREATE MATERIALIZED VIEW LOG

Purpose

This statement is used to create a materialized view log.

A materialized view log (MLOG) records incremental updates to the base table to support fast refreshes of materialized views. An MLOG is a tracking table that monitors changes to the base table and applies these changes to the corresponding materialized view to enable fast refreshes.

Privilege requirements

To create a materialized view log, you must have the CREATE TABLE privilege and the SELECT privilege on the base table. For more information about privileges in OceanBase Database, see Privilege types in MySQL-compatible mode.

Syntax

CREATE [OR REPLACE] MATERIALIZED VIEW LOG ON [database.] table
    [parallel_clause]
    [with_clause]
    [mv_log_purge_clause];

parallel_clause:
    NOPARALLEL
    | PARALLEL integer

with_clause:
    WITH [ {PRIMARY KEY | ROWID | SEQUENCE}
            [ { , PRIMARY KEY | , ROWID | , SEQUENCE }]... ]
        (column_name [, column_name]...)
        [new_values_clause]

new_values_clause:
    {INCLUDING | EXCLUDING} NEW VALUES

mv_log_purge_clause:
    PURGE {IMMEDIATE [ SYNCHRONOUS ]
        | START WITH datetime_expr [NEXT datetime_expr]
        | [START WITH datetime_expr] NEXT datetime_expr
        }

Parameters

parallel_clause

• NOPARALLEL: The default configuration, with a parallelism level of 1.

• PARALLEL integer: Specifies the parallelism level, where integer must be greater than or equal to 1.

with_clause

• PRIMARY KEY: Indicates that the materialized view log records the primary key columns of the base table. This option is only effective for tables with a primary key. If not specified, the system will automatically add this attribute to the materialized view log, meaning it will default to including the primary key columns.

• ROWID: Indicates that the materialized view log records the row identifiers (ROWID) of the base table. If not specified, the system will automatically add this attribute to the materialized view log, meaning it will default to including the M_ROW$$ column.

• SEQUENCE: Indicates that the materialized view log records the sequence numbers (seq_no) of multiple-row updates within a transaction. The system will automatically add this attribute to the materialized view log, meaning it will default to including the SEQUENCE$$ column.

new_values_clause

• INCLUDING: The default setting, indicating that both new and old values can be saved in the materialized view log. If you want the materialized view to support fast refreshes, you must specify INCLUDING NEW VALUES.

• EXCLUDING: Indicates that new values will not be recorded in the materialized view log. Do not use EXCLUDING NEW VALUES, as it will result in an error.

mv_log_purge_clause

• IMMEDIATE: Specifies that the corresponding materialized view log will be purged immediately after each refresh. The default value is SYNCHRONOUS.

  • SYNCHRONOUS: Optional. Specifies that the purge will be executed synchronously.

• START WITH datetime_expr [NEXT datetime_expr]:

  • START WITH datetime_expr: Specifies the initial purge time for the materialized view log.
  • [NEXT datetime_expr]: Optional. Specifies the next purge time for the materialized view log. Used to set the next purge time.

• [START WITH datetime_expr] NEXT datetime_expr: If only the NEXT datetime_expr parameter is specified and not the START WITH datetime_expr parameter, the initial purge time for the materialized view log will be set to the value of the NEXT datetime_expr parameter.

We recommend using sysdate() to represent the current time in the current time zone. Here is an example of a time expression:

START WITH sysdate() NEXT sysdate() + INTERVAL 1 DAY

The above clause indicates that expired materialized view log records will be purged every 1 day starting from the current time (sysdate()).

Examples

1. Create a table named test_tbl1.

   obclient> CREATE TABLE test_tbl1 (col1 INT PRIMARY KEY, col2 VARCHAR(20), col3 INT);
   

2. Create a materialized view log on the test_tbl1 table. Specify a parallelism of 5 for processing the materialized view log and record changes to the col2 column, including both the old and new values. Configure the materialized view log to start from the current date and to clean up expired records every 1 day.

   obclient> CREATE MATERIALIZED VIEW LOG ON test_tbl1
       PARALLEL 5
       WITH SEQUENCE(col2) INCLUDING NEW VALUES
       PURGE START WITH sysdate() NEXT sysdate() + INTERVAL 1 DAY;
   

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

   obclient> DESC mlog$_test_tbl1;
   

   The return result is as follows:

   +------------+-------------+------+------+---------+-------+
   | Field      | Type        | Null | Key  | Default | Extra |
   +------------+-------------+------+------+---------+-------+
   | col1       | int(11)     | NO   | PRI  | NULL    |       |
   | col2       | varchar(20) | YES  |      | NULL    |       |
   | SEQUENCE$$ | bigint(20)  | NO   | PRI  | NULL    |       |
   | DMLTYPE$$  | varchar(1)  | YES  |      | NULL    |       |
   | OLD_NEW$$  | varchar(1)  | YES  |      | NULL    |       |
   +------------+-------------+------+------+---------+-------+
   5 rows in set
   

4. Change the materialized view log on the test_tbl1 table to record changes to the col2 and col3 columns.

   obclient> CREATE OR REPLACE MATERIALIZED VIEW LOG ON test_tbl1
       PARALLEL 5
       WITH SEQUENCE(col2 ,col3) INCLUDING NEW VALUES
       PURGE START WITH sysdate() NEXT sysdate() + INTERVAL 1 DAY;
   

5. View the information about the materialized view log on the test_tbl1 table.

   DESC mlog$_test_tbl1;
   

   The return result is as follows:

   +------------+-------------+------+------+---------+-------+
   | Field      | Type        | Null | Key  | Default | Extra |
   +------------+-------------+------+------+---------+-------+
   | col1       | int(11)     | NO   | PRI  | NULL    |       |
   | col2       | varchar(20) | YES  |      | NULL    |       |
   | col3       | int(11)     | YES  |      | NULL    |       |
   | SEQUENCE$$ | bigint(20)  | NO   | PRI  | NULL    |       |
   | DMLTYPE$$  | varchar(1)  | YES  |      | NULL    |       |
   | OLD_NEW$$  | varchar(1)  | YES  |      | NULL    |       |
   +------------+-------------+------+------+---------+-------+
   6 rows in set
   

References

Materialized view logs