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 the fast refresh feature of materialized views. An mlog is a record table that tracks changes in the base table and applies these changes to the corresponding materialized view to enable fast refresh.

Privilege requirements

To create a materialized view log, you must have the CREATE TABLE and SELECT privileges on the base table. For more information about the privileges in OceanBase Database, see Privilege types in MySQL 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. The parallelism is 1.

• PARALLEL integer: specifies the parallelism. The value of 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 effective only for tables with a primary key. If you do not specify this option, the system automatically adds this attribute to the materialized view log, meaning that the materialized view log 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 you do not specify this option, the system automatically adds this attribute to the materialized view log, meaning that the materialized view log 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 automatically adds this attribute to the materialized view log, meaning that the materialized view log will default to including the SEQUENCE$$ column.

new_values_clause

• INCLUDING: the default setting. Indicates that both the old and new values can be recorded in the materialized view log. If you want to enable fast refresh for the materialized view, you must specify INCLUDING NEW VALUES.

• EXCLUDING: indicates that new values are not recorded in the materialized view log. Do not use EXCLUDING NEW VALUES, or an error will be returned.

mv_log_purge_clause

• IMMEDIATE: specifies that the corresponding materialized view log is purged immediately after the materialized view is refreshed. The default value is SYNCHRONOUS.

  • SYNCHRONOUS: optional. Specifies that the purge is 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. It is used to set the next purge time.

• [START WITH datetime_expr] NEXT datetime_expr: if you do not specify the START WITH datetime_expr parameter but specify the NEXT datetime_expr parameter, the initial purge time for the materialized view log is set to the value of the NEXT datetime_expr parameter.

We recommend that you use 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 preceding clause indicates that expired materialized view log records are 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. Set the parallelism for processing the materialized view log to 5, record changes to the col2 column, and include both the old and new values of the changed column. Configure the materialized view log to start from the current date and clear 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 of the materialized view log on the test_tbl1 table.

   obclient> DESC mlog$_test_tbl1;
   

   The returned 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 of the materialized view log on the test_tbl1 table.

   DESC mlog$_test_tbl1;
   

   The returned 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