Refresh a materialized view

When the data in the base table is updated, the data in the materialized view may become inconsistent with that in the base table. To maintain the data in the materialized view, OceanBase Database refreshes the materialized view.

OceanBase Database supports full refresh and incremental refresh for materialized views. You can also manually or automatically refresh a materialized view.

Full refresh

OceanBase Database performs a full refresh by using remote refresh. Specifically, a hidden table is created, the refresh statement is executed on the hidden table, and the original table is then switched with the hidden table. Therefore, a full refresh requires additional space and will rebuild all indexes (if any).

Considerations

• A full refresh may be a very time-consuming process, especially when a large amount of data needs to be read and processed. Therefore, you should always consider the time required for a full refresh before performing it.
• A full refresh is applicable only if the column types of the base table match those of the materialized view. Otherwise, a full refresh cannot be performed.
• If a materialized view is fully refreshed, any dependent materialized views (nested materialized views) must also be fully refreshed before they can be incrementally refreshed. Otherwise, an error will be returned.

Incremental refresh

Currently, incremental refresh is supported for materialized views that use the following types of SQL statements: non-aggregated single-table queries, aggregated single-table queries, multi-table join queries, multi-table join aggregated queries, and UNION ALL queries. For SQL statements that do not fall into these five categories, incremental refresh is not supported. For more information about the requirements for incremental refresh, see the following sections.

Non-aggregated incremental refresh on a single table

Example of non-aggregated incremental refresh on a single table

1. Create the tbl1 table.

   CREATE TABLE tbl1 (col1 NUMBER PRIMARY KEY, col2 NUMBER, col3 NUMBER, col4 NUMBER);
   

2. Create a materialized view log on the tbl1 table.

   CREATE MATERIALIZED VIEW LOG ON tbl1
       WITH SEQUENCE (col2, col3, col4) INCLUDING NEW VALUES;
   

3. Create an incremental refresh materialized view mv_tbl1 based on the tbl1 table.

   CREATE MATERIALIZED VIEW mv_tbl1
       REFRESH FAST ON DEMAND
       AS SELECT col1, col2
          FROM tbl1;
   

Single-table incremental refresh of aggregate views

The following are the basic requirements for single-table incremental refresh of aggregate views:

• The FROM table must be a base table, not an inline view or a regular view.

  Note

  Nested materialized views do not support real-time materialized views. Therefore, when the FROM table is a materialized view, real-time materialized views cannot be used.

• Only one table exists in the FROM clause.

• An mlog is created on the FROM table, and the columns used in the view are all included in the mlog.

• The view definition does not contain subqueries.

• Window functions are not supported.

• The view definition does not contain the ROLLUP, HAVING, DISTINCT, ORDER BY, LIMIT, or FETCH clause.

• If the query contains DISTINCT, the output columns of the materialized view that can be incrementally updated must be unique. Therefore, you can directly disable the DISTINCT keyword or remove it.

• Statements without the GROUP BY clause must be scalar aggregates.

• For materialized views with the GROUP BY clause, only the SUM and COUNT aggregate functions are supported, and only simple columns can be used in the aggregate functions. The requirements for GROUP BY are as follows:

  Aggregate function	The SELECT clause must contain the dependent columns
  COUNT( expr )	N/A
  SUM ( expr )	COUNT( expr ) or expr is not null
  AVG ( expr )	SUM ( expr ),COUNT( expr )
  STDDEV ( expr )	SUM ( expr ),COUNT( expr ),SUM ( expr * expr )
  VARIANCE ( expr )	SUM ( expr ),COUNT( expr ),SUM ( expr * expr )
  Other aggregate functions that can be decomposed into SUM and COUNT... (The calculation methods may change, which may affect the precision)	SUM (col1),COUNT(col1)
  MAX(expr)/MIN(expr)	COUNT(expr)

  1. The GROUP BY clause must be in the standard GROUP BY syntax and must not contain ROLLUP or HAVING.
  2. The SELECT clause must contain all the GROUP BY columns.
  3. The aggregate functions must not contain the DISTINCT keyword.
  4. The SELECT clause must contain the dependent columns and the COUNT(*) column corresponding to the aggregate functions, in addition to the aggregate function columns. For example, if you use the SUM(expr) aggregate function, you must also include the COUNT(*) and COUNT(expr) columns.

Requirements for incremental refresh of aggregate views with MIN/MAX aggregate functions:

If the basic requirements for single-table incremental refresh of aggregate views are met, the following requirements apply when you use the MAX and MIN aggregate functions: an index with the GROUP BY columns as the prefix must exist on the base table of the materialized view.

Single-table aggregate incremental refresh example

1. Create a table named test_tbl1.

   CREATE TABLE test_tbl1 (col1 NUMBER PRIMARY KEY, col2 NUMBER, col3 NUMBER, col4 NUMBER);
   

2. Create a materialized view log on the test_tbl1 table.

   CREATE MATERIALIZED VIEW LOG ON test_tbl1
     WITH SEQUENCE (col2, col3, col4) INCLUDING NEW VALUES;
   

3. Create a materialized view with an incremental refresh method.

   • Create a materialized view named mv1_test_tbl1. Set the refresh method to incremental refresh, and allow manual refresh triggers as needed. The query part of the materialized view selects the col2 column from the test_tbl1 table and calculates the aggregate results of count(*), count(col3), and sum(col3), grouped by the values in the col2 column.

     CREATE MATERIALIZED VIEW mv1_test_tbl1
       REFRESH FAST ON DEMAND
       AS SELECT col2, count(*) cnt, count(col3) cnt_col3, sum(col3) sum_col3
          FROM test_tbl1
          GROUP BY col2;
     

   • Create a materialized view named mv2_test_tbl1. Set the refresh method to incremental refresh, and allow manual refresh triggers as needed. The query part of the materialized view calculates the aggregate results of count(*), count(col3), and sum(col3) from the test_tbl1 table.

     CREATE MATERIALIZED VIEW mv2_test_tbl1
       REFRESH FAST ON DEMAND
       AS SELECT count(*) cnt, count(col3) cnt_col3, sum(col3) sum_col3
          FROM test_tbl1;
     

   • Create a materialized view named mv3_test_tbl1. Set the refresh method to incremental refresh, and allow manual refresh triggers as needed. The query part of the materialized view calculates the results of count(col3) and sum(col3) from the test_tbl1 table.

     CREATE MATERIALIZED VIEW mv3_test_tbl1
       REFRESH FAST ON DEMAND
       AS SELECT count(col3) cnt_col3, sum(col3) sum_col3
          FROM test_tbl1;
     

   • Create a materialized view named mv4_test_tbl1. Set the refresh method to incremental refresh, and allow manual refresh triggers as needed. The query part of the materialized view selects the col2 and col3 columns from the test_tbl1 table and calculates the aggregate results of count(*), count(col3), and sum(col3), grouped by the values in the col2 and col3 columns.

     CREATE MATERIALIZED VIEW mv4_test_tbl1
       REFRESH FAST ON DEMAND
       AS SELECT col2, col3, count(*) cnt, count(col3) cnt_col3, sum(col3) sum_col3
          FROM test_tbl1
          GROUP BY col2, col3;
     

   • Create a materialized view named mv5_test_tbl1. Set the refresh method to incremental refresh, and allow manual refresh triggers as needed. The query part of the materialized view selects the col2 column from the test_tbl1 table and calculates the aggregate results of count(*), count(col3), sum(col3), and avg(col3), as well as some custom columns calcol1 and calcol2, grouped by the values in the col2 column.

     CREATE MATERIALIZED VIEW mv5_test_tbl1
       REFRESH FAST ON DEMAND
       AS SELECT col2, count(*) cnt, count(col3) cnt_col3, sum(col3) sum_col3, avg(col3) avg_col3, avg(col3) * sum(col3)/col2 calcol1, col2+sum(col3) calcol2
          FROM test_tbl1
          GROUP BY col2;
     

   • Create a materialized view named mv6_test_tbl1. Set the refresh method to incremental refresh, and allow manual refresh triggers as needed. The query part of the materialized view selects the col2 column from the test_tbl1 table and calculates the aggregate results of count(*), count(col3), sum(col3), count(col3*col3), sum(col3*col3), and STDDEV(col3), grouped by the values in the col2 column.

     CREATE MATERIALIZED VIEW mv6_test_tbl1
       REFRESH FAST ON DEMAND
       AS SELECT col2, count(*) cnt, count(col3) cnt_col3, sum(col3) sum_col3, count(col3*col3) cnt_col3_2, sum(col3*col3) sum_col3_2, STDDEV(col3) stddev_col3
          FROM test_tbl1
          GROUP BY col2;
     

   • Create a materialized view using the MAX and MIN aggregate functions.

     1. Create an index named idx_test_tbl1 on the test_tbl1 table based on the col1 and col2 columns.

        CREATE INDEX idx_test_tbl1 ON test_tbl1(col1, col2);
        

     2. Create a materialized view named mv7_test_tbl1. Set the refresh method to incremental refresh, and allow manual refresh triggers as needed. The query part of the materialized view selects the col1 and col2 columns from the test_tbl1 table and calculates the aggregate results of count(*), the sum of the minimum value of col3 and the maximum value of col4 in each group, grouped by the combination of col1 and col2 values.

        CREATE MATERIALIZED VIEW mv7_test_tbl1
            REFRESH FAST ON DEMAND
            AS SELECT
                col1,
                col2,
                count(*) cnt,
                MIN(col3) + MAX(col4) AS min_max_val
                FROM test_tbl1
                GROUP BY col1, col2;
        

   • Use non-basic columns in the GROUP BY clause.

     1. Create a table named m_tbl1.

        obclient> CREATE TABLE m_tbl1(col1 NUMBER PRIMARY KEY, col2 DATE, col3 NUMBER, col4 NUMBER, col5 NUMBER, col6 NUMBER);
        

     2. Create a materialized view log on the m_tbl1 table.

        obclient> CREATE MATERIALIZED VIEW LOG ON m_tbl1
            WITH PRIMARY KEY, ROWID, SEQUENCE (col2, col3, col4, col5, col6)
            INCLUDING NEW VALUES;
        

     3. Create an index named idx1_m_tbl1 on the m_tbl1 table based on the col3 and (DATE(col2)) columns.

        obclient> CREATE INDEX idx1_m_tbl1 ON m_tbl1(col3, (ROUND(col2)));
        

     4. Create a materialized view named m_tbl1_mv1 with a single-table aggregate incremental refresh method.

        obclient> CREATE MATERIALIZED VIEW m_tbl1_mv1
            REFRESH FAST ON DEMAND
            AS SELECT
                col3,
                ROUND(col2) gby_2,
                COUNT(*) cnt,
                MAX(col4) max_c4,
                MIN(col5 + col6) min_c5_c6
            FROM m_tbl1
            GROUP BY col3, ROUND(col2);
        

Incremental refresh of materialized views with multiple tables joined

The following requirements must be met for incremental refresh of materialized views with multiple tables joined:

1. The FROM table must be a base table, not an inline view.

2. The FROM table must contain at least two tables.

   Notice

   • The incremental refresh feature of materialized views in OceanBase Database supports outer joins (LEFT JOIN/RIGHT JOIN).
   • Outer join restrictions: The join tree must be a left-deep join tree where INNER JOIN is performed first, followed by LEFT JOIN.
   • If the SELECT clause of an incremental refresh materialized view contains the LEFT JOIN operator, you cannot specify a primary key (PRIMARY KEY) or unique index (UNIQUE INDEX) when you create the materialized view. This is to prevent refresh exceptions caused by constraint conflicts.
   • Aggregated materialized views with outer joins do not support real-time materialized views.

3. Mlogs must be created for all FROM tables, and the columns used in the view must exist in the mlogs.

4. The view definition must not contain subqueries.

5. The view definition must not contain the ROLLUP, HAVING, WINDOW FUNCTION, DISTINCT, ORDER BY, LIMIT, or FETCH clause.

6. The view definition must not contain expressions that generate unstable output values, such as ROWNUM, RAND, and SYSDATE.

Example of incremental refresh of materialized views with multiple tables joined

1. Create base tables t1 and t2.

   CREATE TABLE t1(c1 INT PRIMARY KEY, c2 INT, c3 INT);
   

   CREATE TABLE t2(c1 INT PRIMARY KEY, c4 INT, c5 INT);
   

2. Create materialized view logs on tables t1 and t2.

   CREATE MATERIALIZED VIEW LOG ON t1 WITH PRIMARY KEY, ROWID, SEQUENCE (c2) INCLUDING NEW VALUES;
   

   CREATE MATERIALIZED VIEW LOG ON t2 WITH PRIMARY KEY, ROWID, SEQUENCE (c4) INCLUDING NEW VALUES;
   

3. Create a materialized view mv1_t1_t2 for the join of tables t1 and t2.

   CREATE MATERIALIZED VIEW mv1_t1_t2
     REFRESH FAST
     AS SELECT t1.c1 t1c1, t1.c2, t2.c1 t2c1, t2.c4
       FROM t1 JOIN t2 ON t1.c1=t2.c1;
   

Here is an example of creating indexes for the materialized view and its dependent base tables:

1. (Optional) Execute the following statement to delete the test data.

   You can skip this step if the following database objects do not exist.

   DROP MATERIALIZED VIEW LOG ON t1;
   DROP TABLE t1;
   DROP MATERIALIZED VIEW LOG ON t2;
   DROP TABLE t2;
   DROP MATERIALIZED VIEW rt_mv1;
   

2. Execute the following statement to create table t1 and index idx_t1_c2.

   CREATE TABLE t1(c1 INT GENERATED BY DEFAULT AS IDENTITY, c2 INT, c3 INT, c4 INT, c5 INT, PRIMARY KEY(c1));
   CREATE INDEX idx_t1_c2 ON t1(c2);
   

3. Execute the following statement to create table t2 and index idx_t2_c3.

   CREATE TABLE t2(c1 INT GENERATED BY DEFAULT AS IDENTITY, c2 INT, c3 INT, c4 INT, c5 INT, PRIMARY KEY(c1));
   CREATE INDEX idx_t2_c3 ON t2(c3);
   

4. Execute the following statement to create materialized view logs on tables t1 and t2.

   CREATE MATERIALIZED VIEW LOG ON t1 WITH PRIMARY KEY, ROWID, SEQUENCE (c2, c3, c4) INCLUDING NEW VALUES;
   CREATE MATERIALIZED VIEW LOG ON t2 WITH PRIMARY KEY, ROWID, SEQUENCE (c2, c3, c4) INCLUDING NEW VALUES;
   

5. Execute the following statement to create a real-time materialized view rt_mv1.

   CREATE MATERIALIZED VIEW rt_mv1
     NEVER REFRESH
     ENABLE ON QUERY COMPUTATION
     DISABLE QUERY REWRITE
     AS SELECT t1.c1 AS t1_c1, t2.c1 AS t2_c1, t1.c2 AS t1_c2, t2.c2 AS t2_c2, t1.c3 AS t1_c3, t2.c3 AS t2_c3 
         FROM t1, t2
         WHERE t1.c2 = t2.c3;
   

6. Execute the following statement to create indexes on the primary key columns of the base tables in the materialized view.

   CREATE INDEX idx_mv_t1_c1 ON rt_mv1(t1_c1);
   CREATE INDEX idx_mv_t2_c1 ON rt_mv1(t2_c1);
   

Incremental refresh of aggregated views across multiple tables

The basic requirements for incremental refresh of aggregated views across multiple tables are as follows:

• The basic requirements for incremental refresh of aggregated views across multiple tables are the union of the basic requirements for incremental refresh of aggregated views on a single table and incremental refresh of joined views across multiple tables.
• Supports incremental refresh of aggregated views with outer joins. The limitations for outer joins in aggregated views are the same as those for non-aggregated outer-joined materialized views. The limitations for the aggregated part are the same as those for inner-joined aggregated views. However, aggregated views with outer joins do not support the MIN and MAX aggregate functions, and do not support real-time materialized views.

Example of incremental refresh of aggregated views across multiple tables

1. Create the base tables t3 and t4.

   CREATE TABLE t3(c1 INT, c2 INT, c3 INT, c4 INT, PRIMARY KEY(c1));
   

   CREATE TABLE t4(c1 INT, c2 INT, c3 INT, c4 INT, PRIMARY KEY(c1));
   

2. Create materialized view logs on the t3 and t4 tables.

   CREATE MATERIALIZED VIEW LOG ON t3 WITH PRIMARY KEY, ROWID, SEQUENCE(c2, c3, c4) INCLUDING NEW VALUES;
   

   CREATE MATERIALIZED VIEW LOG ON t4 WITH PRIMARY KEY, ROWID, SEQUENCE(c2, c3, c4) INCLUDING NEW VALUES;
   

3. Create a real-time materialized view mv1_t3_t4 for the incremental refresh of the aggregated join between the t3 and t4 tables.

   CREATE MATERIALIZED VIEW mv1_t3_t4
     REFRESH FAST
     ENABLE ON QUERY COMPUTATION
     AS SELECT t3.c1,
         COUNT(*) cnt,
         COUNT(t4.c4) cnt_c4,
         SUM(t4.c4) sum_c4,
         AVG(t4.c4) avg_c4
       FROM t3, t4
       WHERE t3.c2 = t4.c3
       GROUP BY t3.c1;
   

Incremental refresh of a collection query

A collection materialized view supports the UNION ALL clause and allows incremental refresh. An incremental refresh of a collection materialized view allows you to use all materialized views that support incremental refresh in the collection branches, except for the excepted join materialized views.

The following requirements must be met for an incremental refresh of a collection query:

• A materialized view with a collection query does not support real-time materialization.

• The top-level query is UNION ALL, and the view definition does not contain subqueries or clauses such as ORDER BY, LIMIT, or FETCH.

• The output columns at the same projection positions in the UNION ALL branches must have the same data types. You cannot forcibly convert the data types of the columns in the subbranches to a consistent type by using the CAST clause.

  For example, in a materialized view that joins multiple tables, the primary keys of the joined tables must appear in the SELECT clause. The SELECT output columns cannot be forcibly converted to a consistent type by using the CAST clause because of the UNION ALL clause.

• The UNION ALL branches must have the same data types and values for the columns at the same projection positions. The constant columns are used to identify the branches.

Examples

1. Create the base tables ua_tbl1 and ua_tbl2.

   CREATE TABLE ua_tbl1 (col1 INT PRIMARY KEY, col2 INT, col3 INT, col4 INT);
   

   CREATE TABLE ua_tbl2 (col1 INT PRIMARY KEY, col2 INT, col3 INT, col4 INT);
   

2. Create materialized view logs on the tables ua_tbl1 and ua_tbl2.

   CREATE MATERIALIZED VIEW LOG ON ua_tbl1
       WITH PRIMARY KEY, ROWID, SEQUENCE (col2, col3, col4) INCLUDING NEW VALUES;
   

   CREATE MATERIALIZED VIEW LOG ON ua_tbl2
       WITH PRIMARY KEY, ROWID, SEQUENCE (col2, col3, col4) INCLUDING NEW VALUES;
   

3. Create the collection query materialized view mv_ua_tbl1_tbl2 that allows incremental refresh.

   CREATE MATERIALIZED VIEW mv_ua_tbl1_tbl2
       REFRESH FAST ON DEMAND
       AS SELECT
           a.col1 as a_c1,
           b.col1 as b_c1,
           1 marker,
           a.col2 val
       FROM ua_tbl1 a
               INNER JOIN ua_tbl2 b
               ON a.col2 = b.col3
       UNION ALL
       SELECT
           col1 a_c1,
           col2 b_c1,
           2 marker,
           count(*) val
       FROM ua_tbl1
       GROUP BY col1, col2;
   

Parallelism control for materialized view refresh

OceanBase Database provides a parallelism control mechanism for materialized view refresh: if a user explicitly specifies the parallelism for a refresh operation, the specified value is used; if not, the system variable mview_refresh_dop can be configured to determine the parallelism for the current session.

Set mview_refresh_dop

mview_refresh_dop is a system variable in OceanBase Database that controls the default parallelism for materialized view refresh operations. By appropriately setting its value, you can significantly improve the refresh efficiency and optimize the database performance.

When mview_refresh_dop is set to 0 or 1, parallel refresh is disabled. For more information about the system variable mview_refresh_dop, see mview_refresh_dop.

Here are some examples:

• Set the parallelism for the current session to 5.

  SET mview_refresh_dop = 5;
  

• Set the parallelism for all sessions to 5.

  SET GLOBAL mview_refresh_dop = 5;
  

  Notice

  Setting a global variable does not take effect on the current session. You must log in again to create a new session for the setting to take effect.

View the parallelism of a materialized view

• Use the DBA_MVIEWS view to query the background refresh parallelism of a materialized view.

  Note

  The DBA_MVIEWS view can only be used to query the parallelism specified for a materialized view.

  • When the REFRESH_DOP field is not 0, the background refresh task of the materialized view will use the parallelism specified by REFRESH_DOP.
  • When REFRESH_DOP is 0, the value of the global-level mview_refresh_dop variable will be used.

  Here is an example:

  1. Create a materialized view named mv0_t1.

     CREATE MATERIALIZED VIEW mv0_t1
         REFRESH COMPLETE ON DEMAND
         START WITH current_date
             NEXT current_date + INTERVAL '10' SECOND
         AS SELECT c1, c2
             FROM t1;
     

  2. Query the background refresh parallelism of the materialized view mv0_t1.

     SELECT OWNER, MVIEW_NAME,REFRESH_DOP
     FROM SYS.DBA_MVIEWS
     WHERE OWNER = 'SYS'
     AND MVIEW_NAME = 'MV0_T1';
     

     The query result is as follows:

     +-------+------------+-------------+
     | OWNER | MVIEW_NAME | REFRESH_DOP |
     +-------+------------+-------------+
     | SYS   | MV0_T1     |           0 |
     +-------+------------+-------------+
     1 row in set
     

  3. Change the parallelism of the materialized view mv0_t1 to 8.

     ALTER MATERIALIZED VIEW mv0_t1 PARALLEL 8;
     

  4. Query the background refresh parallelism of the materialized view.

     SELECT OWNER, MVIEW_NAME,REFRESH_DOP
     FROM SYS.DBA_MVIEWS
     WHERE OWNER = 'SYS'
     AND MVIEW_NAME = 'MV0_T1';
     

     The query result is as follows:

     +-------+------------+-------------+
     | OWNER | MVIEW_NAME | REFRESH_DOP |
     +-------+------------+-------------+
     | SYS   | MV0_T1     |           8 |
     +-------+------------+-------------+
     1 row in set
     

• Use the DBA_MVREF_RUN_STATS view to query the historical refresh parallelism of a materialized view.

  Here is an example:

  SELECT REFRESH_ID, MVIEWS, PARALLELISM
  FROM SYS.DBA_MVREF_RUN_STATS
  WHERE MVIEWS = 'SYS.MV0_T1'
  ORDER BY REFRESH_ID;
  

  The query result is as follows:

  +------------+------------+-------------+
  | REFRESH_ID | MVIEWS     | PARALLELISM |
  +------------+------------+-------------+
  |    6752103 | SYS.MV0_T1 |           5 |
  |    6752733 | SYS.MV0_T1 |           5 |
  |    6753371 | SYS.MV0_T1 |           5 |
  |    6753985 | SYS.MV0_T1 |           8 |
  |    6754618 | SYS.MV0_T1 |           8 |
  |    6755249 | SYS.MV0_T1 |           8 |
  +------------+------------+-------------+
  6 rows in set
  

Manually refresh a materialized view

If the refresh mode of a materialized view is ON DEMAND, you can use the DBMS_MVIEW package to manually refresh the materialized view. For a materialized view defined for incremental refresh, you can specify a full refresh when manually refreshing it.

Use the REFRESH procedure to refresh a materialized view

DBMS_MVIEW.REFRESH (
   list                   IN     VARCHAR2,                  -- The name of the materialized view. Multiple materialized views are not supported.
   method                 IN     VARCHAR2       := NULL,    -- The refresh option. Valid values: f, ?, C, c, A, and a. The default value is NULL. For more information, see the description of this parameter.
                                                            --   f: fast refresh
                                                            --   ?: force refresh
                                                            --   C|c: complete refresh
                                                            --   A|a: always refresh, which is equivalent to C
   ----------- The following parameters are not supported. They are provided for compatibility with Oracle. ----------------
   rollback_seg           IN     VARCHAR2       := NULL,
   push_deferred_rpc      IN     BOOLEAN        := true,
   refresh_after_errors   IN     BOOLEAN        := false,
   purge_option           IN     BINARY_INTEGER := 1,
   parallelism            IN     BINARY_INTEGER := 0,
   heap_size              IN     BINARY_INTEGER := 0,
   atomic_refresh         IN     BOOLEAN        := true, 
   nested                 IN     BOOLEAN        := false,
   out_of_place           IN     BOOLEAN        := false,
   skip_ext_data          IN     BOOLEAN        := false,
   ---------------------------------------------------------
   refresh_parallel       IN     BINARY_INTEGER := 0);       -- The refresh parallelism of the materialized view. This parameter is unique to OceanBase Database.

Here is an example:

1. Insert three rows of data into the test_tbl1 table.

   INSERT INTO test_tbl1 VALUES (1, 1, 1, 1),(2, 2, 2, 2),(3, 3, 3, 3);
   

2. View the information of the materialized view mv1_test_tbl1.

   SELECT * FROM mv1_test_tbl1;
   

   The returned result is as follows:

   Empty set
   

3. Manually refresh the materialized view mv1_test_tbl1 by using the DBMS_MVIEW.REFRESH function.

   • Refresh the materialized view by using the refresh option specified for the materialized view:

     CALL DBMS_MVIEW.REFRESH('mv1_test_tbl1');
     

   • Refresh the materialized view by specifying the refresh option:

     CALL DBMS_MVIEW.REFRESH('mv1_test_tbl1', 'c');
     

4. View the information of the materialized view mv1_test_tbl1 again.

   SELECT * FROM mv1_test_tbl1;
   

   The returned result is as follows:

   +------+------+----------+----------+
   | COL2 | CNT  | CNT_COL3 | SUM_COL3 |
   +------+------+----------+----------+
   |    3 |    1 |        1 |        3 |
   |    2 |    1 |        1 |        2 |
   |    1 |    1 |        1 |        1 |
   +------+------+----------+----------+
   3 rows in set
   

Set the refresh parallelism of a materialized view

You can set the mview_refresh_dop system variable to specify the default refresh parallelism of a materialized view.

You can also explicitly specify the refresh_parallel parameter in the DBMS_MVIEW.REFRESH function to set the refresh parallelism of the current refresh.

Here is an example:

1. Set the parallelism of the current session to 5.

   SET mview_refresh_dop = 5;
   

2. Manually refresh the materialized view:

   • Explicitly specify the refresh parallelism as 8. The refresh parallelism of this refresh is 8.

     CALL DBMS_MVIEW.REFRESH('mv1', 'c', 8);
     

   • Do not explicitly specify the refresh parallelism. The refresh parallelism is 5, which is the value of the session variable.

     CALL DBMS_MVIEW.REFRESH('mv1', 'c');
     

Automatic refresh of materialized views

When you create a materialized view, if you specify the START WITH datetime_expr and NEXT datetime_expr clauses, the system automatically creates a background refresh task for the materialized view when the specified conditions are met.

Parallelism for automatic refresh of materialized views

You can specify the parallelism for the automatic refresh of materialized views in the following two ways:

1. Specify the parallelism (Table DOP) when you create the materialized view.

   Here is an example:

   CREATE MATERIALIZED VIEW mv_t1
       PARALLEL 8
       REFRESH COMPLETE ON DEMAND
           START WITH current_date
               NEXT current_date + INTERVAL '10' SECOND
           AS SELECT c1, c2
              FROM t1;
   

2. Set the global session variable mview_refresh_dop as the refresh parallelism.

   Automatic refresh operations are performed using internal sessions. To enable parallel refresh for background sessions, you must set the global-level mview_refresh_dop variable.

   Notice

   If you do not explicitly specify the parallelism when you create the materialized view and the mview_refresh_dop variable is set to 0 or 1, parallel refresh is not enabled for the background refresh task.

   Here is an example:

   1. Set the global session parallelism to 5.

      SET GLOBAL mview_refresh_dop = 5;
      

   2. Create a materialized view:

      • If you specify the parallelism as 8 when you create the materialized view, the background refresh task will use 8 as the refresh parallelism.

        CREATE MATERIALIZED VIEW mv1_t1
            PARALLEL 8
            REFRESH COMPLETE ON DEMAND
                 START WITH current_date
                     NEXT current_date + INTERVAL '10' SECOND
            AS SELECT c1, c2
            FROM t1;
        

      • If you do not specify the parallelism when you create the materialized view, the background refresh task will use the value of the mview_refresh_dop variable, which is 5, as the refresh parallelism.

        CREATE MATERIALIZED VIEW mv2_t1
            REFRESH COMPLETE ON DEMAND
                 START WITH current_date
                     NEXT current_date + INTERVAL '10' SECOND
            AS SELECT c1, c2
            FROM t1;
        

Refresh nested materialized views

Refresh rules for nested materialized views

The refresh methods supported by nested materialized views are the same as those supported by non-nested materialized views. Both types of materialized views support full refresh and incremental refresh. Although refreshing a nested materialized view only requires the user tables and materialized views (along with their mlogs) that it directly depends on, the data consistency of a nested materialized view depends on the materialized views it is based on. This means that if you want to refresh a nested materialized view to keep its data up to date, you must first refresh the materialized views it depends on to ensure their data is up to date.

For example, consider the following scenario: Materialized view mv1 is built on tables tbl1 and tbl2, materialized view mv2 is built on materialized view mv1 and table tbl3, and materialized view mv3 is built on materialized views mv1 and mv2. If you refresh mv1, then mv2, and finally mv3, you can ensure the overall data consistency of the nested materialized views. However, if you refresh mv2 first and then mv1, the data in mv2 will not be up to date (it will lag behind mv1). Similarly, if you refresh mv3 first and then mv2, the data in mv3 will not be up to date (it will lag behind mv2).

OceanBase Database supports cascading refreshes for nested materialized views. Cascading refreshes can be either cascading non-consistent refreshes or cascading consistent refreshes:

• Cascading non-consistent refresh: This method refreshes all materialized views that the nested materialized view depends on, starting from the bottom. Each materialized view is refreshed without data consistency guarantees, and the data points read from the base tables may not be consistent. Cascading non-consistent refreshes are suitable for batch synchronization scenarios, such as when a business party periodically synchronizes data from the upstream source. After the data synchronization is complete, a cascading non-consistent refresh can be performed to ensure the final consistency of the materialized views.

• Cascading consistent refresh: This method ensures that, after the entire cascading refresh is completed, all base tables that the upper-level materialized views depend on are consistent in their data points. Cascading consistent refreshes are suitable for real-time data synchronization scenarios. We ensure that, after each cascading refresh, the data snapshots in the materialized views are at the same data point.

Example of nested materialized view refresh

If a materialized view is fully refreshed, any dependent materialized views must be fully refreshed before they can be incrementally refreshed. Otherwise, an error will occur.

Here is an example:

1. Create a table tbl1 and insert a row of data.

   CREATE TABLE tbl1(id INT, name VARCHAR2(30), PRIMARY KEY(id));
   

   INSERT INTO tbl1 VALUES (1, 'jack');
   

2. Create a table tbl2 and insert a row of data.

   CREATE TABLE tbl2(id INT, age INT, PRIMARY KEY(id));
   

   INSERT INTO tbl2 VALUES (1, 21);
   

3. Create a materialized view log on the tbl1 and tbl2 tables.

   CREATE MATERIALIZED VIEW LOG ON tbl1 WITH PRIMARY KEY (name) INCLUDING NEW VALUES;
   

   CREATE MATERIALIZED VIEW LOG ON tbl2 WITH PRIMARY KEY (age) INCLUDING NEW VALUES;
   

4. Create a materialized view mv1 based on the tbl1 and tbl2 tables.

   CREATE MATERIALIZED VIEW mv1 (PRIMARY KEY (id1, id2))
       REFRESH FAST ON DEMAND
       AS SELECT tbl1.id id1, tbl2.id id2, tbl1.NAME, tbl2.AGE
           FROM tbl1, tbl2
           WHERE tbl1.id = tbl2.id;
   

5. Create a materialized view log on the mv1 materialized view.

   CREATE MATERIALIZED VIEW LOG ON mv1 WITH PRIMARY KEY (name, age) INCLUDING NEW VALUES;
   

6. Create a materialized view (nested materialized view) mv2 based on the mv1 materialized view.

   CREATE MATERIALIZED VIEW mv2
       REFRESH FAST
       AS SELECT COUNT(*) cnt, COUNT(AGE) age_cnt, SUM(AGE) age_sum
           FROM mv1;
   

7. Query the data of the mv1 materialized view.

   SELECT * FROM mv1;
   

   The returned result is as follows:

   +------+------+------+------+
   | ID1  | ID2  | NAME | AGE  |
   +------+------+------+------+
   |    1 |    1 | jack |   21 |
   +------+------+------+------+
   1 row in set
   

8. Query the data of the mv2 materialized view.

   SELECT * FROM mv2;
   

   The returned result is as follows:

   +------+---------+---------+
   | CNT  | AGE_CNT | AGE_SUM |
   +------+---------+---------+
   |    1 |       1 |      21 |
   +------+---------+---------+
   1 row in set
   

9. Insert a row of data into the tbl1 and tbl2 tables.

   INSERT INTO tbl1 VALUES (2, 'rose');
   

   INSERT INTO tbl2 VALUES (2, 19);
   

10. Incrementally refresh the mv1 materialized view.

    CALL dbms_mview.refresh('mv1', 'f');
    

11. Query the data of the mv1 materialized view.

    SELECT * FROM mv1;
    

    The returned result is as follows:

    +------+------+------+------+
    | ID1  | ID2  | NAME | AGE  |
    +------+------+------+------+
    |    1 |    1 | jack |   21 |
    |    2 |    2 | rose |   19 |
    +------+------+------+------+
    2 rows in set
    

12. Incrementally refresh the mv2 materialized view.

    CALL dbms_mview.refresh('mv2', 'f');
    

13. Query the data of the mv2 materialized view.

    SELECT * FROM mv2;
    

    The returned result is as follows:

    +------+---------+---------+
    | CNT  | AGE_CNT | AGE_SUM |
    +------+---------+---------+
    |    2 |       2 |      40 |
    +------+---------+---------+
    1 row in set
    

14. Insert a row of data into the tbl1 and tbl2 tables.

    INSERT INTO tbl1 VALUES (3, 'mary');
    

    INSERT INTO tbl2 VALUES (3, 25);
    

15. Fully refresh the mv1 materialized view.

    CALL dbms_mview.refresh('mv1', 'c');
    

16. Query the data of the mv1 materialized view.

    SELECT * FROM mv1;
    

    The returned result is as follows:

    +------+------+------+------+
    | ID1  | ID2  | NAME | AGE  |
    +------+------+------+------+
    |    1 |    1 | jack |   21 |
    |    2 |    2 | rose |   19 |
    |    3 |    3 | mary |   25 |
    +------+------+------+------+
    3 rows in set
    

17. Incrementally refresh the mv2 materialized view.

    CALL dbms_mview.refresh('mv2', 'f');
    

    The returned result is as follows:

    OBE-12052: cannot fast refresh materialized view
    at package body oceanbase.DBMS_MVIEW.DO_REFRESH , line : 54, col : 1
    at oceanbase.DBMS_MVIEW.REFRESH , line : 72, col : 1
    

    Notice

    Since mv1 has been fully refreshed, directly incrementally refreshing mv2 will result in an error. A full refresh is required before incremental refresh.

18. Query the data of the mv2 materialized view.

    SELECT * FROM mv2;
    

    The returned result is as follows:

    +------+---------+---------+
    | CNT  | AGE_CNT | AGE_SUM |
    +------+---------+---------+
    |    2 |       2 |      40 |
    +------+---------+---------+
    1 row in set
    

19. Fully refresh the mv2 materialized view.

    CALL dbms_mview.refresh('mv2', 'c');
    

20. Query the data of the mv2 materialized view.

    SELECT * FROM mv2;
    

    The returned result is as follows:

    +------+---------+---------+
    | CNT  | AGE_CNT | AGE_SUM |
    +------+---------+---------+
    |    3 |       3 |      65 |
    +------+---------+---------+
    1 row in set
    

Refresh statistics of materialized views

OceanBase Database can collect and store statistics of materialized view refresh operations. These statistics can be queried through specific views. The statistics of current and historical materialized view refresh operations are stored in the database. By analyzing the historical statistics of materialized view refreshes, you can understand and analyze the performance of materialized view refreshes in the database.

The statistics of materialized view refreshes serve the following purposes:

• Reporting: Provides an overview of current and historical statistics of materialized view refresh operations, including the actual time required for each refresh, to help you track and monitor refresh performance.

• Diagnostics: Allows you to analyze the performance of materialized view refreshes based on detailed current and historical statistics. For example, if a materialized view refresh takes a long time, the statistics can help you identify whether the performance degradation is due to increased system load or a larger volume of data changes.

Collect statistics of materialized views

You can collect statistics of materialized views by using the analyze table or call dbms_stats.gather_table_stats('database_name', 'table_name') statement.

• For more information about how to collect statistics of tables and columns, see GATHER_TABLE_STATS.

• For more information about how to manage the collection and retention of statistics of materialized view refresh operations, see DBMS_MVIEW_STATS Overview.

Show materialized view refresh information

References

• Overview of materialized views
• Materialized view logs
• Create materialized views
• Query rewrite on materialized views
• Query materialized views
• Delete materialized views
• Materialized view resource isolation
• Materialized view exception diagnosis