A materialized view (snapshot) is a table segment whose contents are periodically refereshed based on query (against a local or remote table)(针对的本地或者远程表)
The simplest way to achieve replication of data between sites for against remote tables:
materiralized views
Basic Syntax
create materialized view
-- Normal
CREATE MATERIALIZED VIEW view-name
BUILD [IMMEDIATE | DEFERRED]
REFRESH [FAST | COMPLETE | FORCE ]
ON [COMMIT | DEMAND ]
[[ENABLE | DISABLE] QUERY REWRITE]
AS
SELECT ...;
-- Pre-Built
CREATE MATERIALIZED VIEW view-name
ON PREBUILT TABLE
REFRESH [FAST | COMPLETE | FORCE ]
ON [COMMIT | DEMAND ]
[[ENABLE | DISABLE] QUERY REWRITE]
AS
SELECT ...;The build clause options are shown below :
- IMMEDIATE: mv is populated immediately.
- DEFFERED: mv is populated on the first requested refresh.
The following refresh type are avaliable:
- FAST : mv logs are not present for source tables in
- COMPLETE
- FORCE
A fresh can be triggered :
- ON COMMIT: data change in one of the dependent tables be committed
- ON DEMAND: manual request or a schedule task
The QUERY REWRITE clause tells the optimizer if the materialized view should be consider for query rewrite operations.
The ON PREBUILT TABLE clause tells the database to use an existing table segment, which must have the same name as the materialized view and support the same column structure as the query.
Check Privileges
[oracle@oracle-db-19c ~]$ sqlplus / as sysdba
SQL*Plus: Release 19.0.0.0.0 - Production on Sun Nov 27 15:18:35 2022
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
SQL> show user;
USER is "SYS"
SQL>
SQL> ALTER SESSION SET CONTAINER=PDB1;
Session altered.
SQL> grant create materialized view to scott;
Grant succeeded.
SQL> grant create database link to scott;
Grant succeeded.
SQL>
SQL> conn scott/tiger@PDB1
Connected.
SQL> show con_name;
CON_NAME
------------------------------
PDB1
SQL> set pagesize 200
SQL> set linesize 200
SQL>
SQL> select * from session_privs;
PRIVILEGE
----------------------------------------
CREATE SESSION
UNLIMITED TABLESPACE
CREATE TABLE
CREATE CLUSTER
CREATE SEQUENCE
CREATE DATABASE LINK
CREATE PROCEDURE
CREATE TRIGGER
CREATE MATERIALIZED VIEW
CREATE TYPE
CREATE OPERATOR
CREATE INDEXTYPE
SET CONTAINER
13 rows selected.
SQL>
Create DBLINK
SQL> show user
USER is "SYS"
SQL>
SQL> grant CREATE PUBLIC DATABASE LINK,DROP PUBLIC DATABASE LINK to scott;
Grant succeeded.
SQL> column owner for a40
SQL> column object_name for a40
SQL> select owner,object_name from dba_objects where object_type='DATABASE LINK';
OWNER OBJECT_NAME
---------------------------------------- ----------------------------------------
SYS SYS_HUB
SQL> COLUMN owner FORMAT A30
SQL> COLUMN db_link FORMAT A30
SQL> COLUMN username FORMAT A30
SQL> COLUMN host FORMAT A30
SQL> select owner,db_link,username,host from dba_db_links order by owner,db_link;
OWNER DB_LINK USERNAME HOST
------------------------------ ------------------------------ ------------------------------ ------------------------------
SYS SYS_HUB SEEDDATA
SQL> select owner,object_name from dba_objects where object_type='DATABASE LINK';
OWNER OBJECT_NAME
------------------------------ ----------------------------------------
SYS SYS_HUB
SQL>
SQL> show user;
USER is "SCOTT"
SQL>
SQL> create public database link LINK_ORCLPDB1
2 connect to SCOTT identified by "TIGER"
3 using
4 '(DESCRIPTION =(ADDRESS = (PROTOCOL = TCP)(HOST = xxx.xxx.xxx.xxx)(PORT = 1521))(CONNECT_DATA =(SERVER = DEDICATED)(SERVICE_NAME = ORCLPDB1)))';
Database link created.
SQL>
SQL> show user
USER is "SYS"
SQL> select owner,object_name from dba_objects where object_type='DATABASE LINK';
OWNER OBJECT_NAME
------------------------------ ----------------------------------------
SYS SYS_HUB
PUBLIC LINK_ORCLPDB1
SQL>
SQL> show user
USER is "SCOTT"
SQL>
SQL> select * from SCOTT.emp@LINK_ORCLPDB1;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
14 rows selected.
SQL> Create Materialized View
SQL> show user;
USER is "SCOTT"
SQL>
SQL> CREATE MATERIALIZED VIEW emp_mv
2 BUILD IMMEDIATE
3 REFRESH FORCE
4 ON DEMAND
5 AS
6 SELECT * FROM emp@link_orclpdb1;
Materialized view created.
SQL> Remember to gather stats after building the materialized view.
SQL> BEGIN
2 DBMS_STATS.gather_table_stats(
3 ownname => 'SCOTT',
4 tabname => 'EMP_MV');
5 END;
6 /
PL/SQL procedure successfully completed.
SQL> Create Materialized View Logs
Since a complete refresh involves truncating the materialized view segment and re-populating it using the related query, it can be quite time consuming and involve a considerable amount of network traffic when performed against a remote table. To reduce the replication costs, materialized view logs can be created to capture all changes to the base table since the last refresh. This information allows a fast refresh, which only needs to apply the changes rather than a complete refresh of the materialized view.
To take advantage of the of the fast refresh, connect to the master instance and create the materialized view log.
Code:
CREATE MATERIALIZED VIEW LOG ON SCOTT.EMP
TABLESPACE users
WITH PRIMARY KEY
INCLUDING NEW VALUES;Options:
[oracle@MaxwellDBA ~]$ sqlplus SCOTT/TIGER@ORCLPDB1
SQL*Plus: Release 19.0.0.0.0 - Production on Sun Nov 27 16:14:53 2022
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Last Successful login time: Fri Nov 18 2022 19:37:15 +08:00
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
scott@orclpdb1:orclcdb> show user
USER is "SCOTT"
scott@orclpdb1:orclcdb> CREATE MATERIALIZED VIEW LOG ON SCOTT.EMP
2 TABLESPACE users
WITH PRIMARY KEY
4 INCLUDING NEW VALUES;
Materialized view log created.
scott@orclpdb1:orclcdb> Refresh Materialized Views
If a materialized view is configured to refresh on commit, you should never need to manually refresh it, unless a rebuild is necessary. Remember, refreshing on commit is a very intensive operation for volatile base tables. It makes sense to use fast refreshes where possible.
For on demand refreshes, you can choose to manually refresh the materialized view or refresh it as part of a refresh group.
The following code creates a refresh group defined to refresh every minute and assigns a materialized view to it.
Code :
BEGIN
DBMS_REFRESH.make(
name => 'SCOTT.MINUTE_REFRESH',
list => '',
next_date => SYSDATE,
interval => '/*1:Mins*/ SYSDATE + 1/(60*24)',
implicit_destroy => FALSE,
lax => FALSE,
job => 0,
rollback_seg => NULL,
push_deferred_rpc => TRUE,
refresh_after_errors => TRUE,
purge_option => NULL,
parallelism => NULL,
heap_size => NULL);
END;
/
BEGIN
DBMS_REFRESH.add(
name => 'SCOTT.MINUTE_REFRESH',
list => 'SCOTT.EMP_MV',
lax => TRUE);
END;
/Options:
[oracle@oracle-db-19c ~]$ sqlplus scott/tiger@PDB1
SQL*Plus: Release 19.0.0.0.0 - Production on Sun Nov 27 16:56:15 2022
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Last Successful login time: Sun Nov 27 2022 16:50:40 +08:00
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
SQL>
SQL> BEGIN
2 DBMS_REFRESH.make(
3 name => 'SCOTT.MINUTE_REFRESH',
4 list => '',
5 next_date => SYSDATE,
6 interval => '/*1:Mins*/ SYSDATE + 1/(60*24)',
7 implicit_destroy => FALSE,
8 lax => FALSE,
9 job => 0,
10 rollback_seg => NULL,
11 push_deferred_rpc => TRUE,
12 refresh_after_errors => TRUE,
13 purge_option => NULL,
14 parallelism => NULL,
15 heap_size => NULL);
16 END;
17 /
PL/SQL procedure successfully completed.
SQL> BEGIN
2 DBMS_REFRESH.add(
3 name => 'SCOTT.MINUTE_REFRESH',
4 list => 'SCOTT.EMP_MV',
5 lax => TRUE);
6 END;
7 /
PL/SQL procedure successfully completed.
SQL> Information about refresh groups and the materialize views in a refresh group can be queried from the DBA_RGROUP and DBA_RCHILD views respectively.
SQL> column owner for a20
SQL> column name for a20
SQL> column ROLLBACK_SEG for a20
SQL> column JOB_NAME for a20
SQL> select * from dba_rgroup;
REFGROUP OWNER NAME I P R ROLLBACK_SEG JOB PURGE_OPTION PARALLELISM HEAP_SIZE JOB_NAME
---------- -------------------- -------------------- - - - -------------------- ---------- ------------ ----------- ---------- --------------------
5 SCOTT MINUTE_REFRESH N Y Y 0 MV_RF$J_0_S_130
SQL> desc dba_rchild;
Name Null? Type
----------------------------------------------------------------------------------------------------------------- -------- ----------------------------------------------------------------------------
REFGROUP NUMBER
OWNER NOT NULL VARCHAR2(128)
NAME NOT NULL VARCHAR2(128)
TYPE# VARCHAR2(128)
SQL> column owner for a20
SQL> column name for a20
SQL> column REFGROUP for a20
SQL> column TYPE for a20
SQL> select * from dba_rchild;
REFGROUP OWNER NAME TYPE#
---------- -------------------- -------------------- --------------------------------------------------------------------------------------------------------------------------------
########## SCOTT EMP_MV SNAPSHOT
SQL> column REFGROUP default
SQL> select * from dba_rchild;
REFGROUP OWNER NAME TYPE#
---------- -------------------- -------------------- --------------------------------------------------------------------------------------------------------------------------------
5 SCOTT EMP_MV SNAPSHOT
SQL> befor 1 minutes & after 1 minutes;
SQL> select * from emp_mv;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
14 rows selected.
SQL> select * from emp_mv;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 400 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1200 300 30
7521 WARD SALESMAN 7698 22-FEB-81 850 500 30
7566 JONES MANAGER 7839 02-APR-81 2575 20
7654 MARTIN SALESMAN 7698 28-SEP-81 850 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2450 30
7782 CLARK MANAGER 7839 09-JUN-81 2050 10
7788 SCOTT ANALYST 7566 19-APR-87 2600 20
7839 KING PRESIDENT 17-NOV-81 4600 10
7844 TURNER SALESMAN 7698 08-SEP-81 1100 0 30
7876 ADAMS CLERK 7788 23-MAY-87 700 20
7900 JAMES CLERK 7698 03-DEC-81 550 30
7902 FORD ANALYST 7566 03-DEC-81 2600 20
7934 MILLER CLERK 7782 23-JAN-82 900 10
14 rows selected.
SQL>
SQL> A materialized view can be manually refreshed using the DBMS_MVIEW package.
Code:
EXEC DBMS_MVIEW.refresh('EMP_MV');Options:
SQL> select * from emp_mv;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7934 MILLER CLERK 7782 23-JAN-82 1300 10
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7839 KING PRESIDENT 17-NOV-81 5000 10
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
14 rows selected.
SQL>
SQL> EXEC DBMS_MVIEW.refresh('EMP_MV');
PL/SQL procedure successfully completed.
SQL> select * from emp_mv;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7782 CLARK MANAGER 7839 09-JUN-81 2350 10
7499 ALLEN SALESMAN 7698 20-FEB-81 1500 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1150 500 30
7788 SCOTT ANALYST 7566 19-APR-87 2900 20
7876 ADAMS CLERK 7788 23-MAY-87 1000 20
7934 MILLER CLERK 7782 23-JAN-82 1200 10
7654 MARTIN SALESMAN 7698 28-SEP-81 1150 1400 30
7902 FORD ANALYST 7566 03-DEC-81 2900 20
7839 KING PRESIDENT 17-NOV-81 4900 10
7369 SMITH CLERK 7902 17-DEC-80 700 20
7566 JONES MANAGER 7839 02-APR-81 2875 20
7844 TURNER SALESMAN 7698 08-SEP-81 1400 0 30
7698 BLAKE MANAGER 7839 01-MAY-81 2750 30
7900 JAMES CLERK 7698 03-DEC-81 850 30
14 rows selected.
SQL> show user;
USER is "SCOTT"
SQL> Cleaning Up
code:
sqlplus scott/tiger@PDB1
DROP MATERIALIZED VIEW emp_mv;
DROP PUBLIC DATABASE LINK LINK_ORCLPDB1;
BEGIN
DBMS_REFRESH.destroy(name=>'SCOTT.MINUTE_REFRESH');
END;
/
sqlplus SCOTT/TIGER@ORCLPDB1
DROP MATERIALIZED VIEW LOG ON SCOTT.EMP;options:
[oracle@oracle-db-19c ~]$ sqlplus scott/tiger@PDB1
SQL*Plus: Release 19.0.0.0.0 - Production on Sun Nov 27 17:27:17 2022
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Last Successful login time: Sun Nov 27 2022 17:07:11 +08:00
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
SQL> DROP MATERIALIZED VIEW emp_mv;
Materialized view dropped.
SQL>
SQL> DROP PUBLIC DATABASE LINK LINK_ORCLPDB1;
Database link dropped.
SQL> BEGIN
2 DBMS_REFRESH.destroy(name=>'SCOTT.MINUTE_REFRESH');
3 END;
4 /
PL/SQL procedure successfully completed.
SQL>
[oracle@MaxwellDBA ~]$
[oracle@MaxwellDBA ~]$ sqlplus SCOTT/TIGER@ORCLPDB1;
SQL*Plus: Release 19.0.0.0.0 - Production on Sun Nov 27 17:30:30 2022
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle. All rights reserved.
Last Successful login time: Sun Nov 27 2022 17:27:27 +08:00
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0
scott@orclpdb1:orclcdb>
scott@orclpdb1:orclcdb> DROP MATERIALIZED VIEW LOG ON SCOTT.EMP;
Materialized view log dropped.
scott@orclpdb1:orclcdb>
Aggregations and Transformations
Materialized views can be used to improve the performance of a variety of queries, including those performing aggregations and transformations of the data. This allows the work to be done once and used repeatedly by multiple sessions, reducing the total load on the server.
The following query does an aggregation of the data in the EMP table.
Code:
show user;
set autotrace trace explain
select deptno,sum(sal)
from emp
group by deptno;
Options:
SQL> show user;
USER is "SCOTT"
SQL>
SQL> set autotrace trace explain
SQL>
SQL> select deptno,sum(sal)
2 from emp
3 group by deptno;
Execution Plan
----------------------------------------------------------
Plan hash value: 4067220884
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 21 | 4 (25)| 00:00:01 |
| 1 | HASH GROUP BY | | 3 | 21 | 4 (25)| 00:00:01 |
| 2 | TABLE ACCESS FULL| EMP | 14 | 98 | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------
SQL> Create a materialized view to perform the aggregation in advance, making sure you specify the ENABLE QUERY REWRITE clause.
SQL> show user;
USER is "SCOTT"
SQL> CREATE MATERIALIZED VIEW emp_aggr_mv
2 BUILD IMMEDIATE
3 REFRESH FORCE
4 ON DEMAND
5 ENABLE QUERY REWRITE
6 AS
7 SELECT deptno, SUM(sal) AS sal_by_dept
8 FROM emp
9 GROUP BY deptno;
Materialized view created.
SQL> EXEC DBMS_STATS.gather_table_stats(USER, 'EMP_AGGR_MV');
PL/SQL procedure successfully completed.
SQL>
The same query is now rewritten to take advantage of the pre-aggregated data in the materialized view, instead of the session doing the work for itself.
Code:
--ALTER SESSION SET QUERY_REWRITE_INTEGRITY = TRUSTED;
--ALTER SESSION SET QUERY_REWRITE_ENABLED = TRUE;
SET AUTOTRACE TRACE EXPLAIN
SELECT deptno, SUM(sal)
FROM emp
GROUP BY deptno;Options:
SQL> set pagesize 500
SQL> set linesize 500
SQL> SET AUTOTRACE TRACE EXPLAIN
SQL> SELECT deptno, SUM(sal)
2 FROM emp
3 GROUP BY deptno;
Execution Plan
----------------------------------------------------------
Plan hash value: 2456459487
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 21 | 3 (0)| 00:00:01 |
| 1 | MAT_VIEW REWRITE ACCESS FULL| EMP_AGGR_MV | 3 | 21 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
SQL> 
