What is materialized view
A materialized view in Oracle is a database object that contains the results of a query. They are local copies of data located remotely, or are used to create summary tables based on aggregations of a table’s data. Materialized views, which store data based on remote tables are also, know as snapshots.
We have already explained how to create materialized view and materialized view log
Oracle materialized view and materialized view log
Suppose it is already created in the database and you want to query the defination.
The below sql will help in that
set long 2000;
select query from user_mviews where mview_name='MV_TEST'; or select query from user_mviews where mview_name='MV_TEST';
How to monitor the progress of refresh of Materialized views:
Many times it happens that materialized view is not refreshing from the master table(s) or the refresh is just not able to keep up with the changes occurring on the master table(s).
In these cases, we should look at below things
(1)The job that is scheduled to run the materialized view.
(2) The materialized view log in case of fast refresh
(3) The Source table
(4) The target materialized view
First we will need to check at the job which is scheduled to run the materialized view
- It could be done using refresh group
- It could be manually refresh using some cronjob or some other scheduling
For refresh group
The below queries gives the information about group
select * from dba_refresh;select * from dba_refresh_children;
select * from sys.v_$mvrefresh;
Then below query to find the status of job.
SELECT /*+ RULE */
DECODE(SUBSTR(WHAT,INSTR(WHAT,’.’,1,2)+2,INSTR(WHAT,’”‘,1,4)-4-INSTR(WHAT,’.’,1,2)+2),NULL,SUBSTR(WHAT,1,40), SUBSTR(WHAT,INSTR(WHAT,’.’,1,2)+2,INSTR(WHAT,’”‘,1,4)-4-INSTR(WHAT,’.’,1,2)+2)) MVIEW_NAME,
DECODE(BROKEN,’Y’,’YES’,’N’,’NO’,’ ‘) IS_BROKEN,
FROM DBA_JOBS A
LEFT OUTER JOIN (SELECT /*+ RULE */
FROM DBA_JOBS_RUNNING ) B
ON A.JOB = B.JOB
ORDER BY SCHEMA_USER, MVIEW_NAME;
We can find out if the job is broken. How much time last refresh took.All those detail can be find out
We also have to check if job-queue_processes parameter is adequately setup.
show parameter job_queue_processes
alter system set job_queue_processes=20 scope=both;
The next thing to check the MVlog table in the source database. We need to check how many changes happening/every hour
(select count(*) from <owner>.MLOG$_<MASTER TABLE NAME> WHERE DMLTYPE$$ = ‘U’) UPDATES,
(select count(*) from <owner>.MLOG$_<MASTER TABLE NAME> WHERE DMLTYPE$$ = ‘I’) INSERTS,
(select count(*) from <owner>.MLOG$_<MASTER TABLE NAME> WHERE DMLTYPE$$ = ‘D’) DELETES,
(select count(*) from <owner>.MLOG$_<MASTER TABLE NAME>) TOTAL
If the changes are high, the refresh will take time. It may be required to increase the frequency of the refresh so as to have less changes in a refresh
The other thing to check the master table. If many changes happening and many queries running on master table simultaneously with refresh time,then again it will slow down the materialized view refresh
The performance of source and target database and network utlization should also be checked
If the materialized view is being refreshed currently, you can check the progress using
SELECT * FROM V$SESSION_LONGOPS;
If the materialized view refresh is taking time, we can enable trace and find out the explain plan for the execution using below useful articles