Is your Materialized View Refresh speed slow?
A materialized view that is verified to be fast refresh should update relatively fast. But what happens when there are few changes to the master table, no network issues, no aggregation in the snapshot query and the refresh still runs slow? One of the most useful replication and data warehousing features in Oracle is materialized views.
What is the difference between a fast refresh and a complete refresh?
A fast refresh requires having a materialized view log on the source tables that keeps track of all changes since the last refresh, so any new refresh only has changed (updated, new, deleted) data applied to the MV. A complete refresh does what it says: it completely refreshes all data in the MV. No materialized view logs are needed.
How to REFRESH MATERIALIZED VIEW in Oracle?
Refresh Materialized View in Oracle 1 Bulk load into the detail table. 2 Enable parallel DML with an ALTER SESSION ENABLE PARALLEL DML statement. 3 Refresh the materialized view. See More....
What happens in fast refresh of materialized view?
An incremental or fast refresh uses a log table to keep track of changes on the master table. A materialized view log (snapshot log) is a schema object that records changes to a master table's data so that a materialized view defined on that master table can be refreshed incrementally.
What is complete refresh in materialized view?
Complete Refresh It loads the contents of a materialized view from scratch. This means, if the SQL query of the materialized view has an execution time of two hours, the Complete Refresh takes at least two hours as well – or ofter even longer.
Are materialized views fast?
Materialized views are faster than tables because of their “cache” (i.e. the query results for the view); in addition, if data has changed, they can use their “cache” for data that hasn't changed and use the base table for any data that has changed.
What is refresh force on demand in materialized view?
To refresh ON DEMAND materialized views we explicitly call one of the following procedures. DBMS_MVIEW.REFRESH. DBMS_MVIEW.REFRESH_ALL_MVIEWS. DBMS_MVIEW.REFRESH_DEPENDENT.
Can we automate refresh of materialized views?
You can set autorefresh for materialized views using CREATE MATERIALIZED VIEW. You can also use the AUTO REFRESH clause to refresh materialized views automatically. For more information about creating materialized views, see CREATE MATERIALIZED VIEW.
Can we delete data from materialized view?
You cannot delete rows from a read-only materialized view. If you delete rows from a writable materialized view, then the database removes the rows from the underlying container table. However, the deletions are overwritten at the next refresh operation.
What is a limitation of a materialized view?
Materialized views use a restricted SQL syntax and a limited set of aggregation functions. For more information, see Supported materialized views. Materialized views cannot be nested on other materialized views. Materialized views cannot query external tables.
Can we perform DML on materialized view?
Users cannot perform data manipulation language (DML) statements on read-only materialized views, but they can perform DML on updatable and writeable materialized views.
Is materialized view cached?
A materialized view, sometimes called a "materialized cache", is an approach to precomputing the results of a query and storing them for fast read access. In contrast with a regular database query, which does all of its work at read-time, a materialized view does nearly all of its work at write-time.
How do you refresh a materialized view daily?
So if you want to refresh mview daily, you need to keep it refresh on demand and set the next refresh time as sysdate + 1 . You can set any interval although. Once you do this the materialized view is created and a job is set in Oracle that will refresh mview every 24 hrs (sysdate + 1) . Save this answer.
Why does materialized view taking long time to create?
When you create a materialized view, you actually create a copy of the data that Oracle takes care to keep synchronized (and it makes those views somewhat like indexes). If your view operates over a big amount of data or over data from other servers, it's natural that the creating this view can take time.
How do I know if materialized view refresh is running?
You can also use v$mvrefresh dynamic performance view to know which MV is being refresh. Save this answer.
How can you tell when a materialized view was last refreshed?
Try something like the following in an SQLExecutor or SQLCreator:SELECT.OWNER,MVIEW_NAME,to_char(last_refresh_date, 'yyyymmddhh24miss') LAST_REFRESH_DATE.FROM all_mviews.WHERE owner = 'MY_OWNER_NAME'AND mview_name = 'MY_MATERIALIZED_VIEW_NAME'
How do you refresh a materialized view daily?
So if you want to refresh mview daily, you need to keep it refresh on demand and set the next refresh time as sysdate + 1 . You can set any interval although. Once you do this the materialized view is created and a job is set in Oracle that will refresh mview every 24 hrs (sysdate + 1) . Save this answer.
How do I know if materialized view refresh is running?
You can also use v$mvrefresh dynamic performance view to know which MV is being refresh. Save this answer.
What is the purpose of refreshing?
The refresh button, also known as the refresh option, is a function of all Internet browsers. It is used to ask the browser to send you the most updated version of the page you're viewing. See the browser page for additional information about this term and related links.
How does a materialized view refresh work?
Materialized views can be refreshed in two ways: fast or complete. A fast refresh requires having a materialized view log on the source tables that keeps track of all changes since the last refresh, so any new refresh only has changed (updated, new, deleted) data applied to the MV.
What is a complete refresh?
A complete refresh does what it says: it completely refreshes all data in the MV. No materialized view logs are needed. And it takes a little longer.
Do you have to recreate indexes on MV?
Advantage in this approach is, you don’t have to recreate the indexes on the MV (just set unusable before and rebuild after the refresh).
Why are the records in that table not being deleted?
Multiple simple snapshots can use the same snapshot log, meaning that records already used to refresh one snapshot might still be needed to refresh another snapshot. Therefore, Oracle does not delete rows from the log until all snapshots have used them. If the snapshot log has grown very large, then the time to complete a fast refresh will increase as well since more records must be scanned by the consuming site before determining which records to use for the refresh. This explained why a fast refresh with almost no changes on the master table would still take 10-20 seconds to complete.
Why do we use materialized view in Oracle?
Some companies use fast refresh materialized views on remote databases in order to improve performance and security when using distributed computing for online transaction processing. The speed of a fast refresh will be determined by how much data has changed since the last refresh. If the master table's data is updated very often, then the log table will have more recorded changes to process in order to update the materialized view.
How does a materialized view update?
The data in a materialized view is updated by either a complete or incremental refresh. An incremental or fast refresh uses a log table to keep track of changes on the master table. A materialized view log (snapshot log) is a schema object that records changes to a master table's data so that a materialized view defined on that master table can be refreshed incrementally. The frequency of this refresh can be configured to run on-demand or at regular time intervals.
What is materialized view?
One of the most useful replication and data warehousing features in Oracle is materialized views. Materialized views, also known as snapshots, have been a feature of Oracle for several years. A simple way to conceptualize this is to think of a view of a master table that has actual data that can be refreshed. Since the introduction in 8i, Oracle has consistently enhanced the technology for each subsequent release.
How many changes per minute in master table?
Furthermore, the master table had approximately 10-30 changes per minute while the refresh was happening 1-2 times per minute. There was no doubt that a fast refresh was occurring, there were no aggregations in the query, there was a small number of changes to the master table, and network issues were not the problem. So what was causing this fast refresh to go so slow?
Is a materialized view refresh a fast refresh?
However, the materialized view refresh was confirmed to be a fast refresh by querying USER_MVIEWS.
Does Oracle delete rows from snapshots?
Therefore, Oracle does not delete rows from the log until all snapshots have used them. If the snapshot log has grown very large, then the time to complete a fast refresh will increase as well since more records must be scanned by the consuming site before determining which records to use for the refresh.
How does Fast F refresh work?
FAST F Refreshes by incrementally applying changes to the materialized view. For local materialized views, it chooses the refresh method which is estimated by optimizer to be most efficient. The refresh methods considered are log-based FAST and FAST_PCT.
How to refresh materialized view?
A materialized view can be refreshed automatically using the ON COMMIT method. Therefore, whenever a transaction commits which has updated the tables on which a materialized view is defined, those changes are automatically reflected in the materialized view. The advantage of using this approach is you never have to remember to refresh the materialized view. The only disadvantage is the time required to complete the commit will be slightly longer because of the extra processing involved. However, in a data warehouse, this should not be an issue because there is unlikely to be concurrent processes trying to update the same table.
What is the meaning of "back up"?
Making statements based on opinion; back them up with references or personal experience.
Do you need a ROWID to create a materialized view log?
I see that you created the materialized view logs with ROWID, which is not really required as both tables have a primary key so you could try without the ROWID.
What is a fast refresh in Oracle?
With the fast refresh Oracle keeps track of the changes for the base tables and applies the changes to the materialized view when it is refreshed. A complete refresh on the other hand rebuilds the materialized view from scratch. With millions of rows that will be expensive, but again it is impossible to pick the best option without knowing more about your application.
What is the meaning of "back up"?
Making statements based on opinion; back them up with references or personal experience.
Is Fast Refresh faster than Complete Refresh?
Usually Fast Refresh is much faster than Complete Refresh but it has restrictions. You have to define MATERIALIZED VIEW LOG on master tables.
Description
REFRESH MATERIALIZED VIEW completely replaces the contents of a materialized view. To execute this command you must be the owner of the materialized view. The old contents are discarded. If WITH DATA is specified (or defaults) the backing query is executed to provide the new data, and the materialized view is left in a scannable state.
Parameters
The name (optionally schema-qualified) of the materialized view to refresh.
Notes
If there is an ORDER BY clause in the materialized view's defining query, the original contents of the materialized view will be ordered that way; but REFRESH MATERIALIZED VIEW does not guarantee to preserve that ordering.
Examples
This command will replace the contents of the materialized view called order_summary using the query from the materialized view's definition, and leave it in a scannable state:
