Is it possible to select and view the existing data from materialized view while a complete refresh happens?
That depends on whether the refresh is atomic or not.
In an atomic refresh, Oracle behind the scenes will delete the existing data and re-insert the results. That takes longer but queries that start before the refresh finishes will see the old data. In a non-atomic refresh, Oracle behind the scenes will truncate the existing data and re-insert. That makes for a faster refresh but it means that the materialized view will be empty during the refresh.
If you are using dbms_mview.refresh, atomic is the default refresh method.
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I have an Oracle materialized view refreshed on demand. Since the view is pretty large, it takes a while to update. I am wondering: when do the updates really become visible to the queries that try read the view? Does RDBMS update the view in a buffer and then makes it visible when update is complete? Or will the changes gradually become visible to the reader? Is this possible to control this?
Materialized views can be refreshed on commit or on demand.
Oracle's documentation is quite good
https://docs.oracle.com/cd/B19306_01/server.102/b14200/statements_6002.htm#i2116410
To have materialized views refresh incrementally you must use the FAST option and you must have materialized view logs to capture changes on the source tables.
If you schedule the demanded refreshes frequently there will be less work to be done for each refresh. If you schedule them less frequently there will be more work to be done. You need to investigate how changes on the source tables are distributed over time to choose well.
Changes will be visible to users when the refresh (whatever size) commits.
I have materialized view in my own schema and the materialized view source is the other schema table but , this master table has 900 rows and i did some dml operations on this master table after that i will refresh this materialized view as you know , and i did some resarch " how can i refresh my own materialized view " and it says " you can do complete refresh or fast refresh " but i didnt understand these solutions meaning so my question is simple ;
What is the difference between complete refresh and fast refresh in MV ?
P.S:If my master table has 1 million or more rows , which one i should choose? (fast or complete)
Thank you for your reply.
"Complete Refresh" means you truncate entire materialized view and insert new data.
"Fast Refresh" means you update (or insert/delete) only the rows which have been changed on master tables.
And just as information "Force Refresh" mean, Oracle tries to make a Fast Refresh and if this is not possible then do "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.
Here is a full list of restrictions General Restrictions on Fast Refresh, there are quite many.
As always it depends, so if possible try both and measure for your application. As a general rule the fast refresh is likely to be much faster provided that only a small part of the data has changed. If all the data has changed a complete refresh is better.
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.
We have to create a materialised view in our database from a remote database which is currently in production. The view has about 5 crore records and is taking a long time. In between , the connection has snapped once and not even a single record persisted in our database. Since the remote database is production server, we get a very limited window to create the view.
My question is, can we have something like auto commit /auto start from where left at last time while the view is created so that we don't have to do the entire operation in one go ?
We are tying to make the query such that records can be fetched in smaller numbers as a alternate. But the data is read only for us and doesn't really have a where clause at this point which we can use.
Due to sensitive nature of data, I cannot post the view structure or the query.
No, you can not commit during the process of creating the view. But why don't you import the data to a table instead of a view? This would provide the possibility to commit in between. Furthermore this might provide the possibility, to load only the delta of changes maybe on daily basis - this would reduce the required time dramatically.
I have a materialized view that brings back 2 columns and roughly 300m rows.
The materialized view is based on joins between multiple views.
Currently it takes roughly 10 minutes for this to refresh and I am trying to improve the refresh time since there are usually very few changing records in this data set.
I currently run a "Complete" refresh - is there a way to run a fast refresh since this materialized view is based on other views and not tables?
Thank you
See https://docs.oracle.com/database/121/DWHSG/basicmv.htm#i1007007 for fast refresh requirements and limitations.
Use DBMS_MVIEW.EXPLAIN_MVIEW for analyzing your materialized view query to see why the fast refresh is not possible to use.
I am creating an oracle view basically as a read-only summary of data from other tables. It is not necessary to update the view every time if there is any updates to associated tables otherwise the workload will be huge and a waste. It is better to refresh the view every 5 minutes and then the front end can receive the latest data that will be all.
However, how to setup the oracle view refreshing interval? Do we have to settle this constraints in PL/SQL?