I know that deleting a row from view will delete that row from base table also. But I think it will be more surprised to know that how does deleting a row from base table affect on view?
Deleting a row in a table will affect the results from regular views.
Views are not executed when they are created. They are executed when they are referenced. Each time you reference the view in the query, it is run again. So, any updates to the data in the underlying tables will affect the view.
This does not apply to materialized views, which are executed when they are created. To reflect changes in the underlying data, you need to refresh a materialized view.
Related
What is the difference between Views and Materialized Views in Oracle?
Materialized views are disk based and are updated periodically based upon the query definition.
Views are virtual only and run the query definition each time they are accessed.
Views
They evaluate the data in the tables underlying the view definition at the time the view is queried. It is a logical view of your tables, with no data stored anywhere else.
The upside of a view is that it will always return the latest data to you. The downside of a view is that its performance depends on how good a select statement the view is based on. If the select statement used by the view joins many tables, or uses joins based on non-indexed columns, the view could perform poorly.
Materialized views
They are similar to regular views, in that they are a logical view of your data (based on a select statement), however, the underlying query result set has been saved to a table. The upside of this is that when you query a materialized view, you are querying a table, which may also be indexed.
In addition, because all the joins have been resolved at materialized view refresh time, you pay the price of the join once (or as often as you refresh your materialized view), rather than each time you select from the materialized view. In addition, with query rewrite enabled, Oracle can optimize a query that selects from the source of your materialized view in such a way that it instead reads from your materialized view. In situations where you create materialized views as forms of aggregate tables, or as copies of frequently executed queries, this can greatly speed up the response time of your end user application. The downside though is that the data you get back from the materialized view is only as up to date as the last time the materialized view has been refreshed.
Materialized views can be set to refresh manually, on a set schedule, or based on the database detecting a change in data from one of the underlying tables. Materialized views can be incrementally updated by combining them with materialized view logs, which act as change data capture sources on the underlying tables.
Materialized views are most often used in data warehousing / business intelligence applications where querying large fact tables with thousands of millions of rows would result in query response times that resulted in an unusable application.
Materialized views also help to guarantee a consistent moment in time, similar to snapshot isolation.
A view uses a query to pull data from the underlying tables.
A materialized view is a table on disk that contains the result set of a query.
Materialized views are primarily used to increase application performance when it isn't feasible or desirable to use a standard view with indexes applied to it. Materialized views can be updated on a regular basis either through triggers or by using the ON COMMIT REFRESH option. This does require a few extra permissions, but it's nothing complex. ON COMMIT REFRESH has been in place since at least Oracle 10.
Materialised view - a table on a disk that contains the result set of a query
Non-materiased view - a query that pulls data from the underlying table
Views are essentially logical table-like structures populated on the fly by a given query. The results of a view query are not stored anywhere on disk and the view is recreated every time the query is executed. Materialized views are actual structures stored within the database and written to disk. They are updated based on the parameters defined when they are created.
View: View is just a named query. It doesn't store anything. When there is a query on view, it runs the query of the view definition. Actual data comes from table.
Materialised views: Stores data physically and get updated periodically. While querying MV, it gives data from MV.
Adding to Mike McAllister's pretty-thorough answer...
Materialized views can only be set to refresh automatically through the database detecting changes when the view query is considered simple by the compiler. If it's considered too complex, it won't be able to set up what are essentially internal triggers to track changes in the source tables to only update the changed rows in the mview table.
When you create a materialized view, you'll find that Oracle creates both the mview and as a table with the same name, which can make things confusing.
Materialized views are the logical view of data-driven by the select query but the result of the query will get stored in the table or disk, also the definition of the query will also store in the database.
The performance of Materialized view it is better than normal View because the data of materialized view will be stored in table and table may be indexed so faster for joining also joining is done at the time of materialized views refresh time so no need to every time fire join statement as in case of view.
Other difference includes in case of View we always get latest data but in case of Materialized view we need to refresh the view for getting latest data.
In case of Materialized view we need an extra trigger or some automatic method so that we can keep MV refreshed, this is not required for views in the database.
When I create a table from a query, it seems like the table is only populated with data available from query at the time of creation. Is this correct?
If new data gets added to the dependent tables, then the created table wont sync with the updated data?
Is there anyway to make it so it always updates from the query?
Yes, that is correct.
If you want the "table" to be automatically updated, then use a view. This stores the query and runs it each time you reference it.
You can also create a materialized view. This is like a view but it is usually faster.
Is it possible to delete rows from a View?
Usually, a view isn't something you can delete from; it's kind of a virtual table, which shows you the rows from one or more real tables in the database. If you want a row to disappear from a view, you need to either delete the data from the real tables behind the view, or alter the view-creating SQL so that that particular row won't be shown in the view. With some simpler views you can DELETE FROM (and update) a view; however, even so the data is actually deleted from the real table.
You also cannot generally add anything to a view; if you need completely new data, it has to be added in the real table(s) from which the view is created.
For view basics, see for example http://www.w3schools.com/sql/sql_view.asp
If your view is updatable - really depends on a database you are using and the way view was created. General rule (again, varies from one DB to another) there should be one table and no aggregates in the select statement, creating the view.
Here is details for MySQL: http://dev.mysql.com/doc/refman/5.7/en/view-updatability.html
And for SQL Server: https://msdn.microsoft.com/en-CA/library/ms187956.aspx
InterSystems Caché: http://docs.intersystems.com/cache20152/csp/docbook/DocBook.UI.Page.cls?KEY=GSQL_views#GSQL_views_update
I have a question about views. Consider we have a view that I insert a record in it's base table. Does my view update after this insert or I should do a SELECT to update?
I think my question obvious - is view just a SELECT or it's result save in database and if it's base table, so when it's updated it then becomes update?
Normal Views are not persisted. If an updateable view is inserted into then selecting from the view (or the affected underlying tables) will show your results.
Not entirely sure what problem you are trying to solve. Views (non-indexed) suffice for most applications.
Have a look at Indexed Views: Improving Performance with SQL Server 2008 Indexed Views:
In the case of a nonindexed view, the portions of the view necessary
to solve the query are materialized at run time. Any computations such
as joins or aggregations are done during query execution for each
query referencing the view. After a unique clustered index is
created on the view, the view's result set is materialized immediately
and persisted in physical storage in the database, saving the overhead
of performing this costly operation at execution time.
The typical use of an indexed view is when you have expensive aggregations to perform.
Think of a view as a select statement. Instead of having to write out the entire select statement, you just select the view and it runs that select statement for you. So yes, anything you do to the underlying tables will automatically be visible in the view.
What is the difference between Views and Materialized Views in Oracle?
Materialized views are disk based and are updated periodically based upon the query definition.
Views are virtual only and run the query definition each time they are accessed.
Views
They evaluate the data in the tables underlying the view definition at the time the view is queried. It is a logical view of your tables, with no data stored anywhere else.
The upside of a view is that it will always return the latest data to you. The downside of a view is that its performance depends on how good a select statement the view is based on. If the select statement used by the view joins many tables, or uses joins based on non-indexed columns, the view could perform poorly.
Materialized views
They are similar to regular views, in that they are a logical view of your data (based on a select statement), however, the underlying query result set has been saved to a table. The upside of this is that when you query a materialized view, you are querying a table, which may also be indexed.
In addition, because all the joins have been resolved at materialized view refresh time, you pay the price of the join once (or as often as you refresh your materialized view), rather than each time you select from the materialized view. In addition, with query rewrite enabled, Oracle can optimize a query that selects from the source of your materialized view in such a way that it instead reads from your materialized view. In situations where you create materialized views as forms of aggregate tables, or as copies of frequently executed queries, this can greatly speed up the response time of your end user application. The downside though is that the data you get back from the materialized view is only as up to date as the last time the materialized view has been refreshed.
Materialized views can be set to refresh manually, on a set schedule, or based on the database detecting a change in data from one of the underlying tables. Materialized views can be incrementally updated by combining them with materialized view logs, which act as change data capture sources on the underlying tables.
Materialized views are most often used in data warehousing / business intelligence applications where querying large fact tables with thousands of millions of rows would result in query response times that resulted in an unusable application.
Materialized views also help to guarantee a consistent moment in time, similar to snapshot isolation.
A view uses a query to pull data from the underlying tables.
A materialized view is a table on disk that contains the result set of a query.
Materialized views are primarily used to increase application performance when it isn't feasible or desirable to use a standard view with indexes applied to it. Materialized views can be updated on a regular basis either through triggers or by using the ON COMMIT REFRESH option. This does require a few extra permissions, but it's nothing complex. ON COMMIT REFRESH has been in place since at least Oracle 10.
Materialised view - a table on a disk that contains the result set of a query
Non-materiased view - a query that pulls data from the underlying table
Views are essentially logical table-like structures populated on the fly by a given query. The results of a view query are not stored anywhere on disk and the view is recreated every time the query is executed. Materialized views are actual structures stored within the database and written to disk. They are updated based on the parameters defined when they are created.
View: View is just a named query. It doesn't store anything. When there is a query on view, it runs the query of the view definition. Actual data comes from table.
Materialised views: Stores data physically and get updated periodically. While querying MV, it gives data from MV.
Adding to Mike McAllister's pretty-thorough answer...
Materialized views can only be set to refresh automatically through the database detecting changes when the view query is considered simple by the compiler. If it's considered too complex, it won't be able to set up what are essentially internal triggers to track changes in the source tables to only update the changed rows in the mview table.
When you create a materialized view, you'll find that Oracle creates both the mview and as a table with the same name, which can make things confusing.
Materialized views are the logical view of data-driven by the select query but the result of the query will get stored in the table or disk, also the definition of the query will also store in the database.
The performance of Materialized view it is better than normal View because the data of materialized view will be stored in table and table may be indexed so faster for joining also joining is done at the time of materialized views refresh time so no need to every time fire join statement as in case of view.
Other difference includes in case of View we always get latest data but in case of Materialized view we need to refresh the view for getting latest data.
In case of Materialized view we need an extra trigger or some automatic method so that we can keep MV refreshed, this is not required for views in the database.