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.
Related
I create a table from a junction between 3 tables.
The issue that I'm facing is that this table becomes outdated once a new modification affects once of the original table used to supply its data.
How should solve this problem? Is a trigger the only solution?
You have several options:
Live with the outdated data and periodically (say once a day or once an hour) update the table.
Use a trigger to update the associated values.
Use a view.
Use an indexed view if possible (see here).
The last option is a way to define a view that SQL Server updates automatically -- what is called a materialized view in other databases. That said, not all queries are supported in materialized views.
"There are two problems in computer science. Cache coherency, and naming things." Tables derived from other tables exemplify both.
SQL Server supports materialized views, if you actually need them. Before you decide you do, do what every good carpenter does: measure twice and cut once. That is, write an ordinary view, and see how fast it is. If it's too slow -- however defined -- find out why. Most of the time, slow views can be sped up using indexes. If you have 100s of millions of rows, you might convert the view to a function that takes an argument applied to the WHERE clause, making it a "parameterized" view.
You pay per size of data queried. So, would be a better alternative to use views from the cost point of view?
Views are not materialized in bigquery (currently), so the cost of querying from a view is identical to writing the more complex query on the underlying table.
You can, of course, create your own "materialized views" by running a query and saving it as a table. Then you can run subsequent queries against that table. This may be more cost effective if the saved table is smaller than the underlying table. That takes a bit more manual bookkeeping, however.
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.
Possible Duplicate:
Difference Between Views and Tables in Performance
What is the main difference between view and table in SQL. Is there any advantage of using views instead of tables.
A table contains data, a view is just a SELECT statement which has been saved in the database (more or less, depending on your database).
The advantage of a view is that it can join data from several tables thus creating a new view of it. Say you have a database with salaries and you need to do some complex statistical queries on it.
Instead of sending the complex query to the database all the time, you can save the query as a view and then SELECT * FROM view
Table:
Table is a preliminary storage for storing data and information in RDBMS.
A table is a collection of related data entries and it consists of columns and rows.
View:
A view is a virtual table whose contents are defined by a query.
Unless indexed, a view does not exist as a stored set of data values in a database.
Advantages over table are
We can combine columns/rows from multiple table or another view and have a consolidated view.
Views can be used as security mechanisms by letting users access data through the view, without granting the users permissions to directly access the underlying base tables of the view
It acts as abstract layer to downstream systems, so any change in schema is not exposed and hence the downstream systems doesn't get affected.
I thought this was best explained by the article "SQL - What is a View" published by 1Keydata:
A view is a virtual table. A view consists of rows and columns just like a table. The difference between a view and a table is that views are definitions built on top of other tables (or views), and do not hold data themselves. If data is changing in the underlying table, the same change is reflected in the view. A view can be built on top of a single table or multiple tables. It can also be built on top of another view. In the SQL Create View page, we will see how a view can be built.
Views offer the following advantages:
Ease of use: A view hides the complexity of the database tables from end users. Essentially we can think of views as a layer of abstraction on top of the database tables.
Space savings: Views takes very little space to store, since they do not store actual data.
Additional data security: Views can include only certain columns in the table so that only the non-sensitive columns are included and exposed to the end user. In addition, some databases allow views to have different security settings, thus hiding sensitive data from prying eyes.
In view there is not any direct or physical relation with the database.
And Modification through a view (e.g. insert, update, delete) is
not permitted.Its just a logical set of tables
A view helps us in get rid of utilizing database space all the time. If you create a table it is stored in database and holds some space throughout its existence. Instead view is utilized when a query runs hence saving the db space. And we cannot create big tables all the time joining different tables though we could but its depends how big the table is to save the space. So view just temporarily create a table with joining different table at the run time. Experts,Please correct me if I am wrong.
SQL Views:
View is a virtual table based on the result-set of an SQL statement and that is Stored in the database with some name.
SQL Table:
SQL table is database instance consists of fields (columns), and rows.
Check following post, author listed around seven differences between views and table
https://codechef4u.com/post/2015/09/03/sql-views-vs-tables
Table:
Table stores the data in database and contains the data.
View:
View is an imaginary table, contains only the fields(columns) and does not contain data(row) which will be framed at run time
Views created from one or more than one table by joins, with selected columns.
Views are created to hide some columns from the user for security reasons, and to hide information exist in the column.
Views reduces the effort for writing queries to access specific columns every time
Instead of hitting the complex query to database every time, we can use 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.