What is the difference between Views and Materialized Views in Oracle?

Question

What is the difference between Views and Materialized Views in Oracle?

Kailakaile

Answer 1

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.

Secondguess

Also when you need performance on data that don't need to be up to date to the very second, materialized views are better, but your data will be older than in a standard view. Usually BI reports gain a lot of benefit from materialized views. – Gust

@Gust - that is correct except in the case of a materialized view which is REFRESH ON COMMIT - the MV will return exactly the data that has been committed. – Pascha

What is the meaning of DISK based? Is it mean table is not part of DISK? Is it stored in a file and DISK access is faster that File access ....? – Ariew

@KanagaveluSugumar "DISK based" means the data is physically stored on the computer's hard disk. – Secondguess

@Secondguess Thanks! but that i got it, my question is; Only MView is part of DISK and actual DB tables is not part of DISK..? – Ariew

@KanagaveluSugumar Yes, the actual tables are written to disk also. – Secondguess

@Secondguess Thanks! i think you mean to say other than DB tables in the DISK; this MVIEW also maintain a table by resolving all the joins. So that in the runtime single table access is enough; and no need to query multiple tables for join conditions which is usually done by the normal view. Thank you! – Ariew

what will happen if materialize view's data no longer met where condition ? will the data be deleted upon refresh time ? – Talent

Answer 2

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.

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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.

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Materialized views also help to guarantee a consistent moment in time, similar to snapshot isolation.

Chokefull

+1 for the detailed explanation, but what is the downside of query rewrite? If query rewrite will allow Oracle to optimize the query even further then we should ALWAYS enable query rewrite, no? – Trix

@Rosdi, he said it: "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" – Pascha

@Mike thx u , is there a book or course that explains how to automate this materalized view refresher? – Galton

@ERJAN, I am not sure if there is a book or course specifically for this. But I would just use a daemon or something similar that can run in the background. I also suspect that specific database engines have specific ways to run scheduled jobs. – Chokefull

@Galton Here are some options – Chemical

Answer 3

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.

Yawn

There is a "REFRESH ON COMMIT" option that can be specified on a Materialized View – Kurrajong

Thanks! I'll update the answer. Do you know when this functionality was added to Oracle? – Yawn

Answer 4

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

Frannie

Answer 5

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.

Forwards

What is the meaning of DISK based? Is it mean table is not part of DISK? Is it stored in a file and DISK access is faster that File access ....? – Ariew

Answer 6

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.

Ann

Answer 7

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.

Lohner

The tables that support a materialized view don't take the same name as the view. – Pascha

@JeffreyKemp, are you sure? The accepted answer here stackoverflow.com/a/33552513 claims otherwise. Although yeah, this may have been different in 2010 when you commented... – Snodgrass

My comment above must have been on an older version (probably 9i). You're right, MVs get a table with the same name at least in modern versions. – Pascha

Answer 8

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.

Kei