Databases Reference
In-Depth Information
This evolution previously presented a problem when table structures were changed as
applications changed. Typically, a new release of an application would be run in test
mode, and this testing is most valid if it can run on production data. Prior to EBR, this
requirement would force testers to run on a duplicate copy of a potentially large data‐
base, and perform their testing outside of a real world production scenario.
With EBR, you can create a new version of a table with a different data structure that
resides in the Oracle Database at the same time as a previous version. An application
can access a specific edition of a table, so a new version of an application can run si‐
multaneously with a previous one that uses a table with a different structure.
Not only can organizations run two versions of an application at the same time, but
developers can roll back the changes in a new version by simply dropping the edition
for that new version. In the same way, a new version can be made permanent by dropping
a previous version and pointing applications to the newer version, which means an
upgrade can take place without any downtime.
You need to do some initialization work to use editions, as well as creating edition-
related triggers that will duplicate data manipulation actions in multiple versions.
Views
A
view
is an Oracle data structure defined through a SQL statement. The SQL statement
is stored in the database. When you use a view in a query, the stored query is executed
and the base table data is returned to the user. Views do not contain data, but represent
ways to look at the base table data in the way the query specifies.
You can use a view for several purposes:
• To simplify access to data stored in multiple tables.
• To implement specific security for the data in a table (e.g., by creating a view that
includes a WHERE clause that limits the data you can access through the view).
Starting with Oracle9
i
, you can use
fine-grained access control
to accomplish the
same purpose. Fine-grained access control gives you the ability to automatically
limit data access based on the value of data in a row.
• To isolate an application from the specific structure of the underlying tables.
A view is built on a collection of
base tables
, which can be either actual tables in an
Oracle Database or other views. If you modify any of the base tables for a view so that
they no longer can be used for a view, that view itself can no longer be used.
In general, you can write to the columns of only one underlying base table of a view in
a single SQL statement. There are additional restrictions for INSERT, UPDATE, and
DELETE operations, and there are certain SQL clauses that prevent you from updating
any of the data in a view.
