Also, it's important to note that 11g virtual
columns only work within the specified table, and you cannot
reference columns within other tables.
Virtual columns are expressions that are based on one or more
existing columns in the table. When using Virtual Column-Based
Partitioning, a virtual column in a table provides advantages in disk
space utilization. A virtual column is only stored as metadata. It
does not consume physical space, but it can be indexed. The virtual
column also contains optimizer statistics and histograms.
Using a virtual column also simplifies the use of derived columns.
Transparently derived values do not require the application to
calculate and insert an additional value. This also prevents the need
to use a trigger on the table to provide an alternate implementation
of this functionality. Using virtual columns in tables also eliminates
the need to use views to display derived column values.
A new table with a virtual column can be derived using the
create table <table_name>(
[generated always] as (<column_expression>) [virtual]
Adding a virtual column to an existing table is accomplished with
the following alter table syntax:
alter table <table_name>
add (<column_name> [<data_type>] [generated always]
as (<column_expression>) [virtual]);
When defining a virtual column in a table, it is possible to
either include the datatype or let the database determine the datatype
based on the expression.
Optionally, the phrases ?generated always? and ?virtual? can
be used to help clarify the syntax. The column expression must
reference columns defined on the same table; however, the column
expression can refer to a PL/SQL function if the function is
designated DETERMINISTIC during its creation.
example, a virtual column might be helpful in a table that
stores employee information:
SQL> create table employees(
3 start_date date,
5 hourly_rate generated always as (annual_salary/2080),
6 annual_salary number,
7 active as (case when end_date is null
then 'Y' else 'N' end));
7 ('C. TESTER', '01-JAN-2011',
1 row created.
SQL> select * from
EMPLOYEE_NAME START_DATE END_DATE
HOURLY_RATE ANNUAL_SALARY ACTIVE
--------- ----------- ------------- ----------
C. TESTER 01-JAN-11
48.0769231 100000 Y
statement required to create a new record in this
table only requires four values, but the query of this table displays
all six values. This includes the two virtual columns derived from
other columns. The hourly rate virtual column is an expression of the
annual salary divided by 2080 working hours per year. The second
virtual column displays if the employee is active by examining the
row?s end date.
Virtual columns can be used for partitioning, indexing,
constraints and foreign keys. However, virtual columns cannot be used
for index-organized, external, object, cluster or temporary tables.
There are several error related to virtual columns, including
the ORA-12996 error.
oerr utility shows this for the ORA-12996 error:
ORA-12996: cannot drop
system-generated virtual column
Cause: An attempt was made to drop a virtual
column generated by the system.