Friday, July 10, 2015

Virtual Columns

Oracle 11g allows you to create virtual columns in a table. Values are not stored for these columns, Oracle calculates them at runtime. You can see the expression used to generate the value in the DATA_DEFAULT column of the USER_TAB_COLUMNS view.

As you might expect, you cannot UPDATE virtual columns directly. If you try, you get an ORA-54017.
 
It isn’t quite so obvious but you cannot use a virtual column to generate the value of another virtual column either. If you try, you get an ORA-54012
 
SQL> create table tab1
  2  (col1 number,
  3   col2 number,
  4   col3 number generated always
  5   as (col1 + col2) virtual)
  6  /
 
Table created.
 
SQL> insert into tab1(col1,col2) values(1,2)
  2  /
 
1 row created.
 
SQL> select * from tab1
  2  /
 
      COL1       COL2       COL3
---------- ---------- ----------
         1          2          3
 
SQL> column column_name format a15
SQL> column data_default format a15
SQL> select column_name, data_default
  2  from user_tab_columns
  3  where table_name = 'TAB1'
  4  /
 
COLUMN_NAME     DATA_DEFAULT
--------------- ---------------
COL1
COL2
COL3            "COL1"+"COL2"
 
SQL> update tab1 set col3 = 4
  2  /
update tab1 set col3 = 4
       *
ERROR at line 1:
ORA-54017: UPDATE operation disallowed on virtual
columns
 
SQL> drop table tab1
  2  /
 
Table dropped.
 
SQL> create table tab1
  2  (col1 number,
  3   col2 number,
  4   col3 number generated always
  5   as (col1 + col2) virtual,
  6   col4 number generated always
  7   as (col1 + col3) virtual)
  8  /
col3 number generated always
*
ERROR at line 4:
ORA-54012: virtual column is referenced in a column
expression
 
SQL>

1 comment:

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