11g Enhanced Read-only tables

Oracle 11g has introduced a much simpler method for enabling read-only tables that even protect the table from unintentional DML by the table’s owner.  Prior to 11g, a read-only table was achieved by creating a table under a restricted account and granting select privileges to the appropriate users.  However, using this method, it is possible for the owner of the table to unintentionally modify the table. 

Also see my related notes on locking and read only tablespaces.
 
Oracle Database 11g introduces new ALTER TABLE syntax. For example, a table can be set to read-only by issuing the following command:
 
ALTER TABLE READ ONLY;
 
A table can be returned to read and write using the following command:
 
ALTER TABLE READ WRITE;
 
In read-only mode, the following operations are permitted on the table:

•  Select
•  Management indexes, constraints, supplemental log
•  Dropping and deallocation of unused columns
•  Renaming and moving of the table
•  Altering the table for physical property changes, row movement, and shrinking the segment
•  Drop table

The following operations are disabled on a table in read-only mode:

•  DML on table or any table partitions
•  Truncation of table
•  Select for update
•  Adding, removing, renaming, dropping, or setting a column to unused
•  Dropping a partition or sub partition belonging to the table
•  Online redefinition
•  Flashback on the table

For example, consider a transactional table that stores a customer’s account, sales amount, and transaction date:
 
SQL> desc account_sales
 


 Name                                      Null?    Type
 ----------------------------------------- -------- ------
 
 ACCOUNT_ID                                         NUMBER
 SALES_AMOUNT                                       NUMBER
 TRX_DATE                                           DATE
 
The business might request that this table be used to keep a historical record of accounts that were active during each month.  If it is November 1st, 2007, the DBA might run the following command to meet this requirement and then make the table read-only since the table is only for a historical record:
 
SQL> create table
  2     account_sales_october2007
  3  as
  4  select
  5    *
  6  from
  7     account_sales
  8  where
  9     trx_date between ('01-OCT-07') and ('31-OCT-07');
 
Table created.
 
SQL> alter table account_sales_october2007 read only;
 
Table altered.
 
A table’s read-only status is available from the read_only column of the dictionary views for [user|all|dba]_tables.  For example:
 
SQL> select
  2     table_name,
  3     read_only
  4  from
  5     user_tables
  6  where
  7     table_name = 'ACCOUNT_SALES_OCTOBER2007';
 
TABLE_NAME                     READ_ONLY
------------------------------ ----------
ACCOUNT_SALES_OCTOBER2007      YES
 
Any attempts to insert, delete, or update the data from this table while it is in read-only status results in the following error:
 
SQL> insert into
  2     account_sales_october2007
  3  values (3, 100, '31-OCT-2007');
   account_sales_october2007
   *
ERROR at line 2:
ORA-12081: update operation not allowed on table
"VJ"."ACCOUNT_SALES_OCTOBER2007"
 
 
SQL> delete from
  2     account_sales_october2007
  3  where
  4     account_id = 3;
   account_sales_october2007
   *
ERROR at line 2:
ORA-12081: update operation not allowed on table
"VJ"."ACCOUNT_SALES_OCTOBER2007"
 
 
SQL> update
  2     account_sales_october2007
  3  set
  4     sales_amount = 2*sales_amount
  5  where
  6     customer_id = 3;
   account_sales_october2007
   *
ERROR at line 2:
ORA-12081: update operation not allowed on table
"VJ"."ACCOUNT_SALES_OCTOBER2007"
 
While the example above has been simplified to focus on the new feature, there are many business cases where a read-only table is appropriate.  Most of these business cases share the fact that the data is being stored for historical record or reference, and there is no reason that the table should need to be updated.  In many cases, protecting the integrity of the data is a main priority. 

An example is an OLTP table with transactional data and sales compensation based on the table’s monthly activity.  It would be desirable to have each month’s data stored in a read-only table that is protected from modification.
 
The need for read-only data has existed long before the new release of Oracle.  However, this new feature greatly simplifies the process of enabling and disabling read-only status from a table with its simple syntax. 

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