Subtype Supertype with Oracle Object Type Creation. Limit on the number of subtypes?

Question

I have run into an issue when creating a object type in Oracle 10g that inherits from a supertype. We currently have many object types that inherit from this supertype and recently the compiler started throwing the following errors

ORA-30745: error occured while trying to add column "SYS_NC_ROWINFO$" in table "DATA_CACHE.CACHE_ENTRIES"

ORA-01792: maximum number of columns in a table or view is 1000

Is there a cap on the number of subtypes you can generate that inherit from a supertype?

Answer 1

When you create tables with columns based on user-defined types, Oracle creates additional "secret" columns for you under the covers. For example:

SQL> create type emp_data_t  as object (empno number, ename varchar2(30));
  2  /

Type created.

SQL> create table emp_data_table (id int, emp_data emp_data_t);

Table created.

This table appears to have 2 columns:

SQL> desc emp_data_table
 Name                       Null?    Type
 -------------------------- -------- ------------------------
 ID                                  NUMBER(38)
 EMP_DATA                            EMP_DATA_T

... but it really has four:

SQL> select name
  2  from sys.col$
  3  where obj# = (select object_id 
  4                from user_objects 
  5                where object_name='EMP_DATA_TABLE');

NAME
------------------------------
ID
EMP_DATA
SYS_NC00003$
SYS_NC00004$

As you have seen, Oracle has a limit of 1000 columns per table. This limit will include any of these hidden columns derived from types and supertypes. It looks like your table has exceeded this limit.