Oracle Scratchpad

March 28, 2022

Drop column bug

Filed under: Bugs,Infrastructure,Oracle — Jonathan Lewis @ 11:29 am BST Mar 28,2022

In the previous note about a problem dropping virtual columns the “guilty party” that made it impossible to drop any columns was based on a complex data type owned by the MDSYS (Spatial) schema. This note demonstrates the same problem with a very simple example created from scratch in an ordinary user schema.

rem     Script:         object_virtual_col.sql
rem     Author:         Jonathan Lewis
rem     Dated:          Mar 2022
rem     Last tested 

create type point_type as object(x_coord number, y_coord number);

create or replace function my_point(inpoint in point_type)
return point_type
deterministic as 
        return inpoint;

show errors

create table t1 (id, n1, p1, n2, v1, padding)
with generator as (
                rownum id
        from dual 
        connect by 
                level <= 1e4    -- > comment to avoid WordPress format issue
        rownum                          id,
        rownum                          n1,
        point_type(rownum, rownum)      p1,
        rownum                          n2,
        lpad(rownum,10,'0')             v1,
        lpad('x',100,'x')               padding
        generator       v1
        rownum <= 100   -- > comment to avoid WordPress format issue

                ownname     => null,
                tabname     => 'T1',
                method_opt  => 'for all columns size 1'

alter table t1 add constraint t1_pk primary key(id);

So I’ve declared a type “point” which is an object with two attributes of type number, and I’ve created a function that takes a point as its input parameter and returns a point. Then I’ve created a table which includes a column of type point.

Let’s start with a little reminder of what a pain it is to use even simple object types correctly. What’s going to happen with the following three SQL statements:

select    p1.x_coord from t1    where rownum <= 4;
select t1.p1.x_coord from t1    where rownum <= 4;
select t1.p1.x_coord from t1 t1 where rownum <= 4;

The first two will fail – the first one shouldn’t be too surprising, the second does seem a little unreasonable:

ORA-00904: "P1"."X_COORD": invalid identifier
ORA-00904: "T1"."P1"."X_COORD": invalid identifier

So let’s try adding some virtual columns to pick out the X value:

alter table t1 add x_val generated always as (p1.x_coord) virtual;
alter table t1 add c_val generated always as (cast(p1.x_coord as number)) virtual;

The first call will fail (ORA-54016: Invalid column expression was specified) but the second will succeed. What if we try to hide out point column behind a call to our function:

alter table t1 add fp_val generated always as (my_point(p1)) virtual;
alter table t1 add fx_val generated always as (my_point(p1).x_coord) virtual;

Again the first call will fail (ORA-54004: resultant data type of virtual column is not supported) but that’s a documented restriction – a user-defined type may not be used as the type of a virtual column and I wasn’t at that point trying to return just the one attribute.

The second call, however, will succeed. So I can’t create a virtual column p1.x_coord, but I can create a virtual column my_point(p1).x_coord.

We now have two virtual columns that should return the required values, so that’s do a quick check with a couple of simple queries – cut and paste:

SQL> select fx_val "my_point(p1).x_coord" from t1 where rownum <= 4;


4 rows selected.

SQL> select c_val  "cast(p1.x_coord as -" from t1 where rownum <= 4;

cast(p1.x_coord as -

4 rows selected.

Finally we’ll finish off by demonstrating that I’ve just created a problem that no-one will notice until long after I’ve left the site (maybe):

SQL> alter table t1 drop column n1;
alter table t1 drop column n1
ERROR at line 1:
ORA-00904: "TEST_USER"."MY_POINT": invalid identifier

After creating (and using successfully) the virtual column that calls my function, I can’t drop any of the columns in the table.


The manuals have a stated restriction for virtual columns that they cannot be a user-defined type, and this restriction seems to carry forward to an attribute of a user-defined type unless the attribute has been cast() to a base type.

The same restriction seems to apply to functions returning a user-defined type, but not to the individual attributes of the returned value – it is not necessary to cast() them to a base type. However, if you (accidentally) take advantage of this relaxation of the restriction you will be unable to drop any columns from the table in the future.

1 Comment »

  1. […] Drop column bug (Mar 2022): a follow-up to the above using user-defined types. […]

    Pingback by Bug Catalogue | Oracle Scratchpad — March 28, 2022 @ 11:31 am BST Mar 28,2022 | Reply

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