Why is SQL*Plus' desc table not showing not null constraints after using dbms_redefinition

Question

I have the following table that I want to redefine:

create table tq84_redefinition (
  id number primary key,
  ts1 timestamp not null,
  ts2 timestamp
);

Please note the not null constraint on the column ts1.

Using dbms_redefinition, I specifically use copy_constraints => true.

create table tq84_redefinition_int (
    id number,                            -- Note: no primary key to prevent «ORA-01408: such column list already indexed»
    ts1 date,
    ts2 date,
    duration_minutes as ((ts2 - ts1) * 24 * 60)
);


begin
  dbms_redefinition.start_redef_table(
    user, 'tq84_redefinition', 'tq84_redefinition_int',
   'id, '               ||
   'to_date(to_char(ts1, ''ddmmyyyyhh24miss''), ''ddmmyyyyhh24miss'') ts1, ' ||
   'to_date(to_char(ts2, ''ddmmyyyyhh24miss''), ''ddmmyyyyhh24miss'') ts2');
end;
/



-- set serveroutput on

declare  
  cnt_errors binary_integer;
begin

  dbms_redefinition.copy_table_dependents(
    user, 'tq84_redefinition', 'tq84_redefinition_int', 
    -------------------------------------------------------
    copy_indexes     => dbms_redefinition.cons_orig_params,
    copy_triggers    => true, 
    copy_constraints => true, 
    copy_privileges  => true, 
    ignore_errors    => false, 
    num_errors       => cnt_errors,
    copy_statistics  => true, 
    copy_mvlog       => false);

  if cnt_errors > 0 then
     dbms_output.put_line('There were ' || cnt_errors || ' errors.');
  end if;

end;
/

exec dbms_redefinition.sync_interim_table(user, 'tq84_redefinition', 'tq84_redefinition_int');
exec dbms_redefinition.finish_redef_table(user, 'tq84_redefinition', 'tq84_redefinition_int');

Everything seems to work fine except that desc in SQL*Plus doesn't correctly display the not null constraint:

...> desc tq84_redefinition;
 Name                        Null?    Type
 --------------------------- -------- ---------------
 ID                                   NUMBER
 TS1                                  DATE
 TS2                                  DATE
 DURATION_MINUTES                     NUMBER

However, somewhere, the not null constraints is in place. I can see it by issuing a

select constraint_type, constraint_name, search_condition
  from user_constraints
 where table_name = 'TQ84_REDEFINITION';

Also, if I try to insert a record [insert into tq84_redefinition (id) values (999999)] I get the (correct) error message ORA-01400: cannot insert NULL into ("META"."TQ84_REDEFINITION"."TS1").

Edit: The version (v$version) is:

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
PL/SQL Release 11.2.0.1.0 - Production
CORE    11.2.0.1.0      Production
TNS for 32-bit Windows: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production

Edit 2 @Munchi, the result of your suggested select statement

select 
  column_name as "Name",
  nullable as "Null?",
  concat(concat(concat(data_type,'('),data_length),')') as "Type"
from
  user_tab_columns
where
  table_name = 'TQ84_REDEFINITION'

is

Name                           N Type
------------------------------ - --------------
ID                             Y NUMBER(22)
TS1                            Y DATE(7)
TS2                            Y DATE(7)
DURATION_MINUTES               Y NUMBER(22)

Answer 1

This is a known bug (description is not public unfortunately):

Bug:4396234 ET10.2OREDEF: NULLABLE COL OF *_TAB_COLUMNS TABLE NOT UPDATED AFTER ONLINE REDEF

The NOT NULL constraints are copied as NOVALIDATE, and you have to set them to VALIDATE state manually, e.g:

ALTER TABLE t84_redefenition ENABLE VALIDATE CONSTRAINT constraint_name;

The primary key issue is similar, but there is no bug reported for that. However I have found that disabling and reenabling it solves the problem.

ALTER TABLE t84_redefinition DISABLE PRIMARY KEY;
ALTER TABLE t84_redefinition ENABLE PRIMARY KEY;

Answer 2

The Not Null clause of a table column definition sets a flag in $sys$cols and generates a sys_xxxxxx constraint on the column.

dbms_redef however just copies the sys_xxxxxxx constraint and omits setting the flag. It's this flag that gets decoded into the "Nullable?" display (say) on SQLDeveloper columns tab of a table.

So what's the effect of this -

Well, referential integrity is maintained - trying to insert a null into a column where the constraint exists still fails.

However I read somewhere (I'll have to find that article again) that the CBO looks at the $sys$cols flag (not the constraint) as part of optimisation for certain queries.

Consider an index based on a not null column. This index may be a candidate for satisfying a select count(*) type query rather than doing a FTS. If the $sys$cols is itself null (not set by re-def) that index won't be considered.

I think it is worth the effort of adding steps to your re-def code

After re-def - save the names of not null cols from constraints
- drop not null constraints - reapply via alter table modify col not null clause

Even just to quieten the Java dev moaners who spot the before and after difference.