Re: strange constraint behavior

In your 1st query, very likely the optimizer has injected the constraint text 'FLAG IS NOT NULL' and through transitive closure it then generated a "NULL IS NOT NULL" filter predicate.

If you show us the execution plan, we can verify this.

On Mon, Oct 10, 2016 at 3:48 PM, Anton Bushmelev <djeday84_at_gmail.com> wrote:

> hello, look at new patch from developer and found strange construction,
> here I’ll try to reproduce what they want to do:
>
> create table tt as select level id from dual connect by level <=100;
> alter table tt add flag varchar2(50) default '' not null ;
> Table altered.
>
> select dump (flag,8 ) as dmp from tt ;
>
> DMP
> ----------
> NULL
> NULL
> …..
> NULL
>
>
> select count (*) from tt where flag is null;
>
> COUNT(*)
> ----------
> 0
>
> select count (*) from tt where flag is not null;
>
> COUNT(*)
> ----------
> 100
>
>
> How it is possible ? =)))
>
>
> ps: if I add default *null, *then all goes well:
>
> alter table tt add flag varchar2(50) default null not null ;
> alter table tt add flag varchar2(50) default null not null
> *
> ERROR at line 1:
> ORA-01758: table must be empty to add mandatory (NOT NULL) column
>
> create table tt as select level id from dual connect by level <=100;
> alter table tt add flag varchar2(50) default null not null ;
> alter table tt add flag varchar2(50) default null not null
> *
> ERROR at line 1:
> ORA-01758: table must be empty to add mandatory (NOT NULL) column
>
>
> ps: sorry for my English
>
>

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