Re: False unique constraint violation (exception block)

On Fri, Apr 12, 2013 at 8:44 AM, whiplash <whiplash(at)bss(dot)org(dot)ua> wrote:

>
> CREATE UNIQUE INDEX idx_table0_unique
> ON table0 ( field0, coalesce ( field1, 'INDEX_COLUMN_NULL' ) );
>
> I created function for insert only unique record (part of code):
>
> BEGIN
> INSERT INTO table0 ( field0, field1 ) VALUES ( p_field0, p_field1 )
> RETURNING id INTO v_table0_id;
> EXCEPTION WHEN unique_violation THEN
> SELECT id FROM table0 WHERE field0 = p_field0 AND field1 = p_field1;
> END;
>
> I use this function for add data to table and sometimes I getting false
> unique violation (in block above). Why?
>

if p_field1 is NULL, then the select cannot return any rows, but can still
violate the constraint.

Also, you may have a race, where the row exists when the insert was
attempted, but was gone by the time it tried to do the select.

Cheers,

Jeff