Re: Problem with sequence increment

On Wed, Jan 25, 2012 at 09:42:05AM -0430, Francisco Calderón wrote:
> Hello,
>
> I am having a situation with postgresql 8.3, i have two tables, ta and tb,
> with a relation "one tb has many ta" and... well, i will let the SQL talk
> for me ;)
>
> -----------SQL-----------
> CREATE TABLE tb
> (
>   id serial NOT NULL,
>   descripcion character varying(200) NOT NULL,
>   CONSTRAINT tb_pkey PRIMARY KEY (id)
> )
> WITH (OIDS=FALSE);
> INSERT INTO tb (descripcion) values ('desc 1');
> INSERT INTO tb (descripcion) values ('desc 2');
> CREATE TABLE ta
> (
>   id serial NOT NULL,
>   descripcion character varying(200),
>   tb_id integer default null,
>   CONSTRAINT ta_pkey PRIMARY KEY (id),
>   CONSTRAINT ta_tb_id FOREIGN KEY (tb_id)
>       REFERENCES tb (id) MATCH SIMPLE
>       ON UPDATE NO ACTION ON DELETE NO ACTION
> )
> WITH (OIDS=FALSE);
>
> When i make an insert like this:
>
> INSERT INTO ta (descripcion, tb_id) values ('prueba', 0);
>
> we can expect this error:
>
> ERROR:  insert or update on table "ta" violates foreign key constraint
> "ta_tb_id"
> DETAIL:  Key (tb_id)=(0) is not present in table "tb".
>
> and that is what i am getting but the unusual situation is the sequence
> "ta_id_seq" is incrementing every time i get the "violates foreign key
> constraint" error and i think this is not a good behavior, what do you
> think?
>

That is how sequences work. If you want different behavior, use another
process or method to produce a sequential count. Hint, it will involve
a lot of locking and be much slower than a sequence. It would be better
to have your application handle gaps in the sequence.

Regards,
Ken