Thread: Re: How to enforce the use of the sequence for serial columns

> create table mytable (myid serial primary key constraint
mytable_myid_chk check
(myid = currval('mytable_myid_seq'), mydata varchar(255), ...);


I'd like to clarify that this will not be a full solution, since it will
not allow update of the table unless nextval has been used in the same
sequence already.

There seems to be a hack in case of oracle that allows using currval
without nextval, but it's a hack and I don't know if there's an equivalent
in postgresql (http://rootshell.be/~yong321/computer/sequence.txt).
(Oracle seems to have "disable" option when adding check constraint by
alter table, but the context seem a bit different from ours so may not be
useful in our case, at any rate.)




Regards

Ben K.
Developer
http://benix.tamu.edu




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"Ben K." <bkim@coe.tamu.edu> writes:

> > create table mytable (myid serial primary key constraint
> mytable_myid_chk check (myid = currval('mytable_myid_seq'), mydata
> varchar(255), ...);
>
>
> I'd like to clarify that this will not be a full solution, since it
> will not allow update of the table unless nextval has been used in the
> same sequence already.

It's not a full solution anyway since it prevents any kind of update
on the table due to check constraints firing  even if target field not
updated.  See below;


jerry@jerry#
= create table foo (a serial check (a = currval('foo_a_seq')), b int);
NOTICE:  CREATE TABLE will create implicit sequence "foo_a_seq" for serial column "foo.a"
CREATE TABLE
jerry@jerry#
= insert into foo values (default, 1);
INSERT 0 1
jerry@jerry#
= insert into foo values (default, 1);
INSERT 0 1
jerry@jerry#
= insert into foo values (default, 1);
INSERT 0 1
jerry@jerry#
= insert into foo values (default, 1);
INSERT 0 1
jerry@jerry#
= select * from foo;
 a | b
---+---
 1 | 1
 2 | 1
 3 | 1
 4 | 1
(4 rows)

jerry@jerry#
= update foo set b=2;
ERROR:  new row for relation "foo" violates check constraint "foo_a_check"
jerry@jerry#
=


>
> There seems to be a hack in case of oracle that allows using currval
> without nextval, but it's a hack and I don't know if there's an
> equivalent in postgresql
> (http://rootshell.be/~yong321/computer/sequence.txt). (Oracle seems to
> have "disable" option when adding check constraint by alter table, but
> the context seem a bit different from ours so may not be useful in our
> case, at any rate.)
>
>
>
>
> Regards
>
> Ben K.
> Developer
> http://benix.tamu.edu
>
>
>
>
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>
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>

--
-------------------------------------------------------------------------------
Jerry Sievers   305 854-3001 (home)     Production Database Administrator
                305 321-1144 (mobil    WWW E-Commerce Consultant

> It's not a full solution anyway since it prevents any kind of update
> on the table due to check constraints firing  even if target field not
> updated.  See below;

Right. It was not a valid idea ...


Regards,

Ben K.
Developer
http://benix.tamu.edu